Speed and Velocity Speed is simply how fast something is going . we measure it in metres per second (written as m/s or ms -1 ) Example: if I walk 6m and it takes me 3 seconds, what is my speed? Answer: speed = distance divided by time, = 6 divided by 3, = 2 metres per second

Avg speed = Total Distance Average speed is really easy to work out: total distance divided by total time for the journey. Avg speed = Total Distance Total time

Velocity Velocity is speed in a particular direction. Ex : a car has a speed of 10 m/s, and go on to say that it's velocity is 10 m/s northwards

(i) Over what distance was the race run?

(ii) How long did the runner take to cover this distance?

iii) How can we conclude from the graph that the runner ran at a steady speed during the race?

iv) What was the runner’s average speed?

(v) On the same axes plot a graph for a runner who ran 96 m in 12 seconds at a steady speed.

Acceleration: changing velocity Acceleration (a) of a body is the rate of change of velocity with respect to time Acceleration = Change in velocity Time Velocity is measured in metres per second (m/s), so acceleration is measured in metres per second per second (written as m/s/s or m/s2 )

Example: a car accelerates from 8 m/s to 20 m/s, and takes 6 seconds to do it. What is the acceleration? Answer: change in velocity is 20 minus 8 = 12 m/s. Time taken for change is 6 seconds. Acceleration = 12 divided by 6 = 2 m/s/s

Forces : pushes and pulls A force is a push or a pull. We measure forces in "Newtons" (N) Forces can change:- the speed of an object the direction that an object is moving in the shape of an object. Make a stationary body move

Gravity Gravity is a force that acts towards the centre of the Earth The gravitational pull of the Earth is what gives objects weight. Thus weight is a force - it's how hard the Earth is pulling on an object. The Earth pulls on every kilogramme with a force of ten Newtons.In other words, an object with a mass of 1kg has a weight of 10N.

Gravity Questions How hard does the Earth's gravity pull on a 2kg object? The Earth pulls on every kilogramme with a force of 10N. This force is the object's weight.So the weight of a 2kg object is:- 2 x 10 = 20 Newtons.

2. What is the weight of a 4kg rock? The Earth pulls on every kilogramme with a force of 10N. This force is the object's weight.So the weight of a 4kg rock is:- 4 x 10 = 40 Newtons.

3. What is the weight of a 100 gramme plastic toy? Firstly, we need to get the mass into kilogrammes. There are 1,000 g in a kg, so 100 g is 0.1 kg. So the weight of a 0.1kg toy is:- 0.1 x 10 = 1 Newton.

Friction: slowing things down Friction is the force that resists the motion of one surface relative to another with which it is in contact .

1. "Static" or "sliding" friction The frictional force depends only on:- the type of surfaces how hard the surfaces are pressed together

2. "Fluid" friction Fluid friction is also known as "drag". On aircraft it's also called "air resistance". t depends on:- how thick the fluid is   the shape of the object the speed of the object

Aircraft and car designers want to reduce drag, so that the vehicle can go fast without having to waste too much fuel.

Terminal velocity When an object falls, it accelerates. As its speed increases, the air resistance increases . Eventually, the force from the air resistance will equal the force from the weight of the object

Reducing friction We can reduce friction by oiling ("lubricating") the surfaces. This means that the surfaces no longer rub directly on each other, but slide past on a layer of oil. It's now much easier to move them.

Air Hockey" tables in amusement arcades use the same principle Air Hockey" tables in amusement arcades use the same principle. The puck goes a long way when you hit it, because there's almost no friction to slow it down.

Using friction or Increasing Friction We use friction to help us grip. This means that our shoes grip the floor, so we don't fall over. Right now you're using a mouse, which works because of friction between the ball and the mouse mat.

If it wasn't for friction between the tyres and the road, driving a car would be like trying to drive on an ice rink. This would make cornering and stopping very difficult!

1 What does dry friction depend upon? The frictional force depends only on:- the type of surfaces how hard the surfaces are pressed together.

2. What is another name for fluid friction? Fluid friction is the force that tries to slow objects down when they move through a liquid or a gas. It's also known as "drag", or "air resistance".

3.What does fluid friction depend upon? Fluid Friction depends on:- how thick the fluid is the shape of the object the speed of the object Remember that "fluid" means a liquid (e.g. water) or a gas (e.g.air).

4. Why do we oil machinery? We oil ("lubricate") machinery to reduce friction. This reduces wear, reduces frictional heating, increases efficiency and extends the life of the machine.

5. Why does a falling object reach a "terminal velocity"? As an object falls faster and faster, the air resistance will get more and more. Eventually, the air resistance will be equal to the object's weight, so the object will stay at that speed, and stop accelerating. This speed is called the terminal velocity.

Balanced Forces

Unbalanced Force

Upthrust Upthrust is the force that pushes an object up and makes it seem to lose weight in a fluid. (Remember, a fluid means a liquid or a gas).

Mass Mass (m) of a body is the measure of amount of matter in the substance. SI unit of mass is Kilogram (Kg)

Weight The weight (w) of a body is the gravitational force exerted on it by Earth

Gravity Gravity Gravity is a force that for us is always directed downwards. But to say that gravity acts downwards is not correct. Gravity acts down, no matter where you stand on the Earth. It is better to say that on Earth gravity pulls objects towards the centre of the Earth.

Weightlessnes Feeling "Weightless" When You Go "Over the Hump"

Circular Motion

Centrifugal force: The force which acts towards the centre of the circular path .