# As Fast As You Can

Revision cards for the topic As Fast As You Can in the Physics P2 exam

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## Speed

• Speed is how fast something is moving
• Speed is measured in m/s, km/h or mph.
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## Velocity

• Velocity is speed with direction.
• The direction of velocity is usually indicted with a (+) or (-) sign, e.g +40mph and -40mph are opposite directions.
• VELOCITY = DISTANCE / TIME
• Distance is sometimes described as displacement.
• Displacement is used to describe the distance covered in a certain direction.
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## Acceleration

• The acceleration of an object is the rate in which the velocity changes. In other words it is how quickly an object is speeding up/ slowing down.
• Acceleration is measured in metres per second per second (m/s squared)
• To work out the acceleration of an object we need to know: change in velocity, time taken for change in velocity.
• ACCELERATION = CHANGE IN VELOCITY / TIME TAKEN FOR CHANGE
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## Velocity-Time Graph

• The slope of a velocity time graph represents the acceleration.
• Steeper gradient = greater acceleration
• The area under the line shows the total distance travelled.
• Straight horizontal line = the object is not accelerating
• Diagonal straight line upward = constant acceleration
• Diagonal straight line downward = constant decceleration
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## Forces

• Forces are pushes or pulls measured in NEWTONS
• If a girl is stood on the ground, she is exerting a downward force and the ground in return exerts an upward force on her which is equal in size and opposite in direction.
• The movement of an object depends on forces acting on it.
• If a car is moving forward, the driving force is in the direction of movement and the resistive forces are air resistance and friction.
• Accelerating = driving force GREATER than resistive force
• Braking = driving force SMALLER than resistive force
• Constant speed = driving force EQUAL to resistive force
• RESULTANT FORCE = DRIVING FORCE - RESISTIVE FORCES
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## Force, Mass and Acceleration

• If an unbalanced force acts on an object then the acceleration of an object will depend on:

- the size of the unbalanced force: bigger force = greater acceleration

- the mass of the object: bigger mass = smaller acceleration

• FORCE = MASS x ACCELERATION
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## Terminal Velocity

• Falling objects experience: downward force of weight (always stays the same), and the upward force or air resistance (or drag).
• When a skydiver jumps out of a plane, the descent is considered before and after the parachute is opened:

- Before: initially accelerates due to gravity, as he falls experiences air resistance in the opposite direction (not as big as weight so accelerates). Eventually air resistance is equal to weight so the resultant force is zero and his speed becomes constant. This is terminal velocity.

- After: unbalanced forces act air resistance increased to bigger than weight.  So, his speed and the air resistance decreases. Eventually air resistance and weight once again become equal. He falls at a steady speed again (slower than before). The new terminal velocity.

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## Stopping Distances

- The stopping distance of a vehicle depends on:

• thinking distance: distance travelled by vehicle while the driver realises they need to apply the brakes and them actually applying them.
• stopping distance: distance travelled by vehicle between the brakes being applied and the vehicle stopping.

THINKING DISTANCE + BRAKING DISTANCE = STOPPING DISTANCE

- 30MPH: 6m thinking, 6m braking....40MPH: 9m, 14m....50MPH: 12m, 24m

- The mass of a vehicle affects the braking distance

- The conditions of the road affect the braking distance as different conditions will affect the friction between the road the tyres.

- The drivers reaction time affects the thinking distance. Things like drinking alcohol, drugs, fatigue and distractions can affect reaction time.

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## Momentum

• Momentum is a measure of the state of movement of an object and depends on: mass and velocity

MOMENTUM = MASS x VELOCITY

• When a force acts on a moving object or a stationary object, it will experience a change in momentum.

CHANGE IN MOMENTUM = FORCE x TIME TAKEN FOR CHANGE

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