Physics P4 (OCR21)
- Created by: Alex
- Created on: 07-05-10 09:40
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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:
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- Gradient = speed
- Flat sections = stationary
- Steep gradient = faster speed
- 'Downhill' = travelling in opposite direction
- Curves = Acceleration/deceleration
- Steepening curve = speeding up
- Levelling off curve = slowing down
- Gradient = Δy/Δx
- 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:
- Gradient = acceleration
- 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…
- Friction between solid surfaces which are gripping (static).
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