Physics P5

• Created by: abbiedye
• Created on: 29-07-18 11:53

What are vector quantities?

• Have a magnitude and a direction
• Include - force, velocity, displacement, acceleration, momentum
• Represented by an arrow
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What are scalar quantities?

• Only have a magnitude and no direction
• Include - speed, distance, mass, temperature, time
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What are contact forces?

• When two objects have to be touching for a force to act
• Contact forces = friction, air, resistance, tension in ropes, normal contact force, etc
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What are non-contact forces?

• If the objects do not need to be touching for the force to act
• Non-contact forces = magnetic forces, gravitational force, electrostatic force, etc
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What is an interaction pair?

• A pair of forces that are equal and opposite and act on two interacting objects (basically Newton's Third Law)
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What is gravity?

• Attracts all masses
• Important effects - makes all things fall towards the ground and gives everything a weight
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Differences between weight and mass

• Mass = amount of 'stuff' in an object, same mass whether on earth or the moon and it is not a force (measured in kg with a mass balance)
• Weight = force acting on an object due to gravity depends on the gravitational field at the location and it is a force measured in Newtons by a calibrated spring (newton metre)
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How are mass and weight linked?

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)

The gravitational field strength of the earth = 9.8N/kg

• Mass and weight are directly proportional
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What is a resultant force?

• When a force moves an object through a distance, energy is transferred and work is done on the object

Work done (J) = force (N) x distance (moved along the line of the force) (m)

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How do you calculate forces?

• Use scale drawings to find resultant forces
• An object is in equilibrium if the forces on it are balanced
• You can split a force into components
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Stretching, compressing and bending in energy

• Applying a force may cause it to stretch, compress or bend
• Work done = force stretches or compresses an object - if it is elastically deformed - all energy transferred to the elastic potential energy store
• elastically deformed = the object can go back to its original shape
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How does extension link to force?

• The extension of a stretched spring (or another elastic object) is directly proportional to the load or force applied

Force (N) = spring constant (N/m) x extension (m)

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Link between extension and force

• There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionality
• When the force is great enough, the extension and force are not directly proportional
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Energy stored for linear relationships

• As long as a spring is not stretched past its limit of proportionality, the work done in stretching (or compressing) a spring can be found using

elastic potential energy (J) = 1/2 x spring constant (m) x extension squared (N/m)

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What is distance and displacement?

• Distance = how far an object has moved and it is a scalar quantity that doesn't involve direction
• Displacement = vector quantity and measures the distance and direction in a straight line (starting point)
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What are speed and velocity?

• Speed and velocity both measure how fast you're going, but speed is a scalar quantity and velocity is a vector quantity
• Speed is just how fast you're going with no regard to the direction. Velocity is speed in a given direction.
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How do you calculate speed?

distance travelled (m) = speed (m/s) x time (s)

s = v x t

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What are typical everyday speeds?

• A person walking = 1.5 m/s
• A person running = 3 m/s
• A person cycling = 6 m/s
• A car = 25 m/s
• A train = 30 m/s
• A plane = 250 m/s
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What is acceleration?

• Not the same as velocity or speed
• Change in velocity in a certain amount of time

acceleration = change in velocity / time

deceleration = negative acceleration

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How do you calculate uniform acceleration?

final velocity - initial velocity = 2 x acceleration x distance

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What is terminal velocity?

• Friction is always there to slow things down
• Drag increases as speed increases
• Objects falling through fluids reach a terminal velocity
• Terminal velocity depends on shape and area (streamline)
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What is Newton's First Law?

• If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it'll just carry on moving at the same velocity (same speed and direction)
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What is acceleration linked to?

• The larger the resultant force acting on an object, the more the object accelerates. The force and the acceleration are directly proportional
• Acceleration is also inversely proportional to the mass of the object
• Newton's second law = resultant force = mass x acceleration
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What is Inertia?

• Inertia is the tendency for the motion to remain unchanged
• An object's inertial mass measures how difficult it is to change the velocity of an object
• Inertial mass can be found using Newton's Second Law rearranged into m = F / a
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What is Newton's Third Law?

• Newton's Third Law says - When two objects interact, the forces they exert on each other are equal and opposite
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How do you calculate stopping distance?

stopping distance = thinking distance + braking distance

typical car braking distances - 14m at 30mph, 55m at 60mph and 75m at 70 mph

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What factors effect thinking distance?

• Your speed
• Your reaction time
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What factors effect braking distance?

• Your speed
• The weather or road surface
• The conditions of your tyres
• How good your brakes are
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How do reaction times differ?

• Everyone's reaction time is different, but a typical reaction time is between 0.2 and 0.9 seconds. This can be affected by tiredness, drugs or alcohol
• Distractions can also affect your ability to react
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What is momentum?

• Momentum is mainly about how much 'oomph' an object has (it is a vector quantity)
• greater the mass/velocity = more momentum
• momentum = mass x velocity
• momentum before = momentum after
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