forces

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  • Created by: ellie_225
  • Created on: 20-05-18 11:07
what is a scalar
magnitude only.
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what is a vector
have magnitude and an associated direction
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what can vectors be represented by
an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction of the vector quantity.
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examples of scalars
temperature, distance, speed, time, mass, energy, power
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examples of vectors
Force, weight, displacement, velocity, acceleration, momentum,
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drawing vectors
You can represent a vector using an arrow. The length of the arrow shows the magnitude of the vector. The direction of the arrow shows the direction of the vector quantity.
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what is a force
A force is a push or pull that acts on an object due to the interaction with another object. All
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contact force
the objects are physically touching e.g. include friction, air resistance, tension and normal contact force.
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non contact force
the objects are physically separated. e.g. gravitational force, electrostatic force and magnetic force.
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what is a resultant force
A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is called the resultant force.
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can a single force be resolved into two components
A single force can be resolved into two components acting at right angles to each other. The two component forces together have the same effect as the single force.
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how can the size of the resultant force be found
by using geometry or by making a scale drawing and measuring it.
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resolving a force
force involves finding 2 perpendicular forces that have a resultant force which is the same as the original force. Forces are usually resolved in to horizontal and vertical components.
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balanced forces
equal in size and opposite in direction
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what happens if forces are balanced
there is no resultant force. The object will continue doing whatever it is doing. It will stay still or continue moving at a constant velocity.
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what happens if the forces are unbalanced
there is a resultant force. The object will accelerate (change direction) in the direction of the unbalanced force.
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gravity
Weight is the force acting on an object due to gravity. The force of gravity close to the Earth is due to the gravitational field around the Earth. • The weight of an object depends on the gravitational field strength at the point where the object
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weight =
mass x gravitational field strength
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weight equation units
weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg
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center of mass
The weight of an object may be considered to act at a single point
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weight and mass
The weight of an object and the mass of an object are directly proportional.
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what is weight measured in
using a calibrated spring-balance (a newtonmeter).
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what is mass
is a measure on the amount of matter in an object. It is measured in kg.
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work done
When a force causes an object to move through a distance work is done on the object. So a force does work on an object when the force causes a displacement of the object.
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work done =
force x distance moved along the line of action of the force
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what is the same as 1 joule
One joule of work is done when a force of one newton causes a displacement of one metre
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work done against the frictional forces acting on an object can cause a
rise in the temperature of the object
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power is the rate
at which energy is transferred. A powerful machine can do work and transfer energy quickly.
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The extension of an elastic object, such as a spring, is directly proportional
to the force applied, provided that the limit of proportionality is not exceeded
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force =
spring constant x extension
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force equation units
force, F, in newtons, N spring constant, k, in newtons per metre, N/m extension, e, in metres, m
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a force that strenches or compressed
a spring does work and elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal.
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elastic potential energy =
0.5 x specific constant x extension2
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elastic stretch
Elastic materials can be stretched or squashed and then return to their original shape. Plastic materials are permanently deformed when a force is applied to them.
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what happens when the force is applied
If a force is applied to a spring, the spring stretches. The extension is the new length minus the original length When the spring is stretched it stores elastic potential energy. It returns to its original length when the load is removed.
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the extension is the length -
original length
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extension graph
The first section of the graph of extension against load is a straight line through the origin. This shows that extension is proportional to load. The spring obeys Hooke's Law.
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hookes law
The extension of a spring is directly proportional to the load, provided the limit of proportionality has not been exceeded.
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if a large load is applied to the spring
it exceeds the elastic limit. If the load is removed the spring will not return to its original length - it is permanently deformed.
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do elastic bands obey hookes law
Elastic bands do not obey Hooke's Law. A graph of extension against load is curved.
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do metal wires obey hookes law
Metal wires obey Hook's Law, but will reach a point where they stretch rapidly and then break.
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the extension of a spring is proportional to
the load as long as the limit of proportionality is not exceeded. • This means a graph of load against extension is a straight line through the origin. • The equation of the graph is of the form F = kx where k is a constant.
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force =
spring constant x extension
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what does a large spring constant mean
that it requires a large force to stretch the spring so the spring is ‘stiff’.
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what happens when a spring is streached
work is done on it by the load. This means that energy is transferred to it. This is (stored) elastic potential energy.
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what is a force
A force or a system of forces may cause an object to rotate
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what is the turning effect
The turning effect of a force is called the moment of the force.
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moment of a force =
distance x force
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turning effect - what if an object is balalnced
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
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center of gravity
¥ The mass of an object can be thought of as being concentrated at a point. ¥ This point is called the centre of gravity or mass. ¥ The weight of a body acts through the centre of gravity
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to find the centre of gravity of an irregular object
1. Make a small hole in the card 2. Suspend the card from a long pin 3. Suspend a plumb-line from the pin. 4. Mark the position of the plumb-line on the card 5. Repeat 2 or 3 times 6. The centre of gravity is where the lines cross
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middle of a regular shape
¥ If the card is a regular shape, the centre of gravity will be on the axis of symmetry. ¥ It should be possible to balance the card at the centre of gravity
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stability
When an object is tilted ¥ If the centre of gravity is over the base of the object, the object will return to its original position (a). It is stable ¥ If the centre of gravity is outside the base, the object will topple over (C). It is unstable.
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moments
¥ A moment is the turning effect of a force ¥ Moments are measured in Nm ¥ Perpendicular means at right angles
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moment of force =
force x perpendicular distance from the pivot
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if an object is balanced
If an object is balanced (in equilibrium) then the moments turning it in one direction must cancel out the moments in the other direction.
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anticlockwise moments =
clockwise moments
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what do levers and gears emit
these both transmit rotational forces
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levers as force multipliers
If d2 is much larger than d1, F2 will be much smaller than F1 .
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gears
The pedals turn a gear which transmits the force to the wheels. The different number of teeth on the gear make the greas combination into a force multiplier. You put the bike in a low gear to go up hill, where you need lots fo force, but move slowly.
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gears 2
On the flat, you put the bike in a high gear so that you can move fast, but you don’t need so much force.
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pressure in fluids
• A fluid can be either a liquid or a gas. • The pressure in fluids causes a force normal (at right angles) to any surface. The pressure at the surface of a fluid can be calculated using the equation:
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pressure =
force normal to a surface / areas of that surface
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what is pressure
Pressure is a measure of how spread out a force is • Pressure is measured in N/cm2 or N/m2 • 1 N/m2 = 1 Pascal (Pa)
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pressure =
height of the column x density of the liquid x gpe
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A partially (or totally) submerged object experiences
greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.
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what is a fluid
liquid or gas
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how is pressure in a fluid caused
by the particles bumping in to each other and their container and exerting a force on it.
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the atmosphere
The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.
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what creates atmospheric pressure (ap)
molecules colliding w/ surface create ap. The no. of air molecules above a surface decreases as the height of the surface above ground level +. height increases there is always less air above a surface than their is at lower height.
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general atmospheric pressure
atmospheric pressure decreases with an increase in height.
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what is distance
Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity.
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what is displacement
Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity.
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is displacement and distance vectors or scalars
Distance is a scalar and has size; displacement is a vector and has both size and direction.
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is speed a scalar
Speed does not involve direction. Speed is a scalar quantity.
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is speed constant
The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing
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what factors affect speed
The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled.
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what things vary in speed
It is not only moving objects that have varying speed. The speed of sound and the speed of the wind also vary.
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value of sound
330n m/s
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for constant speed - distance =
speed x time
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velocity of an onject
is its speed in a given direction. Velocity is a vector quantity
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do objects travel at a constant speed
no. For example a car will speed up and slow down depending on road conditions.
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if an object moves along a straight line
the distance travelled can be represented by a distance–time graph.
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if an object is accelerating
If an object is accelerating, its speed at any particular time can be determined by drawing a tangent and measuring the gradient of the distance–time graph at that time.
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how can speed be found from a graph
The speed can be found from the gradient of the graph. • Where the graph is horizontal, the object is not moving
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if the speed is changing
find the instantaneous speed by drawing the tangent to the curve and then finding the gradient of the tangent.
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the average acceleration =
change in velocity / time taken
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an object that slows down
is decelerating
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how can the distance travelled by an object be calculated
area under a velocity–time graph.
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final velocity2 - initial velocity2
2 x acceleration x distance
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acceleration
Acceleration is the rate of change of speed
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What does it mean if the graph is steeper
the higher the acceleration, so the object is speeding up.
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Acceleration on the graph
is the gradient of the graph? If the gradient is negative, the object is slowing down.
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What does the horizontal part of the graph show
the object is moving at a constant velocity (speed)
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how can the distance travelled?
from the area under the graph. If the graph is horizontal, you can use distance = speed x time, but if the graph slopes, the speed is changing so you must find the area.
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you can use an equation if the acceleration is constant
Final velocity2 = initial velocity2 –2 x acceleration x displacement
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Newton’s First Law:
If the resultant force acting on an object is zero and the object is stationary, the object remains stationary the object is moving, the object continues to move at the same speed and in the same direction. the object continues to move at the same v
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Newton’s law in a vehicle
when a vehicle travels at a steady speed, the resistive forces balance the driving force. the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
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Newton’s First Law on an object
An object at rest stays at rest + object that is moving continues to move at a constant speed, unless acted on by an unbalanced force. If balanced, so that the resultant force is zero it will continue to move with constant speed and direction
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Newton's Second Law
The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.
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Equation of second law
resultant force = mass x acceleration
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units for resultant force
force, F, in newton’s, N mass, m, in kilograms, kg acceleration, a, in metres per second squared, m/s2
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what happens If the forces on an object are balanced,
the resultant force is zero and it will continue moving at a constant speed. If it is not moving, it will remain stationary.
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What happens If the forces on an object are unbalanced,
the object will accelerate in the direction of the resultant force.
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What is the acceleration near the earth surface
Near the Earth’s surface any object falling freely under gravity has an acceleration of about 9.8 m/s2.
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Objects falling through a fluid
initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
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What is the drag force caused by?
the drag force is caused by air resistance. the faster an object falls, the larger the air resistance the weight of the falling object does not change
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Newton's Third Law
Whenever two objects interact, the forces they exert on each other are equal and opposite.
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Stopping distance of vehicle
is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) + the distance travels under the braking force. For a given braking force the greater the speed of the vehicle, the + the stop distance
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Reaction time
Reaction times vary from person to person. Typical values range from 0.2 s to 0.9 s. A driver’s reaction time can be affected by tiredness, drugs and alcohol. Distractions may also affect a driver’s ability to react.
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The braking distance of a vehicle can be affected
by adverse road and weather conditions and poor condition of the vehicle.Adverse road conditions include wet or icy conditions. Poor condition of the vehicle is limited to the vehicle's brakes or tyres.
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When a force is applied to the brakes of a vehicle
, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
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The greater the speed of a vehicle the greater
the braking force needed to stop the vehicle in a certain distance.
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The greater the braking force the greater
the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control.
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Thinking distance
is the distance the car moves while the driver is thinking about pressing the brake. It is the distance travelled during the driver's reaction time.
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Thinking distance increases if:
the car is going faster - the driver is tired - the driver has had alcohol or drugs the driver is distracted (egg mobile phone)
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Braking distance
is the distance the car moves after the brakes have been pressed
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Braking distance increases if:
the car is going faster - the car is heavier - the car brakes are worn - the car tyres are worn - the road surface is slippery, egg ice, rain,
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stopping distance =
thinking distance + braking distance
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what happens for stopping distance
• When the brakes are on, the car slows down, because of friction. • Kinetic energy is transferred to thermal energy in the surroundings. • Work is done by friction so energy is transferred.
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the faster the car is travelling
more kinetic energy it has and so the more energy that must be transferred. More work must be done, so the braking force must act over a longer distance
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momentum =
mass x velocity
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In a closed system, the total momentum before an event is equal
to the total momentum after the event. This is called conservation of momentum.
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Units for momentum:
kg m/s
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is momentum a vector
Momentum is a vector quantity – the direction is important
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Conservation of momentum:
The total momentum of a closed system in a specific direction remains constant as long as no external forces act on the system.
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What happens when two objects collide
the total momentum before the collision is the same as the total momentum afterwards.
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Elastic and inelastic collisions
• In any collision momentum and total energy are conserved • In a perfectly elastic collision the kinetic energy is conserved • In an inelastic collision, kinetic energy is not conserved
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Changes in momentum
When a force acts on an object that is moving, or able to move, a change in momentum occurs.
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force =
rate of change of momentum.
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examples of momentum:
1. Crash helmets, crumple zones on cars, seat belts that stretch slightly. These all: • increase the time it takes an object to stop and so • reduce the force needed • to give the same change in momentum.
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examples of momentum: 2
Hitting a golf ball or tennis ball hard To get the largest effect from a given force you follow through with the hit so that you: • Increase the time the force is exerted on the ball which • Increases the change in momentum • For a given force
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