1. FORCES

2. Forces are essential to our understanding of the world. 
Without forces, nothing would change— stationary things would stay still and moving things would keep moving,  never slowing down and never stopping. 
Without forces, the Earth would not revolve around the Sun— it would break apart and head out into space. 
No work would get done, no new inventions could be created, no plants would grow and no animals, including us, would exist.

3. How do we recognise a force?
Gravity
Friction
Air Resistance
Thrust
Fig 7.1 What forces are at work here?

4. What forces can you ‘see’ at work in Figure 7.1?
Can you think of anything else in your everyday life that you can’t see but you know is there?
Are any forces acting on you right now?

5. How do we recognise a force?
When objects have forces acting on them they may:
Begin to move
Speed up
Slow down or stop moving
Change the direction of motion
Change shape
Remain still

6. Fig 7. 5 Begins to move: The golf club pushes the ball
Fig 7.5 Begins to move: The golf club pushes the ball. The club exerts a force on the ball, causing it to begin to move. If you miss, there is no force on the ball from the club.

7. Fig 7.6 Speeds up: The air consists of tiny invisible Particles that rapidly move about.  When there is a wind, the particles mostly move in the direction of the wind. When they  hit objects in their path, the air particles push the objects. Yachts have a very large  area of sail, so that as many air particles as possible are able to hit the sails and give them a push.

8. Fig 7.7 Slows down or stops moving: The air particles hitting and pushing on  the huge area of parachute from in front slowdown space shuttles so they  can land safely. This force is called air resistance.

9. Fig 7.8 Changes direction: The tennis racquet pushes the ball in a different direction.

10. Fig 7.9 Changes shape: The hands push the plasticine into a different shape. When you stop pushing,  the plasticine no longer changes.

11. Fig 7.10 Remains still: The gravitational force pulling down And the weight of air above are in balance  with the force of the CHAIR pushing up on the boy.

12. Fig 7. 15 A tennis ball cannot throw or roll itself
Fig 7.15 A tennis ball cannot throw or roll itself. It needs an agent to exert a force on it. But even while it is still on the ground, agents outside itself are exerting  forces on it. What are they?

13. WHAT DO YOU KNOW ABOUT IDENTIFYING FORCES?
OXFORD Big Ideas
Questions 1-6, page 228

14. TYPES OF FORCES:
THRUST: This can be any driving force, it may be a push or pull, but it could be an engine which forces an object forward.
LIFT: This is the force required to raise an object.
AIR RESISTANCE: This is a force which acts in the air, it can slow objects down when they are moving against it. Or if harnessed could be used to move an object along.
FRICTION: This force acts on objects when they are in contact with a surface, such as the ground. It can be reduced by ensuring both surfaces are smooth.
GRAVITY: This force affects every object on Earth. It is a force which pulls everything to the centre of the earth.

15. Can you come up with an example of each one of these forces we have just defined?
Write the answer into the table on your sheet

16. How do we represent forces?
Group Activity: What forces are acting on this car which is driving along? (label the diagram with arrows showing the direction and type of force)
For bonus points: Can you make the size of the arrows proportional to the force(s) acting on the car?

17. This force is always present
This force is always present. It goes towards the centre of the earth, so will always be pointing from the top of the object
This acts against the car, as it moves forward and is acting against the propulsion.
Thrust
Thrust
This is the most powerful force, this should be your largest arrow, it should be pointing in the direction the car is going …
This force is only present where the car is in contact with the floor, it acts against the propulsion.

18. Try the example questions on the bottom of your worksheet.
The aeroplane and kicking the soccer ball……..

19. Revision from last lesson:
Label the diagram below with the relevant forces

21. Forces Add Forces Cancel! Total Force Force #2 Force #1 Force #1
4/18/2008
Total Force
Force #2
Force #1
Force #1
Total Force = 0
Force #2
Forces Add
Forces Cancel!
Lecture 8

22. Notes:
When more than one force acts on an object, the forces COMBINE to form a NET FORCE
2. Forces may be BALANCED or UNBALANCED
3. If the effects of the forces cancel each other out, and do not cause an object to move, the forces are said to be BALANCED
4. If the forces don’t cancel each other out – 1 force is stronger than the others – the forces are UNBALANCED and will cause ACCELERATION

23. Two forces can ADD together to produce a LARGER net force.
Examples:
Two forces can ADD together to produce a LARGER net force.
Two forces can ADD to produce a SMALLER net force in the direction of the larger force.
C. Two forces may CANCEL and produce NO NET FORCE. + = + = + =

24. Balanced and Unbalanced Forces:
LINKS ON WEEBLY:

25. Work on it’s a Balancing Act Booklet