P2a revision (part 1)

Page 1

Velocity

Speed and Velocity are how fast you are going

Measured in m/s, km/h or mph

Speed is how fast you are going, no matter the direction

Velocity is the same but a direction must be specified

Distance-time Graphs

Gradient tells you the speed

(Rise/run)

Curves represent acceleration or deceleration

Steepening curve means it's going up

Levelling off curve shows it's slowing down

Acceleration

How quickly velocity is changing

this can be change in speed, direction or both

Change in velocity is final velocity minus the original

Measured in

Velocity-time Graphs

Area under any section is equal to the distance travelled in that time interval

A curve is changing acceleration

Acceleration (at a sloping section is the gradient)

When it is constant just read it off the graph

Gravity

Gravity is the force of Attraction between all masses

Has 2 important effects

On the surface of the planet, it makes all things accelerate towards the ground

It gives everything a weight

Mass

the amount of stuff in an object

It's the same anywhere in the universe

Not a force

Measured in Kg

Weight

Caused by the pull of gravitational forces

Mostly the weight of an object is just the force of gravity pulling it towards the centre of the earth

It is different depending on the strength of the gravitational forces

A force measured in newtons

Measured using a newton metre

Measured in Newtons

On earth the gravitational strength is 10 N/kg

Resultant Forces

The overall force on a point or object

mostly there are two forces acting

Overall force decides motion of the object

If the forces act along the same line the overall force is found by adding or subtracting them

If there is a resultant force acting on an object

The object will change it's state of rest or motion

It would change the object's velocity

e.g A mug on a table

Gravity is acting downwards

Causes reaction force on the surface pushing up

This makes it balance

The resultant force would be 0

10N-10N=0N

Forces and Acceleration

If the resultant force is 0

The object will stay stationary

There has to be a resultant force to move

If there is o resultant force on an object ti will carry on moving at the same velocity

e.g. a car or train

To move at a constant velocity the forces will be balanced

If there is a resultant force (not 0)

The object will accelerate in the direction of force

This acceleration can be in 5 different forms

1. Starting

2. Stopping

3. Speeding up

4. slowing down

5. changing direction

on a diagram the arrows will be unequal

Reaction Forces

When 2 forces interact

The forces they exert on each other are equal and opposite

If you push on something the object will 'push back' twice as hard

The 2 forces are acting on different objects

The sized forces will be the same

But the mass might be different and so there is an acceleration

Friction

if there is not a force making it move

Friction causes everything to slow down and stop

Friction acts in the opposite direction to movement

To travel at a steady speed the driving force need to balance the driving force

You get friction between 2 surfaces in contact

Or when the object passes through a fluid

Resistance in Fluids

mostly cause by air resistance or 'drag'

This can be reduced by giving a streamline shape

like sports cars

Drag increases as speed increases

At 70mph an engine has to wok harder than at 30mph to maintain a steady speed

Terminal Velocity

Objects falling through fluids reach a terminal velocity

when things start falling gravitational force is more than frictional forces

So it accelerates

So friction builds up

This reduces acceleration

Until they are equal

It won't accelerate anymore

It has reached its terminal velocity

Depends on shape and area

The accelerating force on falling objects is gravity

It would make them fall at the same rate

if it wasn't for air resistance

Causes things to fall at different speeds

Terminal velocity is determines by it's drag in comparison to its weight

The frictional force depends on the shape and area

e.g a skydiver

Without parachute there is a small area and a force pulling him down

He reaches a terminal velocity of about 120mph

With the parachute there is more air resistance

there is still the same force pulling him down

The terminal velocity goes down to 15mph

This is a safe speed

Stopping Distances

Thinking Distance + Breaking Distance = Stopping Distances

The total stopping distance is timed from when the driver first spots the hazard to when it stops

Thinking Distance

is affected by:

How fast you're going - whatever reaction time the faster you re going the further you'll travel

How alert you are - tired? Drugs? Alcohol? these all affect reaction time

Braking Distance

is affected by:

How fast you're going - faster you're going the further it takes you to stop

how good the brakes are - brakes must be checked and maintained, faulty brakes make it harder to stop quickly

How good are the tyres - Must hve a dred depth of 1.6mm to be able to get rid of wayer in wet conditions. Stuff on road can make tyres skid

How good is the grip - depends on road durface, weather conditions and tyres

Bad visibility and distractions don't help

e.g. rain and plying with the radio

Driver doesn't notice hazard

Doesn't affect thinking distance it just means there is lees time to think and act

Wet or icy roads make the stopping distance longer as prevent the tyres from getting grip and stopping quickly

Velocity and Acceleration

Gravity, weight and Mass

Acceleration and forces

Terminal velocity

Stopping Distances