# Physics P4 (OCR21)

• Created by: Alex
• Created on: 07-05-10 09:40

SPEED

• Speed = Distance/Time
• EXAMPLE: A cat skulks 20 metres in 40 seconds. Find: a) its speed, b) how long it will take to skulk 75m.
• a) speed = 20/40 = 0.5ms^-1
• b) time = 75m/0.5ms^-1 = 150s
• Pretty rare in real life for an object to stay at exactly the same speed for a long period of time.
• Usually want to find the average speed if the speed varies constantly for a long period of time.
• Speed cameras take an instantaneous speed of a car: Evenly spaced lines are printed on the road and the speed camera measures the time in which the car travels over the lines.

SPEED AND VELOCITY

• On a distance time graph:
• Flat sections = stationary
• Steep gradient = faster speed
• 'Downhill' = travelling in opposite direction
• Curves = Acceleration/deceleration
• Steepening curve = speeding up
• Levelling off curve = slowing down
• Always use standard units: m, kg, l etc.
• Distances can be positive or negative.
• 0 is always start point, +ve = one direction, --ve = other direction.
• Speed is the gradient of a distance/time graph.
• Speed = how fast something is going, it does not have a direction .
• Velociy describes the speed and direction of an object.
• Speed = scalar quantity (mass, temperature, time, length).
• Velocity = vector quantity (force, acceleration, momentum).

VELOCITY

• Velocity can be positive or negative.
• Travel one direction at 20ms^-1, turn around and travel in the opposite direction at -20ms^-1.
• If two objects heading in opposite directions, one can be said to have positive veolcity while the other have a negative velocity.
• On a velocity/time graph:
• Flat sections = steady speed
• Steeper sections = greater acceleration/deceleration
• 'Uphill' = acceleration
• 'Downhill' = deceleration.
• Area under any section (or all of) graph is equal to the distance travelled in that time interval.
• Curve = changing accelerations
• Tachographs plot speed/time when direction isn't important.
• Tachographs are found in lorries to tell managers how long a driver has gone without a break or if they have been speeding.

FORCES AND FRICTION

• Forces occur when two objects interact
• When an object exerts a force on another subject, it interacts with an opposing force: an 'interaction pair'.
• If you push against a wall, the wall will push back just as hard.
• As soon as you stop pushingthe wall, so does the wall.
• If there was no opposing force, you and the wall would fall down.

• If you exert a force of 10N, the wall's results force will be 10N.
• NEWTON'S THIRD LAW!: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
• Moving object usually experience friction.
• When an object is moving relative to another, both objects experience a force in the direction that opposes the movement -- FRICTION!
• Friction between solid surfaces which are gripping (static).
• The Earth's tectonic plates trying to move but friction is so strong they stay put.
• Friction between solid

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# Physics P4 (OCR21)

• Created by: Alex
• Created on: 07-05-10 09:40

SPEED

• Speed = Distance/Time
• EXAMPLE: A cat skulks 20 metres in 40 seconds. Find: a) its speed, b) how long it will take to skulk 75m.
• a) speed = 20/40 = 0.5ms^-1
• b) time = 75m/0.5ms^-1 = 150s
• Pretty rare in real life for an object to stay at exactly the same speed for a long period of time.
• Usually want to find the average speed if the speed varies constantly for a long period of time.
• Speed cameras take an instantaneous speed of a car: Evenly spaced lines are printed on the road and the speed camera measures the time in which the car travels over the lines.

SPEED AND VELOCITY

• On a distance time graph:
• Flat sections = stationary
• Steep gradient = faster speed
• 'Downhill' = travelling in opposite direction
• Curves = Acceleration/deceleration
• Steepening curve = speeding up
• Levelling off curve = slowing down
• Always use standard units: m, kg, l etc.
• Distances can be positive or negative.
• 0 is always start point, +ve = one direction, --ve = other direction.
• Speed is the gradient of a distance/time graph.
• Speed = how fast something is going, it does not have a direction .
• Velociy describes the speed and direction of an object.
• Speed = scalar quantity (mass, temperature, time, length).
• Velocity = vector quantity (force, acceleration, momentum).

VELOCITY

• Velocity can be positive or negative.
• Travel one direction at 20ms^-1, turn around and travel in the opposite direction at -20ms^-1.
• If two objects heading in opposite directions, one can be said to have positive veolcity while the other have a negative velocity.
• On a velocity/time graph:
• Flat sections = steady speed
• Steeper sections = greater acceleration/deceleration
• 'Uphill' = acceleration
• 'Downhill' = deceleration.
• Area under any section (or all of) graph is equal to the distance travelled in that time interval.
• Curve = changing accelerations
• Tachographs plot speed/time when direction isn't important.
• Tachographs are found in lorries to tell managers how long a driver has gone without a break or if they have been speeding.

FORCES AND FRICTION

• Forces occur when two objects interact
• When an object exerts a force on another subject, it interacts with an opposing force: an 'interaction pair'.
• If you push against a wall, the wall will push back just as hard.
• As soon as you stop pushingthe wall, so does the wall.
• If there was no opposing force, you and the wall would fall down.

• If you exert a force of 10N, the wall's results force will be 10N.
• NEWTON'S THIRD LAW!: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
• Moving object usually experience friction.
• When an object is moving relative to another, both objects experience a force in the direction that opposes the movement -- FRICTION!
• Friction between solid surfaces which are gripping (static).
• The Earth's tectonic plates trying to move but friction is so strong they stay put.
• Friction between solid