# OCR 21st Century Physics, P4

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• Created by: hena
• Created on: 11-04-11 11:39

P4-Explaining Motion!

Speed is the distance travelled in a certain time

SPEED = DISTANCE / TIME

- speed normally changes

- more useful to know average speed

Velocity is speed, with direction

e.g 30 mph north is 30 mph

30 mph south in -30 mph

DISTANCE-TIME GRAPHS

GRADIENT ( / OR \ ) = SPEED

* GRADIENT = VERTICAL / HORIZONTAL

------ = STOPPED

/ = MOVEMENT (STEEPER = FASTER)

\ = BACK TO START POINT

STEEP CURVE = ACCELERATION

LEVELLING CURVE = DECELERATION

VELOCITY-TIME GRAPHS

GRADIENT ( / OR \ ) = ACCELERATION

/ = ACCELERATION

\ = DECELERATION

AREA UNDER ANY SECTION OF THE GRAPH = DISTANCE TRAVELLED IN THAT TIME INTERVAL

CURVE - CHANGING ACCELERATION

Forces!

- when an object exerts a force, a EQUAL force is returned

- if you PUSH a wall, it will PUSH back just as hard

- when you stop pushing, so does the wall

this is called..

REACTION FORCE

3 TYPES OF FRICTION!

1. When SOLID SURFACES are GRIPPING

e.g. earths crust,

trying to slide, causes friction, force gets to much, earthquake!

2. SOLID SURFACES that are SLIDING

e.g. moving parts in a car engine

lubricant ( oil or grease ) can reduce to friction

3. RESISTANCE from FLUIDS

e.g a car driving

'fluids' - any liquid or gas

a swimmer through WATER

a bike through AIR

arrows show the size of forces

if a object moves at a steady speed, forces must be balanced

RESULTANT FORCE is the overall force acting on an object!

this force decides the motion of the object

accelerate means change VELOCITY

REMEMBER- velocity is speed AND direction,