Physics - Topic 5 - Forces

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  • Created by: hannah.o
  • Created on: 10-02-18 09:52
What do vector quantities have?
Magnitude and direction.
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What are some vector quantities?
Force, velocity, displacement, acceleration, momentum.
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What do scalar quantities have?
Just magnitude.
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What are some scalar quantities?
Speed, distance, mass, temperature, time.
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What is a force?
A push or a pull on an object that is caused by it interacting with something.
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What is a contact force?
When two objects have to be touching for a force to act.
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What are some examples of contact forces?
Friction, air resistance, tension in ropes, normal contact force.
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What is a non-contact force?
If the objects do not need to be touching for the force to act.
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What are some examples of non-contact forces?
Magnetic force, gravitational force, electrostatic force.
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What is an interaction pair?
It is a pair of forces that are equal and opposite and act on two interacting objects.
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What does gravity attract?
All masses, but you only notice it when one of the masses is really big.
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What are the 2 effects of gravity?
On the surface of a planet, it makes all things fall towards the ground. It gives everything a weight.
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What is mass?
Mass is just the amount of 'stuff' in an object. For any given object, this will have the same value anywhere in the universe.
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What is weight?
Weight is the force acting on an object due to gravity. Close to Earth, this force is caused by the gravitational field around the Earth.
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When does gravitational field strength vary?
With a different location. It's stronger the closer you are to the mass causing the field, and stronger for larger masses.
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What does weight depend on?
The strength of the gravitational field at the location of the object. This means that the weight of an object changes with its location.
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What is centre of mass?
A point at which you assume the whole mass is concentrated.
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How is weight measured?
Using a newtonmeter.
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What is the equation for weight?
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
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What does increasing the mass of an object do?
Increases its weight. If you double the mass, the weight doubles too, so they are directly proportional.
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Draw the free body diagram for a skydiver.
.
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What do the size and direction of the arrow show?
The sizes of the arrows show the relative magnitudes of the force and the directions show the directions of the forces acting on the object.
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What is a resultant force?
If you have a number of forces acting at a single point, you can replace them with a single force - the resultant force.
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How is the resultant force found if the forces all act along the same line?
The overall effect is found by adding those going in the same direction and subtracting any going in the opposite direction.
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What happens when a force moves an object through a distance?
When a force moves an object through a distance, energy is transferred and work is done on the object.
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What is the equation for work done?
Work done (J) = Force (N) x Distance (m)
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How do you draw a scale drawing?
1. Draw all the forces acting on an object, to scale. 2. Draw a straight line from the start of the first force to the end of the last force - this is the resultant force. 3. Measure the length of the resultant force on the diagram
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to find the magnitude and the angle to find the direction of the force.
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What happens when you apply a force to an object? What do you need to do this?
You may cause it to stretch, compress or bend. You need more than one force acting on the object.
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When has an object been elastically deformed?
If it can go back to its original shape and length after the force has been removed. These are called elastic objects.
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When has an object been inelastically deformed?
If it doesn't return to its original shape and length after the force has been removed.
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What is the extension of a stretched spring directly proportional to?
The force applied - F e
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What is the equation for force?
Force (N) = Spring constant (N/m) x Extension (m)
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What does the spring constant depend on?
It depends on the material that you are stretching - a stiffer spring has a greater spring constant.
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What is the graph for force and extension?
.
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What does point P show?
The limit of proportionality - it shows that extension is no longer proportional to force.
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What is the equation for elastic potential energy?
Ee = 1/2ke2
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What is a moment?
The turning effect of a force.
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What is the equation for moment?
Moment of a force (Nm) = Force (N) x Distance (m)
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What would a larger force or a longer distance mean?
A larger moment.
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What do levers do?
Levers increase and decrease the distance from the pivot at which the force is applied. Since M=Fd this means less force is needed to get the same moment. This means levers make it easier to do work.
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What are gears?
Gears are circular discs with 'teeth' around their edges. Their teeth interlock so that turning one cause another to turn in the opposite direction.
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What are gears used for?
They are used to transmit the rotational effect of a force from one place to another.
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How can the moment of a force be changed?
Using different sized gears. A force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater. The larger gear will turn slower than the smaller gear.
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What are fluids?
Fluids are substances that can 'flow' because their particles are able to move around.
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What is pressure?
Pressure is force per unit area.
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What does the pressure of a fluid mean?
It means a force is exerted at normal (at right angles) to any surface in contact with the fluid.
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What is the equation for the pressure at the surface of a fluid?
Pressure (Pa) = Force (N) / Area (m2)
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What happens as the depth of the liquid increases?
The number of particles above that point increases. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth.
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What is the equation for the pressure at a certain depth due to the column of liquid above?
Pressure (Pa) = Height (m) x Density (kg/m3) x Gravitational field strength (N/kg)
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What happens when an object is submerged in a fluid (either partially or completely)?
The pressure of a fluid exerts a force on it from every direction. Pressure increases with depth, so the force exerted on the bottom of the object is larger than the force acting on the top of the object, causing a resultant force upwards (upthrust).
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What is the upthrust equal to?
The weight of the fluid that has been displaced by the object.
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What happens if the upthrust on the object is equal to the object's weight?
Then the forces balance and the object floats.
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What happens if an object's weight is more than the upthrust?
The object sinks.
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How do submarines make use of upthrust?
To sink, large tanks are filled with water to increase the weight of the submarine so that it is more than the upthrust. To rise to the surface, the tanks are filled with compressed air to reduce the weight so that it's less than the upthrust.
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What is the atmosphere?
A layer of air that surrounds Earth. It is thin compared to the size of the Earth.
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How is atmospheric pressure created?
It is created on a surface by air molecules colliding with the surface.
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What happens to atmospheric pressure at the altitude increases?
Atmospheric pressure decreases because as the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface.
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What else contributes to atmospheric pressure?
There are fewer air molecules above a surface as the height increases. This means that the weight of the air above it decreases with altitude.
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What is displacement?
It is a vector quantity. It measures the distance and direction in a straight line from an object's starting point to its finishing point.
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What is speed?
Speed is just how fast you're going with no regard to the direction.
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What is velocity?
Velocity is speed in a given direction.
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What is the equation for distance travelled?
Distance travelled (m) = Speed (m/s) x Time (s)
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What is the typical speed for a person walking?
1.5m/s
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What is the typical speed for a person running?
3m/s
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What is the typical speed for a person cycling?
6m/s
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What is the typical speed for a car?
25m/s
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What is the typical speed for a train?
55m/s
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What is the typical speed for a plane?
250m/s
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What is acceleration?
The change in velocity in a certain amount of time.
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What is the equation for acceleration?
Acceleration (m/s2) = Change in velocity (m/s) / Time (s)
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What is deceleration?
Negative acceleration.
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What is the equation for uniform/constant acceleration?
v2 - u2 = 2as Finall velocity2 - Initial velocity2 = 2 x acceleration x distance
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On a distance-time graph, what does the gradient represent?
Speed.
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On a distance-time graph, what do the flat sections represent?
It's stationary - it's stopped.
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On a distance-time graph, what do the straight uphill sections mean?
It is travelling at a steady speed.
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On a distance-time graph, what do curves represent?
Acceleration or deceleration.
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On a distance-time graph, what does a steepening curve mean?
It's speeding up.
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On a distance-time graph, what does a levelling off curve mean?
It's slowing down.
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Show all of these on a distance-time graph?
.
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On a velocity-time graph, what does the gradient represent?
Acceleration.
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On a velocity-time graph, what do flat sections represent?
Travelling at a steady speed.
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On a velocity-time graph, what do uphill sections represent?
Acceleration
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On a velocity-time graph, what do downhill sections represent?
Deceleration.
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On a velocity-time graph, what happens if the graph is steeper?
The acceleration or deceleration is greater.
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On a velocity-time graph, what does the area under any section of the graph equal to?
The distance travelled in that time interval.
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What does friction always act?
It always acts in the opposite direction to movement.
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What must the driving force be to travel at a steady speed?
The driving force needs to balance the frictional forces.
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When do you get friction?
Between two surfaces in contact or when an object passes through a liquid (drag).
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What is drag?
It is the resistance you get in a fluid. Air resistance is a type of drag. Drag increases as speed increases.
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What is the most important factor for reducing drag?
Keeping the shape of the object streamlined. This is where the object is designed to allow fluid to flow easily across it, reducing drag.
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How does an object falling through a fluid reach terminal velocity?
When a falling object first sets off, the force of gravity is much more than the frictional force slowing it down, so it accelerates. As the speed increases the friction builds up. This gradually reduces the acceleration until eventually the
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frictional force is equal to the accelerating force (so the resultant force is 0). It will have reached its maximum speed or terminal velocity and will fall at a steady speed.
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What is the graph to support this?
.
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What is the terminal velocity of any object determined by?
Its drag in comparison to its weight.
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What does the frictional force depend on?
Its shape and area.
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What does Newton's first law say?
If the resultant force on a stationary object is 0, the object will remain stationary. If the resultant force on a moving object is 0, it'll just carry on moving at the same velocity.
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What will a non-0 resultant force produce?
Acceleration or deceleration in the direction of the force.
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What are the 5 different forms this acceleration can take?
Starting, stopping, speeding up, slowing down and changing direction.
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What happens if there is a larger resultant force?
The larger the resultant force acting on an object, the more the object accelerates - the force and the acceleration are directly proportional.
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What is the equation that describes Newton's second law?
Resultant force (N) = Mass (kg) x Acceleration (m/s2)
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What is inertia?
The tendancy for motion to remain unchanged.
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What does an object's inertial mass measure?
It measures how difficult it is to change the velocity of an object.
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What does Newton's third law say?
When two objects interact, the forces they exert on each other are equal and opposite.
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What is the equation for stopping distance?
Stopping distance = Thinking distance + Braking distance
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What is the thinking distance?
How far the car travels during the driver's reaction time (the time between the driver seeing a hazard and applying the brakes).
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What is the braking distance?
The distance taken to stop under the braking force (once the brakes are applied).
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What are typical car braking distances?
14m at 30mph. 55m at 60mph. 75m at 70mph.
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What is thinking distance affected by?
Your speed - the faster you're going the further you'll travel during the time you take to react. Your reaction time - the longer your reaction time, the longer your thinking distance.
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What is braking distance affected by?
Your speed-given braking force, the faster a vehicle travels, the longer it takes to stop. The weather or road surface-if it is wet or icy, or there are leaves or oil on road, there is less grip (less friction) between tyres+road, causing skidding.
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The condition of tyres-if the tyres are bald then they can't get rid of water, leading to skidding on top of water. How good brakes are-worn or faulty brakes won't be able to apply as much force as well maintained brakes, can be dangerous.
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What does braking rely on?
Friction between the brakes and wheels.
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Explain how friction is produced between brakes and wheels.
When brake pedal is pushed, it causes brake pads to be pressed onto wheels. This contact causes friction, which causes work to be done. The work done between brakes and wheels transfers energy from kinetic energy stores of wheels to thermal energy
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stores of brakes. Brakes increase in temperature. The faster a vehicle is going, the more energy it has in its kinetic stores, so more work needs to be done to stop it. Means that a greater braking force is needed to make it stop within a distance.
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What is a typical reaction time?
It is between 0.2s and 0.9s.
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How can your reaction time be affected?
By tiredness, drugs, alcohol or distractions.
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What happens to the thinking distance as a car speeds up?
The thinking distance increases at the same rate as speed. The graph is linear. This is because the thinking time stays pretty constant - but the higher the speed, the more distance you cover in that same time.
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What happens to the braking distance as a car speeds up?
Braking distance increases faster the more you speed up. The work done to stop the car is equal to the energy in the car's kinetic energy store. So as speed doubles, kinetic energy increases by 4 times, and so the work done to stop the car increases
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by 4 times. Since W=Fs and the braking force is constant, the braking distance increases by 4 times.
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What does a stopping distance graph look like?
.
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How can an object have more momentum?
If it has a greater mass or a greater velocity.
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What is the equation for momentum?
Momentum (kgm/s) = mass (kg) x velocity (m/s)
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What is the momentum like in a closed system?
The total momentum before an event is the same as after the event. This is called conservation of momentum.
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When is there a change in momentum?
When a non-zero resultant force acts on a moving object, it causes its velocity to change.
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What is the equation for force?
Force (N) = Change in momentum (kgm/s) / Change in time (s)
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What is the force causing the change equal to?
The rate of change of momentum.
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What does a larger force mean?
A faster change in momentum.
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What safety features do cars have?
Crumple zones-crumple an impact, increasing time taken for car to stop. Seat belts-stretch slightly, increasing time taken for wearer to stop. Air bags-inflate before you hit dashboard of a car. Compressing air inside slows you down more gradually.
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How do bike helmets make you more safe when riding a bike?
They contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash. This reduces the impact on your brain.
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How do crash mats and cushioned playground flooring make a child more safe?
They increase the time taken for you to stop if you fall on them. This is because they are made from soft, compressible materials.
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Card 2

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What are some vector quantities?

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Force, velocity, displacement, acceleration, momentum.

Card 3

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What do scalar quantities have?

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Card 4

Front

What are some scalar quantities?

Back

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Card 5

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What is a force?

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Comments

Grace

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Great cards, are these 9-1?

Anss

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How do you print

zazi

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Great resource thanks

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