# Physics Mock

June 2011

## Speed, velocity, acceleration

velocity is a vector quantity (measured in both size and direction)

velocity + - to show different ways that something is travelling

(Average) velocity (m/s) = displacement (distance) (m)/time taken (s)

the acceleration of an object is the rate at which its velocity changes. how quickly an object speeds up or slows down. acceleration = m/s squared

to work out the acceleration of any moving object, we need to know 2 things

-the change in velocity -the time taken for change

acceleration = change in velocity(m/s) / time taken for change(s)

decceleration = negative acceleration . increasing velocity by the same amount each second, distance travelled each second is increasing

change in velocity=final velocity-starting velocity

## Velocity time graphs

the slope of a velocity-time graph represents the acceleration of an object: the steeper the gradient, the greater the acceleration. The area underneath the graph represents the distance travelled.

## Forces(newtons) between two interacting object

equal in size if nothing is moving

when object A pushes/pulls object B this is called an action force

when object b pulls/pushes object A with a force that is equal in size and opposite in direction this is called a reaction force

## How forces affect movement

the movement of an object depends on the forces acting upon it. The balance of these forces is called the resultant force and this force affects any subsequent motion of an object

car:

in this diagram, the car exerts a driving force. The air resistance and friction are resistive forces. The balance of these two types of forces dictates the motion of the car.

accelerating - when the driving force is greater than the resistive force (i.e the resultant force is not zero), the car is accelerating. An unbalanced force acts on the car causing it to speed up, ie accelerate.

braking- when the resistive force is greater than the driving force (ie the resultant force is not zero), the car is deccelerating. An unbalanced force acts on the car causing it to slow down, ie decelerate.

Moving at a constant speed - When the driving force is equal to the resistive force (ie the resultant force IS zero), the car is moving at a constant speed. The forces acting on the car are now balanced.

calculating forces - resultant force (F) = driving force - Resistive forces

## free body force diagrams

free body force diagrams show the forces acting on an object with balanced forces

## Force, mass acceleration

if an unblanced force acts on an object then the acceleration of the object will depend on...

-the size of the unbalanced force-the bigger the force, the greater the acceleration

-the mass of the object- the bigger the mass, the smaller the acceleration

example-

one boy pushes the trolley , exerting an unbalanced force which causes it to move and accelerate

if 2 boys push the same trolley it moves with a greater acceleration

However, if one boy pushes a trolley with a bigger mass, it moves with a smaller acceleration than the first trolley

force(n) = mass (kg) x acceleration (m/s squared)

## terminal velocity

falling objects experience two forces..

-the downward force of weight, which always stays the same

-the upward force of air resistance or drag

when a skydiver jumps ou of an aeroplane, the speed of his descent can be considered in 2 seperate parts: before and after the parachute opens

## Before the parachute opens

1.when the skydiver jumps he initially accelerates due to the force of gravity. G is a force of attraction that acts between bodies that have mass eg skydiver and earth. the weight of an object is the force exerted on it by gravity. (N)

2. however as the sky diver falls he experiences the frictional force of air resistance in the opposite direction, but not as great as w so he continues to go in the same direction.

3. as speed increases, so does the air resistance acting on him

4. it increases until R is equal to W. This means the resultant force acting on him is now zero and his falling speed becomes constant. this speed is called the TERMINAL VELOCITY

## after the parachute opens

5. when the parachute opens unbalanced forces act again, because the upward force is now greatly increased and is bigger than W.

6.This causes his speed to decrease and as his speed decreases so does R

7. Eventually R decreases until it is equal to w the forces acting are once again balanced and for the second time he falls at a steady speed , slower than before but at a terminal velocity.

In the absence of air resistance , all falling bodies accelerate at the same rate. If you dropped a feather and a hammer from the same height at the same time on the moon both would reach the floor at the same time.

## Stopping distances

the stopping distance of a vehicle depends on

the thinking distance- the distance travelled by the vehicle from the point when the driver realises they need to apply brakes to when they actually apply them

the braking distance- the distance travelled by the vehicle from the point when the driver applies the brakes to when the vehicle actually stops

stopping distance=thinking distance+braking distance

## Factors affecting stopping distance

The speed of the vehicle -affects both the thinking distance and the braking distance.

the mass of the vehicle- affects braking distance only. if the mass of the vehicle is increased it has greater kinetic (movement) energy, which increases the braking distance.

the conditions of the vehicle on the road-the vehicle may have worn tyres, or the road conditions maybe wet,icy or uneven. all these conditions affect the friction between the tyres and the road and therefore the braking distance

the driver reaction time-affects the thinking distance only. reasons like -drinking alcohol -taking drugs -being tired -being distracted by the surroundings could affect this

## momentum

momentum is a measure of the state of movement of an object. it is dependant on two things -the mass of an object -the velocity of the object

momentum (kg m/s) = mass (kg) x velocity (m/s)

vector quantity

when a force acts on a moving object , or on a stationary object that is capable of moving, the object will experience a change in momentum. Force and momentum are related by the following equation

Force (N) = change in momentum (kg m/s) /time taken for change (s)

mass must be in kg - 1 tonne =1000kg

## collisions & safety technology

in the event of a collsion, such as in a car or on a theme park ride, all of thr momentum (and energy) of the impact has to be absorbed. If the momentum before impact can be reduced then the forces during impact can also be reduced(and the energy lowered) In a car this is achievable using safety features like seatbelts, air bags and crumple zones: instead of coming to an immediate halt, there are a few seconds in which momentum is reduced. The force on the passengers is then reduced, resulting in fewer injuries

## Safety Technology

Cars have lots of safety feature to minimise injury and reduce deaths...

-crumple zones within car structure help absorb momentum, meaning the force exerted on the people inside the car will be reduced, which results in fewer injuries

-powere assisted steering and anti-lock brake systems help the driver control direction and speed which can reduce momentum

-cushioning during impact (airbags,soft seats etc.)

-seatbelts which lock when the car brakes , exerting a force to counteract the momentum of the people wearing them

wearing a seatbelt whilst travelling in a motor vehicle greatly reduces the chance of death in the event of an accident. in 1992 it became compulsary for all front passengers to wear seatbelts, and this led to a massive decline in the number of accident fatalities. The number was reduced again following the introduction of compulsary seatbelts for all rere passengers in 1994

## Risk Assesment

risk is a percieved measure of the probability of something happening. it cannot be calculated with any degree of certainty. risks can be imposed (eg coal mining operating machinery) or voluntary (smoking, rock climbing etc). Peoples perception of risk may differ widely due to their degree of familiarity with that risk. For example, a person working in a mine every day is familiar with the risk and therefore does not consider it to be much of a risk as others might.

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