1. Triple Physics Unit 5 Revision
Forces

2. Forces Monday, 19 November 2018
Scalars and Vectors
Learning Outcomes
By the end of this lesson you will be able...
To state the difference between scalars and vectors
To give examples of scalars and vectors
To represent vectors using arrows
Keywords
Magnitude, resultant
Starter: List as many ways to describe direction as you can think of

3. Directions Left/right, up/down, forwards/backwards
30o above horizontal
North, south, east, west

4. Thanks Disney
Something has both direction AND magnitude The length of an arrow is proportional to it’s size

5. Scalars and Vectors
A scalar is a physical quantity such as mass or energy that has a magnitude A vector has a magnitude and a direction

6. What is the resultant force?
Thrust force
What is the resultant force?
30N
10N
Gravity/Weight

7. Forces Monday, 19 November 2018 Contact and non-contact forces
Learning Outcomes
By the end of this lesson you will be able...
To describe the difference between a contact and non-contact forces.
To describe the interaction between two objects and the force produced on each object. To
Keywords
interaction
Starter: Is a force a scalar or vector?

8. Contact or non contact? Friction Gravity/weight Tension Magnetic
Air resistance
Normal contact
Electrostatic
Upthrust
Lift
Thrust

9. Friction/ air resistance
Forces occur in pairs
Normal contact
Friction/ air resistance
Thrust
Weight

10. Car travelling along
A resultant force if not needed to keep an object moving.
When a car travels at a steady speed along a straight road, the force from the engine is equal to the resistive forces (air resistance and friction).
The resultant force = zero, but the car is still moving at a steady speed.

11. Car travelling along
A resultant forces can change the way something moves, if it is already moving.
If the driver reduces the engine force by taking their foot off the accelerator, to car decelerates, slows down
If the driver pushes his foot down, then the resultant force is forwards and the car accelerates.

12. Forces Monday, 19 November 2018
Weight
Learning Outcomes
By the end of this lesson you will be able...
To describe that weight is a force acting on an object due to gravity.
To describe that weight depends on the gravitational field strength at the point where the object is.
Keywords: Newton, gravitational field strength
Starter: What is the difference between weight and mass?

13. Gravity
Gravity is a force between ALL objects!
Gravity

14. Gravity Gravity is a very weak force.
The force of gravitational attraction between two people (when 1 metre apart) is only around Newtons!

15. Gravity
The size of the force depends on the mass of the objects. The bigger they are, the bigger the force!
Small attractive force
Bigger attractive force

16. Gravity
The size of the force also depends on the distance between the objects.

17. Gravity
We only really notice the gravitational attraction to big objects!
Hola! ¿Como estas?

18. Gravity
The force of gravity on something is called its weight. Because it is a force it is measured in Newtons.
Weight

19. Woah my god! W
m x g

20. A. Tom has a mass of 50 kg. What is his weight?
The equation linking weight, mass, and gravitational field
strength is:
W = m × g
weight of object = mass of object × gravitational field strength
(newtons, N) (kilograms, kg) (newtons per kilogram, N/kg)
The Earth’s gravitational field strength is about 10 N/kg.
Now try these three:
A. Tom has a mass of 50 kg. What is his weight?
B. A car has a mass of 1450 kg. What is its weight?
C. An object has a weight of 12 N. What is its mass?

21. Forces Monday, 19 November 2018
Work done
Learning Outcomes
By the end of this lesson you will be able...
explain and use the term ‘work’.
To calculate the work done when you know the force and distance.
Keywords
displacement, distance, force, mass, work done
Starter: What is the difference between mass and weight? Can we say that weight is a force? (Hint: think about the units)

22. Tell the person next to you what work you’ve done today
In physics, work is done when something moves in the direction in which the force acts...
now tell them about all the work you’ve done .

23. Work done (J) = force applied (N) x distance (m)
E.g. a constant 500 N braking force is applied to a car. It comes to rest in 30m. What was the work done by the brakes?
500N x 30m = 15000J

24. Forces Monday, 19 November 2018
Hooke’s law
Learning Outcomes
By the end of this lesson you will be able...
To describe how forces acting on an object may cause a change in its shape.
To state and use the relationship between the force applied and the extension of a spring.
Keywords
Elastic potential energy, extension, directly proportional, limit of proportionality.

25. Limit of proportionality
When something is stretched with a steady force the length of the object being stretched will increase steadily as well.
This will be the case until the object is permanently stretched and can no longer retake it's original shape once the force is removed. This is known as the limit of proportionality.
Limit of proportionality

26. Hooke’s Law
The extension of an elastic object (like a spring) is directly proportional to the force applied, provided the limit of proportionality is not reached. – LEARN THIS EQUATION
Force = spring constant x extension
(N) (N/m) (m)
F = k x e F
k x e

27. Hooke’s Law
The extension of an elastic object (like a spring) is directly proportional to the force applied, provided the limit of proportionality is not reached. – LEARN THIS EQUATION
Force = spring constant x extension
(N) (N/m) (m)
Calculate the force applied when the spring constant is 25N/m and the extension is 0.2m.
What is the spring constant for an extension of 0.5m when the force applied is 4N?
Have a go at the questions on the sheet

28. A force that stretches (or compresses) a spring does work and elastic potential energy is stored in the spring.
work done on the spring = elastic potential energy stored Elastic potential energy = 0.5 x spring constant x extension2 𝐸 𝑒 = 1 2 𝑘 𝑒 2 You do not need to learn this one

29. Forces Monday, 19 November 2018
Speed, distance, time
Learning Outcomes
By the end of this lesson you will be able...
To state the difference between speed and velocity.
To calculate speed.
To recall typical values for speed.
Keywords
Displacement
Starter: What is a scalar? What is a vector?

30. What is the difference between distance and displacement?
Distance is how far an object moves. Distance does not involve direction.
It is scalar.
Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish
point and the direction of
that straight line.
It is a vector.

31. s t v ÷ Speed: v Speed = distance time Distance: s Time: t Time =
YOU NEED TO LEARN THIS EQUATION
Speed: v
Distance: s
Time: t
Speed = distance time
Time =
Distance = s t v ÷ x

32. What are the units?!
Well if distance is in metres and time is in seconds then the units are metres per second or m/s.
However if distance is in miles and time is in hours then the units are miles per hour or mph.
The units of distance and time used will give the units to be used for speed.

33. What is the difference between speed and velocity?
Speed does not involve direction.
It is a scalar quantity.
The velocity of an object is it’s speed in a given direction.
It is a vector quantity.

34. Velocity
These cars are travelling at the same speed but different velocities.
+40mph
-40mph

35. Typical values - YOU NEED TO LEARN THESE
Walking ~1.5 m/s
Running ~ 3 m/s
Cycling ~ 6 m/s
What do you think the typical value for a car would be?
An aeroplane?

36. Speed
It is not only moving objects that have varying speed, the speed of sound and the speed of the wind also vary
The speed of sound in air is 330m/s
Do you think it is faster or slower in water? Explain why.

37. Velocity
When an object moves in a circle the direction of the object is constantly changing. This means that it’s velocity is changing, even if it’s speed isn’t changing (constant).

38. Forces Monday, 19 November 2018
Distance-Time Graphs
Learning Outcomes
By the end of this lesson you will be able...
To draw distance time graphs
To explain what is happening in a distance time graph.
Keywords
Acceleration, deceleration, gradient
Starter: What do the keywords mean?

39. 3) Steeper diagonal line =
Distance-time graphs
2) Horizontal line =
Stationary 40 30 20 10
Distance
(metres)
3) Steeper diagonal line =
Faster constant speed
Time/s
Diagonal line =
Constant speed 39

40. The gradient is the slope of the line.
The gradient of a distance-time graph is the speed.
How can we work out gradient?

41. Summary: Distance-time
Gradient of a distance-time graph represents the speed.
To work out the gradient = change in y
change in x

42. Drawing graphs
A man walks along a road for 100m for 20 seconds. Stops for 5 seconds to tie his shoe lace then walks for another 100m in 20 seconds.
200
100
Distance
(m)
time (seconds)

43. Instantaneous speed
Distance (m)
Decelerating… the gradient is getting les steep so the speed is slowing. To find the speed at any given point. Draw on a tangent and find the gradient of the tangent.

44. Forces Monday, 19 November 2018
Acceleration
Learning Outcomes
By the end of this lesson you will be able...
To calculate the acceleration of an object.
Unscramble the key words
deceleration
Speeding up
Slowing down
Stationary
accelerate

45. Definition of acceleration – change in velocity in a given time
a = acceleration (m/s2)
Δv = change in speed (m/s)
t = time (s)
YOU NEED TO LEARN THIS EQUATION
a = Δ v t
Acceleration can be negative if the car is slowing down or moving in the opposite direction.
What does it mean if a car is at rest?

46. Calculating acceleration
Time =
Change in speed =

47. Uniform motion equation – constant acceleration
(𝑓𝑖𝑛𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ) 2 −(𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ) 2 =2×𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 ×𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑣 2 − 𝑢 2 =2𝑎𝑠
YOU ARE GIVEN THIS EQUATION
v – final velocity (m/s)
u = initial velocity (m/s)
a = acceleration (m/s2)
s = distance (m)

48. Forces Monday, 19 November 2018
Velocity-time graphs
Learning Outcomes
By the end of this lesson you will be able...
To draw velocity time graphs
To determine acceleration and distance travelled from a velocity-time graph
To measure when appropriate the area under by counting squares
Keywords: gradient
Starter: Look back at your notes on distance-time graphs

49. Velocity-time graphs 80 60 40 20 Upwards line = Acceleration
Upwards line =
Acceleration
4) Downward line =
Deceleration
Velocity
m/s
3) Upwards line =
Acceleration
2) Horizontal line =
Constant speed
T/s 49

50. Summary: Velocity-time
Gradient of a velocity-time graph represents the acceleration.
The steeper the slope, the greater the acceleration.
The area under the graph represents the distance travelled.
Stationary would be a flat line along the x axis

51. Forces Monday, 19 November 2018
Terminal Velocity
Learning Outcomes
By the end of this lesson you will be able...
To understand how a falling object reaches a terminal velocity.
To draw and interpret velocity–time graphs for objects that reach terminal velocity.
Keywords
terminal velocity, streamlined, air resistance
Starter: What does streamlined mean?

52. W=mg What varies the resistance...? Come up with a list
Shape of the object (aerodynamics)
- cross sectional area
- streamlined
- slipperiness of material (e.g. Teflon swimsuits)
2. Speed you’re travelling at
3. Medium you’re travelling in (have you tried running through water?)
The acceleration due to gravity - g is a constant at the earth’s surface, so for something with a given mass, the downwards force is the same.
W=mg

54. Forces Monday, 19 November 2018
F = ma
Learning Outcomes
By the end of this lesson you will be able...
To understand the effects of forces and what a resultant force is.
To calculate force using mass x acceleration
Keywords
Resultant, Newton
Starter: what is acceleration?
Homework: sheet due next lesson

55. How to calculate resultant force?
F = m x a
Resultant force = mass × acceleration
(N) = (kg) (m/s2 )
YOU NEED TO LEARN THIS EQUATION

56. Inertial mass
Inertial mass is a measure of how difficult it is to change the velocity of an object.
Inertial mass is defined by the ratio of force over acceleration.

57. Forces Monday, 19 November 2018
Newton’s 1st and 3rd law
Learning Outcomes
By the end of this lesson you will be able...
To be able to describe and explain Newton’s 1st and 3rd law
Keywords: equilibrium
Starter: Last lesson you learn Newton’s 2nd law, what was it?
F = m x a

58. Newton’s First Law
A body in motion stays in motion at constant velocity and a body at rest stays at rest unless acted upon by a net external force.
Basically if the resultant force is zero
If the object is stationary it will remain stationary
If the object is moving, the object will continue to move at the same speed and in the same direction.
This law is commonly referred to as the law of inertia.

59. The First Law is Counterintuitive
Aristotle firmly believed this,
but physics students know better!

60. Implications of Newton’s 1st Law
If there is zero net force on a body, it cannot accelerate, and therefore must move at constant velocity, which means
it cannot turn,
it cannot speed up,
it cannot slow down.

61. Diagrams
Draw a force diagram and a free body diagram for a book sitting on a table.
Physics N W
Force Diagram N W
Free Body Diagram

62. Mass and Inertia
Chemists like to define mass as the amount of “stuff” or “matter” a substance has.
Physicists define mass as inertia, which is the ability of a body to resist acceleration by a net force.

63. Newton’s Third Law
For every action there exists an equal and opposite reaction.
If A exerts a force F on B, then B exerts a force of -F on A.

64. Forces Monday, 19 November 2018
Braking
Learning Outcomes
By the end of this lesson you will be able...
To describe what the stopping distance of a vehicle is.
To state what affects the stopping distance.
To describe the energy transfers that take place when a car is braked.
Keywords: Braking distance, reaction time, thinking distance.
Starter: What is tailgating?

65. Stopping distance The distance travelled before a car stops.
What factors could increase this?
Stopping distance = Thinking distance + Braking distance

66. What factors will affect...
Alcohol/ drugs
Tiredness
Thinking distance
Concentration/
distractions
Poor visibility
Driving too fast
No. passengers
Icy / wet roads
Braking distance
Brakes worn
Tyres worn out
Extra challenge: explain why the factors affect braking distance

67. How are stopping distance, thinking distance and braking distance related?+ =

68. Energy transfers
Describe the energy transfer that takes place when a car is braked.
Kinetic energy  Heat energy in the brakes
What will happen to the amount of friction during braking?

69. Stopping distance summary
Braking distance
Thinking distance
The faster the car the greater the thinking distance.
The thinking distance will increase if the driver is:
under the influence of alcohol
under the influence of drugs
Tired
distracted
The faster the car is travelling, the greater the braking distance will be.
The braking distance will also increase if:
The brakes or tyres - worn.
The road is icy or wet.

70. End of Triple Physics Unit 5 Revision
Forces