# Topic 5 - Forces Flashcards

1
Q

What are scalar quantities?

A

Scalar quantities only have a magnitude (size).

2
Q

What are vector quantities?

A

Vector quantities have a magnitude and a direction.

3
Q

What is the difference between speed and velocity?

A

Speed is a scalar quantity, so it only has a magnitude. Velocity is a vector quantity, so it has a magnitude and and a direction.

4
Q

What is the equation for speed?

A

Speed = distance / time - s = d/t - m/s = m / s

5
Q

What is the unit for speed?

A

metres per second - m/s

6
Q

What does the calculations of speed assume?

A

It assumes the speed is constant.

7
Q

Give examples of vector quantities?

A

Acceleration, force, displacement and velocity.

8
Q

Give examples of scalar quantities?

A

Temperature, mass, energy, distance and speed.

9
Q

What is the equation for acceleration?

A

Acceleration = change in velocity / time - a = Δv / t - m/s^2 = Δv / seconds

10
Q

What is the unit for acceleration?

A

metres per second squared - m/s^2

11
Q

How do we work out the resultant force?

A

We use Pythagoras’ Theorem.

12
Q

What is the unit for the resultant force?

A

Newtons - n

13
Q

What is displacement?

A

Displacement is the distance an object moves in a straight line from a starting point to a finishing point.

14
Q

Is displacement a scalar or vector quantity?

A

Displacement is a vector quantity as it contains a direction and a magnitude.

15
Q

What is distance?

A

Distance is how far an object moves.

16
Q

Is distance a scalar or vector quantity?

A

Distance is a scalar quantity as it contains a magnitude but not a direction.

17
Q

What is a force?

A

A force is a push or pull that changes an objects motion.

18
Q

What are the two forces?

A

Contact forces and non-contact forces.

19
Q

What are contact forces?

A

Contact forces are forces that act between two objects that are physically touching.

20
Q

What are non-contact forces?

A

Non-contact forces happen when objects are separated.

21
Q

What are examples of contact forces?

A

Friction, air resistance, tension and normal contact force.

22
Q

What are examples of non-contact forces?

A

Gravitational force, electrostatic force and magnetic force.

23
Q

What is an interaction pair?

A

An interaction pair is a set of 2 forces that are equal and opposite, acting on 2 interacting objects.

24
Q

What is air resistance?

A

Air resistance comes about when an object moves through air and collides with molecules. This creates a force that slows the object down.

25
Q

What is tension?

A

Tension is the pulling force that a string or cable exerts (creates) when something or someone pulls on it.

26
Q

What is the normal contact force?

A

When you push on a table, your hand doesn’t move through it. This is because the normal contact force from the table pushes equally on your hand.

27
Q

What is friction?

A

Friction comes about whenever two surfaces are touching and try to move against each other. Tiny bumps in the surface interlock (overlap or fit together). This creates a frictional force that opposes their motion.

28
Q

What is on the x-axis of a distance time graph?

A

The time (seconds) is on the x-axis of a distance-time graph.

29
Q

What is on the y-axis of a distance time graph?

A

The distance (metres) is on the y-axis of a distance-time graph.

30
Q

What happens when the line of a distance time graph is increases horizontally but not vertically?

A

The object is stationary.

31
Q

How do we work out distance in a speed-time graph?

A

We work out the area below the line.

32
Q

What is mass?

A

The mass of an object is a measure of the amount of matter it contains. The mass of an object is constant.

33
Q

What is inertia?

A

Inertia is how difficult it is to change the object’s motion depending on the objects mass.

34
Q

How does the mass of an object tell you how much inertia it has?

A

An object with a high mass has more inertia than an object with a lower mass.

35
Q

What is the equation for weight?

A

Weight = mass x gravitational field strength - w = mg - newtons = kg x n/kg

36
Q

What is the centre of mass?

A

The centre of mass is the point through which an object’s weight appears to act.

37
Q

Where is the centre of mass in a hanging object?

A

If an object is hung from a string, it will hang with its centre of mass directly below the point that it is hung from.

38
Q

How is the centre of mass important if it depends on if an object falls over or not?

A

An object will fall over if its centre of mass is outside its base. An object will fall off a surface if its centre of mass isn’t over the surface.

39
Q

What is the value of acceleration due to gravity at the Earth’s surface?

A

9.81 m/s^2

40
Q

What is the resultant force?

A

The resultant force is the sum of all the forces acting on an object.

41
Q

What is caused by the resultant force?

A

The change in an object’s motion.

42
Q

What does it mean when the forces acting on an object are unbalanced (not equal) ?

A

It means that the resultant force is acting on the object.

43
Q

What is the equation for the resultant force and what law is it?

A

F = ma - Resultant force = mass x acceleration, this is Newton’s 2nd law, resultant force causes acceleration.

44
Q

What does Newton’s 1st Law say?

A

The velocity of an object will only change if a resultant force is acting on the object. This applies to a stationary or moving object.

45
Q

How does Newton’s 1st Law apply to moving objects?

A

If an object is moving and there is no resultant force acting on it, the object will continue moving in the same direction at the same speed. This means that the object will continue moving at the same velocity. This also means that the velocity of an object will only change if a resultant force is acting on the object.

46
Q

How does Newton’s 1st Law apply to stationary objects?

A

If an object is stationary and there is no resultant force acting on it, it will stay stationary.

47
Q

What happens when the forces on an object are balanced?

A

There is no resultant force.

48
Q

What does Newton’s 3rd Law say?

A

Whenever 2 objects interact, the forces that they exert (apply to) each other are equal and opposite. If one object exerts (applies) a force on another object, then the other object must be exerting (applying) a force back.

49
Q

What is a free body force diagram?

A

A free body force diagram is a diagram showing the forces acting on an object. These are shown as vectors.

50
Q

What are the forces acting on a driving car?

A

The driving force, the weight, the friction/resistive force and the normal contact force

51
Q

What are the forces acting on a skydiver?

A

The air resistance (acting upwards) and the skydiver’s weight (acting downwards).

52
Q

We do we say when the resultant force on an object is zero?

A

The object is in equilibrium.

53
Q

How can we stretch, bend or compress objects?

A

By applying forces to them. There must be two or more forces acting on an object.

54
Q

What happens to an object if only one force is acting on it?

A

The object will just move in the direction of that force.

55
Q

At least how many forces must be acting on an object to stretch, bend or compress it?

A

At least 2.

56
Q

What happens to an elastically deformed object when the forces applied stop?

A

It returns to its original shape, like a spring does.

57
Q

What happens to an inelastically deformed object when the forces applied stop?

A

It will not return to its original shape, like when a car crashes into a tree, it will not return to its original shape obviously.

58
Q

What is on the x-axis of an extension-load graph?

A

Extension measured in cm.

59
Q

What is on the y-axis of an extension-load graph?

A

Force measured in newtons.

60
Q

How does the line change on an extension-load graph?

A

For low forces the graph is a straight line which passes through the origin. When no force acts on the spring, there is no extension. As the force on the spring increases, the spring reaches its limit of proportionality, this is where the line begins to curve on the graph.

61
Q

What does a force acting on an object might change about it?

A

Its size or shape.

62
Q

What is Hooke’s Law?

A

When a spring is stretched, the increase in the length of the spring is called its extension. Hooke’s Law tells us that the extension of a spring is directly proportional to the force applied to the spring.

63
Q

What is the formula for Hooke’s Law?

A

Force = spring constant x extension - F= ke

64
Q

What is the spring constant of a spring?

A

Roughly how stiff the spring is. The higher the spring constant, the stiffer the spring and the more force is needed to stretch it.

65
Q

How does the limit of proportionality affect Hooke’s Law?

A

The limit of proportionality is the point where Hooke’s Law breaks down. If a spring is stretched too much, it will not return to its original length when the force stops acting on the spring.

66
Q

What is the apparatus and procedure for Hooke’s Law experiments?

A

Put a retort stand on a table, with a clamp holding it down on the table. Put a clamp on top of the retort stand and a spring on the end of it. We have a 100g mass hanger added to the spring. We put one 100g object on the hanger and we use a metre ruler to measure the stretched length. Repeat this to find the extension of the spring, and put the data in a graph.

67
Q

What is done when a force stretches or compresses another object?

A

Work is done.

68
Q

What is stored in a spring when it is compressed?

A

Elastic potential energy is stored.

69
Q

What is the elastic potential energy in a spring equal to?

A

It is equal to the work done when stretching it.

70
Q

What is the formula for elastic potential energy stored in a spring?

A

Elastic potential energy = 1/2 x spring constant x extension^2 - U = 1/2ke^2

71
Q

How do we find the elastic potential energy stored in a force-extension graph?

A

The area under the line of the graph.

72
Q

What happens when an object is in free fall?

A

The object’s weight is the only force acting on it.

73
Q

What is the rate of acceleration of an object in free fall?

A

An object in free fall will accelerate at a constant rate. This constant rate is called the acceleration due to gravity (g) which is 9.81 m/s^2 on Earth.

74
Q

Why don’t objects falling accelerate forever?

A

Air resistance slows them down.

75
Q

What is terminal velocity?

A

When the weight force and force due to air resistance are equal, the object stops accelerating, it has reached terminal velocity, meaning it is the fastest an object can fall.

76
Q

What is the equation for momentum?

A

Momentum = mass x velocity - p = mv

77
Q

What is the law of the conservation of momentum?

A

It says that momentum cannot be created or destroyed.

78
Q

What is the stopping distance?

A

The stopping distance is the distance it takes a car to stop in an emergency (like when the car is braking suddenly).

79
Q

What is the equation for stopping distance?

A

Stopping distance = thinking distance + braking distance

80
Q

What is the thinking distance?

A

The time is takes for a driver to react to a situation is their reaction time. During this reaction time, the car carries on moving. The thinking time is the distance travelled between when the driver realises they need to brake and when they apply the brakes.

81
Q

What is the braking distance?

A

The braking distance is the distance the car travels between the driver applying the brakes and the car stopping.

82
Q

What are the factors affecting thinking distance?

A

Tiredness - affects the drivers reaction time
Distractions - affects reaction time - small children and mobile phones
Drugs or Alcohol - affects the drivers reaction time, these all increase reaction time

83
Q

What are the factors affecting braking distance?

A

Car speed - faster the car is travelling, the further it travels before it comes to a stop
The condition of the car - if the brakes or tyres are in a poor condition, it increases the braking distance

84
Q

Why is work done needed for braking a car?

A

When the brake pedal is pushed, the brake pads are pressed onto the wheels. This contact causes friction. This causes work to be done.

85
Q

How does the work done when braking a car increase the temperature of the brakes?

A

The work done between the brakes and wheels converts kinetic energy in the wheels to thermal energy in the brakes, causing the temperature to rise.

86
Q

How does the speed of the car affect the work done needed?

A

The greater the speed of the vehicle, the grater the braking force needed to stop the vehicle in a certain distance. This means that more work needs to be done on the brakes to stop the car.

87
Q

How does the mass of the car affect the work done needed?

A

The greater the mass of the vehicle, the greater the breaking force needed to stop the vehicle. This means that more work needs to be done on the brakes to stop the car.

88
Q

What happens if the force (grip) between the road and the vehicle increases while the same work done is being applied?

A

The stopping distance decreases.

89
Q

What must equal what when a car comes to a stop?

A

Work done = initial kinetic energy

F d = 1/2 m v^2

90
Q

What happens when a family car decelerates at traffic lights?

A

Only a small force is exerted on to the passengers. This is because the deceleration happens over a long period of time. The force does not harm the passengers.

91
Q

What happens when a family car has to have an emergency stop?

A

A greater force is exerted on to the passengers. This is because the deceleration happens over a shorter period of time. Fortunately, the force should not be enough to harm the passengers.

92
Q

What happens when a family car crahses?

A

An even greater force is applied to the passengers. This is because the deceleration happens in less time. This could harm the passengers.

93
Q

What can large decelerations lead to?

A

This causes brakes to overheat and/or the car to skid.