1. FORCE AND MOTION
A REVISION exercise

2. Mass:
The mass of an object is the measure of how much substance is in that object.
Every physical object has mass: you do, the Earth does, and the Sun does.
Mass is measured in kilograms.

3. The Difference Between Weight and Mass
Mass is the amount of matter in an object.
The mass of an object will not change as you move it from place to place.

4. Mass = 1 kg An object's mass is constant where ever it is.
on both the Earth and the Moon.

5. Weight is a force--the force with which gravity "pulls" on an object's mass.
The Earth is bigger than the Moon therefore it applies a bigger pull on objects.
Moon
Earth

6. An object's weight depends upon its location .
Weight = 10 N
Weight = 1.6 N

7. g is approximately 10 N kg-1
Weight is calculated by multiplying the mass by the force due to gravity.
Weight = m x g
On the Earth:
g is approximately 10 N kg-1
On the Moon:
g is approximately 1.6 N kg-1

8. Christian Cullen on the EARTH
Mass = 82 kg
Weight = mass x g
= 82 x 10
= 820 N

9. What would Christian Cullen’s mass and weight be on the Moon?

10. Christian Cullen on the MOON
Mass = 82 kg
(Same as on Earth)
Weight = mass x g
= 82 x 1.6
= N

11. Forces can be:
Pushes
Pulls
Twists

12. Forces at Work

13. Forces in ACTION !

14. When two rough surfaces rub together there is more friction.
Orbital sander for light sanding of wooden surfaces.

15. FRICTION Friction is the force between two surfaces rubbing together.
When two smooth surfaces rub together there is very little friction

16. There is less friction when there is a liquid (e. g
There is less friction when there is a liquid (e.g. oil) between the two surfaces.
Oil is added to a car engine to lubricate the parts.

17. There is more friction if the two surfaces are forced against each other.

18. Friction is a good thing:
Just think of walking on a wet marble floor or an icy pavement with new leather soled shoes: you might slip over! Worse still, if the tyres on your car are worn out and the road is wet and slippery, you will probably skid the car and have an accident.

19. Friction is the force between two surfaces rubbing together
Friction is the force between two surfaces rubbing together. It is high if the surfaces are dry and rough and it is low if the surfaces are smooth and wet.
Friction Force
Pulling Force

20. Friction is also very important for your car's brakes to work properly.
When you put you foot on the brake pedal, some rough pads are squeezed tight against the brake discs.
This friction slows the car down. If oil gets on the discs, the brakes will not work so well.

21. Both your shoes and the car's tyres need good grip so their surfaces are rough. This increases the friction between them and the ground.

22. Friction is a bad thing:
Friction inside a car engine and inside the wheel axles will slow a car down and wear out the metal. To prevent this we put oil or even grease in them. This makes their surfaces more slippery and so reduces friction.

23. When we push or pull on an object, we create a force.
When forces are equal, they are in balance When forces are unequal, an object moves or twists.

24. At a given location on the earth and in the absence of air resistance, all objects fall with the same uniform acceleration.

25. This acceleration is called the acceleration due to gravity and it is refered to commonly as g.
On Earth g = 9.8 m s-2
(Approximately 10 m s-2)

26. If all the forces are in balance, the object will stay as it is - stationary or moving - at a steady speed in a straight line.

27. If the forces are unbalanced, it will:
start to move in the direction of the force
speed up (accelerate)
slow down (decelerate)
change direction.

28. PUSH, PULL and TWIST all at the same time.

29. When driving on the motorway you might travel several kilometres at the same speed.
e.g. Travel 5 km at 100 km h-1
For that short period of time you are travelling at CONSTANT SPEED.

30. On a normal journey you speed up and slow down.
We can calculate your AVERAGE SPEED for the journey.
Distance travelled
Average Speed =
Time taken

31. e.g. Distance from Raumati Beach to Wellington is 50 km
Time to travel from Raumati Beach to Wellington is 45 minutes.
The journey will have fast bits and slow bits. You will travel faster in the four lane parts. You might be slowed down by the Paramata Bridge. You may have to stop for a train at McKay’s Crossing. You will not be able to travel at a constant speed.

32. Now set out your work as follows to calculate the AVERAGE SPEED for the whole journey.
Show all your working !

33. The 3 equations you need are:
For us Speed and Velocity are the same thing. d V t
The 3 equations you need are:
Let: V= velocity
d = distance
t = time d d
V =
t = t V
d =
V x t

34. Average Speed = Average Speed =
e.g. Distance from Raumati Beach to Wellington is 50 km
Time to travel from Raumati Beach to Wellington is 45 minutes.
d = 50 km
t = 45 minutes
= 0.75 hr d
Average Speed = t 50
Average Speed = =
66.7 km h-1
0.75

35. We can represent a journey in graph form.
What will the slope of the graph tell us about our trip?

36. E D C B A
A = fast constant speed B = Slower constant speed C = at rest D= medium constant speed E = at rest

37. The SLOPE of the graph gives us an indication of the speed.
The steeper the slope the greater the speed.
A horizontal line tells us that the object is at rest (or stationary)
From the graph you can calculate the speed.

38. RUN RISE RISE 30 Slope = = 6 ms-1 Slope = 5 RUN Distance Time Graph
Distance (m)
RISE
Time (s)
RISE 30
Slope = =
6 ms-1
Slope = 5
RUN

39. The steeper the slope the faster the speed.
Distance Time Graph
The steeper the slope the faster the speed.
The SLOPE of the graph indicates SPEED.

40. An object that is changing its velocity is accelerating.
Something that slows down or speeds up, is said to accelerate.

41. All accelerations are caused by forces.
A force is basically a push or a pull on an object.

42. Something that is speeding up has a positive acceleration, and something that slowing down (decelerating) has a negative acceleration.

43. The slope of a velocity time shows whether an object is accelerating.
Travelling at a constant speed
Accelerating at a constant rate
Decelerating at a constant rate

44. Change in velocity Acceleration = Change in time Final V - Initial V =
e.g. A car speeds up from rest to ms-1 in 3 seconds
Initial Velocity = 0 ms-1
Final Velocity = 24 ms-1
Time taken = 3 s

45. Final V - Initial V Acceleration = t 24 - 0 = 3 = 8 ms-2
Initial Velocity = 0 ms-1
Final Velocity = 24 ms-1
Time taken = 3 s

46. Final V - Initial V Acceleration = t 10 - 35 = 5 - 25 = 5 = -5 ms-2
e.g. A car slows down from 35 ms to 10 ms-1 in 5 seconds
Final V - Initial V
Acceleration = t = 5
- 25 =
I V = 35 ms-1 5
F V = 10 ms-1 =
-5 ms-2
t = 5 s

47. 2. The forces are unbalanced so the bar accelerates up.
A weightlifter is lifting up the bar.
1. The weightlifter is pushing up with a bigger force than the weight of the bar.
2. The forces are unbalanced so the bar accelerates up.

48. 1. The weightlifter is using a force equal to the weight of the bar.
A weightlifter is holding the bar above his head.
2. The forces are balanced so the bar remains stationary.

49. 1. The weightlifter is using a force less than the weight of the bar.
A weightlifter is lowering the bar to the floor.
2. The forces are unbalanced so the bar accelerates down.

50. air resistance - drag: Here are 4 types of forces:
When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.

51. contact force (support):
happens when two objects are pushed together. They exert equal and opposite forces on each other. The support force from the ground pushes up on your feet as you push down to walk forwards.

52. Support Force
Support Force
Gravitational Force
Gravitational Force

53. gravity:
the force which pulls objects towards the Earth. We call the pull of gravity on an object its weight. The Earth pulls with a force of about 10 Newtons on every kilogram of mass.

54. Air resistance slows the shuttle.
friction:
the force which resists movement between two surfaces which are in contact.
Air resistance slows the shuttle.

55. The 3 equations you need are:
What is the relationship between Force, mass and acceleration.
Force = mass x acceleration
Units: (Newtons) (kilograms) (ms-2)
The 3 equations you need are: F F
m =
a = F a m a m
F =
m x a

56. Problem:
A car has a mass of 700 kg. It is accelerating at 2 m/s Calculate the unbalanced force causing this acceleration.
(See next slide for correct working.)

57. 1. Write down the equation. (learn it.)
2. Put in the values.
3. Work out the answer and write it down (Do not forget the units.)
Force = mass x acceleration
Force = 700 kg x 2 m s-2
Force = 1400 N

58. Try this question on your own paper:
What force would you need to accelerate (or decelerate) a 1500 kg car at 3 m s-2?
(See next slide for correct answer.)

59. Answer:
What force would you need to accelerate (or decelerate) a 1500 kg car at 3 m s-2?
F = m x a
= 1500 x 3
= 4500 N

60. The bigger the unbalanced force the bigger the acceleration.
The bigger the mass, the smaller the acceleration.

61. When the mass is kept constant the acceleration is directly proportional to the force applied.
When the force is kept constant the acceleration is inversely proportional to the mass of the object.

62. What is POWER?

63. Power is the rate of doing work.
It is not about being strong.
Power is the rate of doing work.
Two people do the same task. Bryce takes 30 minutes while Mariah takes 15 minutes.
Who has the greatest power output?

64. Work done Power = Time taken Who has the greatest power output?
Mariah has the greater power output because she complete the same task (same amount of work) in the shortest time.
Work done
(Joules)
Power =
Time taken
(seconds)
(Watts)

65. The 3 equations you need are:
Work done
(Joules)
Power =
Time taken
(seconds)
(Watts)
The 3 equations you need are: W W
P =
t = t P W t P
W =
P x t

66. A crane lifts 600 kg of concrete a height of 20 m in 15 seconds
A crane lifts 600 kg of concrete a height of 20 m in 15 seconds. What is the power output of the crane?
(i) What is the weight of concrete?
Weight = mass x g
= 600 x 10
= N
(ii) What is the work done by the crane?
Work = F x d
= 6000 x 20
Continue
next slide
= J

67. (iii) What is the power of the crane?
= J 20
= 6000 W W t P

68. How much work did she do to climb up to the top of the slide?
Mass of girl
= 20 kg
Height
= 2 m
A Slide
Check your answer on next slide.

69. In climbing to the top of the slide the girl has gained gravitational potential energy. (EP)
Mass of girl
= 20 kg
Height
= 2 m
A Slide
Gain in EP = Work Done in climbing up.
= 400 J

70. Her weight is the force she has to overcome to climb up.
Mass of girl
= 20 kg
Height
= 2 m
F = m x g
= 20 x 10
A Slide
= 200N
W = F x d
= 200 x 2
= 400J

71. (Same answer as before) Alternate way to calculate EP
gravitational potential energy. (EP)
Mass of girl
= 20 kg
Height
= 2 m
EP = m x g x h
=20 x 10 x 2
= 400 J
A Slide
(Same answer as before)
Alternate way to calculate EP