1. By the end of this presentation you should be able to… Describe simply what a force is. Recall the units and the name of the instrument used to measure the size of a force. Recall examples of forces and describe situations in which they act. Understand what is meant by ‘a contact’ or ‘a reaction’ force. Differentiate between balanced and unbalanced forces. FORCES : Introduction

2. Definition A force is a push….. ….. or a pull

3. Units Force is measured in Newtons (N). So you can say that a force is 50 Newtons to the left, or 2N upwards. The unit is named after the famous scientist Sir Isaac Newton whose ideas have influenced our way of life more than we may realise! Read more about Sir Isaac Newton here The instrument used to measure the size of a force is called a Newton-meter, or a force-meter.

4. Examples of forces 1. On a bird flying through the air.

5. 2. On a metal rod or spring

6. 3. On a rocket as it flies through the air.

7. 4. On a person walking on the road. Weight always acts towards the centre of the earth and is caused by gravity pulling on the object’s mass. A contact force is created when the foot strikes the ground. There is a force exerted by the foot on the ground, and an equal but opposite contact force exerted by the ground on the person’s foot. When the foot is no longer touching the floor, the force disappears.

8. Examples of contact forces When a tennis racquet hits a ball, a force is exerted on the ball by the strings (this makes the ball change direction). The ball also exerts a force on the strings and makes them bend. Force exerted on the strings by the ball Force exerted on the ball by the strings

9. More examples of contact forces A reaction force is felt by the fighter who is punching. It is exerted by the other’s stomach! A force exerted on this boxer’s stomach by the other’s hand!

10. Balanced and unbalanced forces If the forces that act on a particular object are balanced, this means that they are equal in size but opposite in direction. As a result they “cancel” out and the behaviour of the object will not change. Weight of clock Table pushing up on clock This clock will remain stationary.

11. Balanced and unbalanced forces If the forces that act on a particular object are balanced, this means that they are equal in size but opposite in direction. As a result they “cancel” out and the behaviour of the object will not change. If the forces provided by the runner’s muscles equals the drag forces (friction and air resistance), he will continue to run at a constant speed.

12. Unbalanced forces… …may cause changes to an objects size and shape, or its speed or its direction. …The total unbalanced force (or resultant) must be worked out to determine the effect on the object. Table pushing up on clock Weight of clock If the weight of the clock was 60N, and the table could only push up with a force of 30N (maybe it was made from a very flimsy material), then there would be a resultant of 60 -30 = 30N acting down. The clock would break the table and fall down. Accelerating as it goes. Unbalanced force = 30N