# AS Physics unit - 2

AQA AS Physics unit - 2 work, kinetics, power...etc

## Motion - SUVAT

Uniform Acceleration = constant Units - ms-2

Acceleration = the rate of change of velocity

a = (v-u)/t (check units i.e. time = in seconds...)

Gravity Acceleration = 9.81ms-2

If deaccelerating the value will be negative

SUVAT EQUATIONS:

## Displacement - Time Graphs

Gradient = VELOCITY

When the velocity is constant the line is straight

The steeper the gradient of the line = the larger the acceleration (if the line is straight it is constant acceleration)

Calculating the velocity (from graph)

Velocity = changing velocity / time taken

(change in y) / change in x)

If the line is curved the acceleration isn't constant = draw a tangent

## Velocity - Time Graphs

The difference between speed-time graphs & velocity-time graphs.

- Velocity shows the direction of the object

Gradient = Acceleration uniform acceleration = straight line

If curved upwards = increasing acceleration

If curved downwards = decreasing deacceleration (negative)

Calculating the acceleration (from graph)

Acceleration = (change in y - change in x) / time

The distance travelled = the area under the graph.

## Free Fall - only gravity

Free Fall = The motion of an object undergoing acceleration at the same rate of 9.81ms-2 on earth with neglective air resistance.

ALL objects will accelerate in free fall at the same rate of 9.81ms-2 despite their weight.

Gravity = 9.81ms-2 on earth ... acceleration = 9.81ms-2

If upward = positive If downwards = negative

The only force acting is weight (mass + gravity)

If calculating a time for free fall remember may need to x2 for upwards and downwards to give a total time.

Final Velocity = 0ms-1 at highest point.

## Projectile Motion

Any object given an initial velocity and then left to fall freely under gravity only is projectile. As long as the initial force is no longer acting on the object or any other force.

- The acceleration of the object is always = to gravity, and is downwards because the force gravity acts downwards
- Therefore the acceleration of the object only effects the vertical component (motion)
- The horizontal velocity of the object is constant, because the acceleration doesn't have a horizontal component.
- This means the vertical & horizontal component are not connected to each other.

Max.projectile range = 45 degrees

## Newtons - Experiment

ALL objects in free fall accelerate at the same rate

Galileo:

Problem to prove this was that the objects fell too quickly to be able to take accurate measurements.

Air resistance effects the rate of which they fall.

Experiment:

He measured the time taken for a ball to roll down a smooth incline plane - becuase it slows the time taken for the ball to fall and reduces air resistance.

By rolling the ball down different fractions of total length of the slope he found that the distance the ball travelled was proportional to the square of the time taken - the ball was accelerating at a constant rate.

## F=ma

Remember - F and A are vectors, the direction of the object accelerates in the same direction as the resultant force.

F must be in Newtons, A = ms-2, M = Kg

## Terminal Speed

The resistive force equals the driving force, resultant force = zero so constant velocity Drag depends on:

- The shape of the object
- Its speed
- Viscosity of what its pasting through (i.e. air, fluid...etc)

Friction = a force that opposes motion - 2 types of friction:

Contact Friction - between solid surfaces

Fluid Friction - drag, in air or liquid...

Friction increases as speed increases, the larger the shape of the object = more friction (or drag).

## Point keys about friction

- Always acts in opposite direction to motion
- Never starts anything moving or speeds something up
- Kinetic energy is converted into heat

Stopping Distance = thinking distance + braking distance

Thinking distance = distance travelled whilst reacting

Braking distance = distance travelled whilst braking

Factors increasing thinking distance:

- Alochol/drugs/tierdness
- Distractions in car/music
- Increased speed

Factors increasing braking distance:

- Wet, Icy surface, increased speed, brake pad condition, mass of vericle

## Car safety features

- ABS - anti-lock braking system - prevent wheel spin/skidding
- Crumple Zones - increase impact time so impact force is reduced
- Seat Belts ^^^
- Airbags ^^^
- Metal reinforced panaling bars on doors - to stop the car being pushed into the driver and passangers area

Impact time - calculated by using S = 1/2 (u+v)t

Impact force - F=ma (only if mass is known)

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