1. Forces
Chapter 7

2. What are forces?
A force is a push, pull or twist that can change an objects motion

3. What can forces do? Acceleration: increase in speed
deceleration: decrease in speed

4. Measuring forces Force is measured using a unit called newtons (N)
The ability of springs to stretch and squash allow a weight force to be measured
The larger he pulling force, the more the string is stretched, the higher the force

6. summary
Not moving
Speed

7. Note that the arrows are the same length
Balanced forces
Note that the arrows are the same length
The downward force of gravity is balanced by the upward support force from the chair. The forces acting on this person are balanced
This is an example of an object that is not moving/stopped

8. Note that the arrows are the same length
Balanced forces
This is an example of an object travelling at the same speed without changing directions
The forward force from the pedalling is cancelled out by the friction force pushing backwards
The forces at this stage are balanced
forward force
Note that the arrows are the same length

9. Unbalanced forces Motion will change when the forces are unbalanced
Forces are unbalanced when:
Start moving (pedalling fast and hard)
Speed up (pedalling fast and hard)
Slows down (using the brakes)
Comes to a stop (using the brakes)
Changes direction (turning the handle bars)

10. Note that the arrows are now different lengths
Unbalanced forces
Note that the arrows are now different lengths

13. Unbalanced forces summary
Decelerate
Accelerate
Arrow is larger therefore moving forward
Arrow is larger therefore moving backwards

14. Driving Force (thrust)
Drag from caravan
question
Driving Force (thrust)
1. State which direction the car is moving
2. Predict what would happen to the speed of the car if the caravan gained extra weight
3. If the car is travelling at a constant speed. Compare the size of the thrust and drag forces acting on the car

15. Driving Force (thrust)
Drag from caravan
Driving Force (thrust)
1. State which direction the car is moving
To left or forward
2. Predict what would happen to the speed of the car if the caravan gained extra weight
Decrease as the drag from the caravan will increase
3. If the car is travelling at a constant speed. Compare the size of the thrust and drag forces acting on the car
The thrust balances the drag. No overall force is acting. (Balanced forces = Constant Speed)

17. Inertia
Inertia is an object’s resistance to changing its state of motion.
The greater the mass of the object the greater the inertia.
Therefore the greater the mass and inertia the harder and more force is required to change the objects state of motion.

18. Inertia
The tendency of an object to resist any change in its motion (unless a force is acted upon it)
If you put your school bag down, it will stay there until something happens to it
Someone could lift it, push it or pull it to make it move. When left alone, the bag will not move

20. Newtons Law of motion
Sir Isaac Newton proposed some statements to explain the way things move
Anything that is not moving will stay that way unless a force makes it move
Anything that is moving will keep moving at the same speed and in the same direction unless a force makes it change

21. The effect of mass
The more massive something is, the greater its inertia
This means it takes a greater force to change its motion
Eg: it takes a smaller force to stop a pebble roll down a hill than to stop a massive rock

22. Types of Forces Non-Contact Contact Contact forces touch an object
Friction
Pull
Push
Non-Contact
Gravity
Magnetic
Electrostatic
Non-contact forces act at a distance

23. Friction
7.2

25. What is friction
Friction occurs whenever one object tries to move over another.
Because it occurs between surfaces in contact, friction is called a contact force.
Friction acts in an opposite direction to motion

26. What affects friction? Friction depends on:
How rough the surfaces in contact are
How hard the surfaces are pushed together.
The greater the weight of a sliding object, the greater the force of friction

27. Reducing friction
By adding grease to ball bearings, or putting oil into a car, we reduce the friction between moving parts (Lubricant)
Removalists use trolleys to shift refrigerators because rolling surfaces produce less friction than sliding surfaces
Polishing a surfboard helps to make its surface smoother and will reduce friction
Vehicles such as cars and aircraft are all designed to have a streamlined shape. These shapes allow air to flow over and around them more freely, and reduce drag.

28. Gravity
7.3

30. What is Gravity?
Gravity is a non-contact force that pulls objects to the centre of the earth
Gravity is this force of attraction
The more mass a pair of objects have, the stronger the pulling force of gravity between them.
distance - The further away from an object you get, the less gravitational pull it will have

31. How do Tides work?
The tides are controlled by the gravitational pull of the moon on planet earth, as well as the gravitational pull of the sun.
We have two high tides and two low tides per day

32. How do tides work?

33. How do tides work?

34. Spring tide
A spring tide is caused by the moon and the sun pulling on the earth from the same side
Spring tides are large because the gravitational pull of both the Moon and the Sun are in the same direction

35. Neap tide
A neap tide is caused by the moon and the sun pulling at the earth at right angles
Neap tides are smaller because the Moon and Sun are pulling in different directions

37. orbits
An orbit is a pathway that a body (planet, moon or star) travels around another body.
It is the gravitational pull of one body on another that holds the smaller body in an orbit

39. Magnetic and electric fields
7.4

41. Magnets
A magnet pulls, or attracts, materials containing the metals iron, cobalt or nickel.
A magnetic field is the space around a magnet where a magnetic force is experienced
The ends of a magnet are called poles (north and south)
The magnetic field is strongest at the poles of a magnet

42. Magnetic fields
A magnetic field is normally invisible to us. Its shape and strength can be determined either by passing a compass around a magnet, or by examining a sprinkling of iron filings around a magnet

43. Attraction and repulsion of poles
Unlike Poles
Magnetic poles may be attracted to each other, or repelled by a magnetic force
Like poles will push away/repel each other
Unlike poles will pull together/attract each other
Like Poles

44. Videos – Magnetic fields

45. Lines of force around magnets

46. Static Electricity
When an object becomes charged (has unequal numbers of protons and electrons), we say that it has static electricity

47. Levers
8.1

48. Machines
Simple
Machines make it easier to do work over a longer period of time
A simple machine can make a task easier in three different ways. It can:
• change the size of a force
• make things speed up
• change the direction of a force.
An effective machine is one that reduces the amount of effort (force) required

49. Key words
Effort is the force applied to a lever to overcome the load or get a job done.
Load what is lifted or moved by a simple machine. The force actually required to do the job.
Fulcrum is the pivot point/balance/turning point of a lever

50. 1st class levers
Leavers increase the size of the effort that you can apply or reduce the effort need to get a job done
First class levers – The fulcrum is between the load and the effort.

51. 1st class levers

52. 2nd class levers
Second class levers – The load is between the effort and the fulcrum

53. 2nd class levers

54. 3rd class levers
Third class levers – The effort is between the load and the fulcrum.

55. 3rd class levers

56. summary