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OCR A-level Physics A > Forces > Flashcards

Forces Flashcards

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1
Q

Define displacement.

A

Displacement is the distance travelled in a particular direction.

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2
Q

Define instantaneous speed.

A

The rate of change of displacement at one particular instant in time.

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3
Q

Define average speed.

A

Average speed = total distance travelled / total time taken

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4
Q

Define velocity

A

Velocity is the rate of change of displacement.

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5
Q

Define acceleration.

A

Acceleration is the rate of change of velocity.

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6
Q

What does the gradient on a displacement-time graph represent?

A

The object’s velocity.

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7
Q

What does the area under a velocity-time graph represent?

A

The object’s displacement.

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8
Q

What does the gradient on a velocity-time graph represent?

A

The object’s acceleration.

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9
Q

What does a horizontal line on an acceleration-time graph represent?

A

Constant acceleration.

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10
Q

Which two quantities in the equations of constant acceleration have the same unit?

A

u and v (both are measured in ms^-1)

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11
Q

Why can’t we apply the equations of constant acceleration to the motion of a skydiver during a skydive?

A

Because a skydiver’s acceleration isn’t constant.

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12
Q

In a rebound question, why must u and v have opposite signs?

A

Because they are in opposite directions.

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13
Q

What is the acceleration g of free fall?

A

The force acting per unit mass

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14
Q

What is the value of g on and near the surface of the earth?

A

9.81 m s^-1

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15
Q

What is thinking distance proportional to?

A

Initial speed.

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16
Q

What is breaking distance proportional to?

A

The square of the speed.

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17
Q

Define 1N.

A

The force required to accelerate a 1kg object at 1ms^-1
1N = 1kg X 1ms^-2

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18
Q

Define the net (resultant) force.

A

The sum of all the forces acting on an object.

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19
Q

What is a projectile?

A

An object in freefall / an object that is only acted on by weight / an object whose acceleration is g

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20
Q

If you have a two-dimensional motion problem with constant velocity in one direction and constant acceleration in a perpendicular direction, how do you solve it?

A
  • Split the page into vertical and horizontal - break the initial motion down into vertical and horizontal components of velocity
  • Use positive and negative to indicate direction (e.g. positive = up, negative = down)
  • Use SUVAT to solve vertically - generally for the time.
  • Use s = v t to solve horizontally
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21
Q

If you are given a SUVAT question and a force starts off at an angle, how should you tackle it?

A

Resolve it into horizontal and vertical components.

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22
Q

If a projectile is launched and lands at the same horizontal level, which two sets of values are the same?

A

Time up = Time down
Vertical take off speed = Vertical landing speed

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23
Q

What is a free body diagram?

A

A diagram that represents the forces or the net (resultant) force acting on an object.

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24
Q

What direction does normal contact force act in?

A

Perpendicular to the surface.

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25
Q

True or false: An object will take the same time to fall a given distance, regardless of the horizontal component of its velocity.

A

True.

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26
Q

Define tension.

A

The resultant of two forces acting on an object in opposite, outwards directions.

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27
Q

Define normal contact force.

A

The reaction force between an object and a surface.

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28
Q

Define upthrust.

A

The upwards force that a fluid applies on an object. The size of the upthrust force can be found by applying Archimede’s principle.

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29
Q

Define friction.

A

The resistive force that one surface encounters when moving over another.

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30
Q

Define drag.

A

The frictional force experienced by an object travelling through a fluid.

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31
Q

What direction do drag forces such as friction act in?

A

The opposite direction to the movement causing it.

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32
Q

List four factors that would affect the size of the drag force exerted on an object.

A
  • The object’s cross sectional area
  • The object’s speed
  • The density of the fluid that the object is travelling through (if applicable)
  • The object’s shape and texture
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33
Q

Describe the motion of an object falling in a uniform gravitational field in the presence of drag.

A
  • Initially, the object accelerates with an acceleration value of g because weight is the only force acting. Initially, there is no velocity.
  • Drag increases as the object falls due to increased speed.
  • Eventually the force of drag will be equal to the weight, the object will no longer accelerate as the net force is 0.
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34
Q

For an object falling in a uniform gravitational field in the presence of drag, what is the initial acceleration when it is stationary?

A

g

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35
Q

For terminal velocity to occur, which two forces must be equal?

A

Drag and weight.

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36
Q

Name two practical techniques that can be used to determine terminal velocity in fluids.

A
  • Ball bearing in a viscous liquid
  • Cupcake case falling through air
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37
Q

What equation can be used to find the moment of a force?

A

Moment = force x perpendicular distance

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38
Q

What is a couple?

A

A pair of forces that together cause a net (resultant) moment, but not a net (resultant) force.

39
Q

State the conditions for a pair of forces acting on the same object to be a couple.

A

The two forces have to be the same size but acting in opposite directions.

40
Q

What is a torque?

A

A force that causes something to turn.

41
Q

State the equation that can be used to find the torque due to a couple.

A

Torque = Force x distance

42
Q

State the principle of moments.

A

For an object to be in equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments exerted on it.

43
Q

What is the centre of gravity of an object?

A

The point through which the weight appears to act.

44
Q

What is the centre of mass of an object?

A

The average position of the object’s mass.

45
Q

State the two conditions required for an object to be in equilibrium.

A

The net resultant force on the object must be zero and the net resultant moment on the object must be zero.

46
Q

Describe the motion of an object that has a net resultant force of zero exerted on it.

A

Its acceleration is zero, however it may be stationary or moving at a constant velocity.

47
Q

Explain what is meant by resolving a force in the horizontal and vertical directions.

A

Finding the horizontal and vertical components of the force.

48
Q

What does coplanar mean?

A

In the same plane.

49
Q

If three coplanar forces add up to zero, what shape will their vector addition diagram make?

A

A triangle.

50
Q

State Archimedes principle.

A

The upthrust exerted on a body immersed in fluid is equal the weight of the fluid that the body displaces. An object will sink if the upthrust is smaller than the weight. For a floating object, its weight is equal to the upthrust.

51
Q

State the equation which can be used to find pressure of fluids in constant density.

A

p = h ρ g

52
Q

What phrase means the same as work done by a force?

A

Energy transferred by the force.

53
Q

Define work done.

A

Work done = force x distance moved in the direction of the force.

54
Q

What is the formula for work done, if the object moves in the same direction as the direction of the force?

A

W = Fx

55
Q

Define power.

A

The rate of energy transfer or the rate of work done.

56
Q

Define 1 Joule.

A

1 Joule is the amount of work when a force of one newton is exerted over a distance of one meter.

57
Q

State the principle of conservation of energy.

A

Energy cannot be created or destroyed, only transferred between different stores and objects.

58
Q

How can you increase an object’s gravitational potential energy store?

A

Lift the object higher in the gravitational field.

59
Q

Why are energy transfers in the real world never 100% efficient?

A

Work done against resistive forces, such as friction and air resistance, means that some energy is always transferred to the thermal energy stores of an object and its surroundings.

60
Q

How the force constant, k, be found from a force extension graph?

A

It is equal to the gradient. For rubber bands it varies and is indicated by the gradient of a tangent to the stretching curve.

61
Q

State Hooke’s law.

A

The extension (or compression) of a spring (or wire) is directly proportional to the force exerted on it, up to its limit of proportionality

62
Q

What is the formula for Hooke’s law?

A

F = kx

63
Q

State the formula for springs in parallel.

A

kc = k1 + k2 + …

64
Q

When two springs are placed in parallel, how does the stiffness constant change?

A

The stiffness constant doubles.

65
Q

When two springs are placed in series, how does the stiffness constant change?

A

The stiffness constant halves.

66
Q

How can work done be found from a force-extension graph?

A

It is the area under the graph.

67
Q

What is a material that behaves elastically?

A

A material that deforms when a force is applied, but returns to its original shape when the force is removed.

68
Q

What is a material that has plastic behaviour?

A

A material that deforms when a force is applied, and stays in its deformed shape when the force is removed.

69
Q

What is a brittle material?

A

A material which breaks through cracks / fracture with little plastic deformation.

70
Q

What are tensile forces?

A

Forces which stretch / pull an object

71
Q

What are compressive forces?

A

Forces that compress an object

72
Q

What is a ductile material?

A

A material which undergoes large plastic deformation under tensile forces.

73
Q

What is a malleable material?

A

A material which undergoes plastic deformation under compressive forces.

74
Q

What is a tough material?

A

A material that can absorb a lot of energy by plastic deformation before breaking.

75
Q

What is a hard material?

A

A material which is resistant to scratching. The harder a material is, the more difficult it is to dislodge atoms from its surface

76
Q

What is a stiff material?

A

A material which requires a lot of force for a little deformation.

77
Q

What is a strong material?

A

A material that requires a large force to break

78
Q

What is a polymeric material?

A

A material made from long chain molecules.

79
Q

Define the elastic limit of an object or material.

A

The point beyond which it becomes permanently deformed and will not return to its original length or shape once any forces are removed.

80
Q

How can the Young’s modulus of a material be found from its stress-strain graph?

A

From the gradient of the initial straight-line section of its stress-strain graph.

81
Q

What does the area under the straight line section of a material’s stress-strain graph represent?

A

The energy stored in the material per unit volume.

82
Q

Define the Ultimate Tensile Stress (UTS) of a material.

A

The maximum stress the material experiences; a measure of the material’s strength.

83
Q

What type of material has a high Young modulus?

A

A stiff material.

84
Q

What type of material has a high UTS and breaking point?

A

A strong material.

85
Q

Describe a stress-strain graph for a typical ductile material.

A

After the limit of proportionality, There is a curve in the line before the elastic limit (from which the material starts to behave plastically).

86
Q

Describe a stress-strain graph for a typical brittle material.

A
  • The graph is linear with a steep slope, indicating that the material obeys Hooke’s Law up to its breaking point.
    • There is little to no plastic deformation, and the material fractures suddenly after a small amount of strain.
87
Q

Describe a stress-strain graph for rubber, a typical polymeric material.

A

The loading and unloading curves for rubber are different - the energy released when the rubber is unloaded is less than the work done to stretch the rubber. This is because some of the elastic potential energy in the rubber is converted to heat.

88
Q

If a material obey’s Hooke’s law, what will its stress-strain graph look like?

A

It will have a straight line through the origin.

89
Q

State Newton’s first law.

A

An object will maintain constant velocity, unless acted on by a resultant force.

90
Q

State Newton’s second law (with the formula and also with words).

A

F = ma

91
Q

State Newton’s third law.

A

If body A exerts a force on body B,then body B exerts the same size force on body A in the opposite direction.

92
Q

What is impulse?

A

The change in momentum:

93
Q

How can impulse be found from a force-time graph?

A

It is equal to the area underneath the graph.

94
Q

What is the difference between elastic and inelastic collisions?

A

In an elastic collision, the kinetic energy before is equal to the kinetic energy afterwards - no energy is lost. However, in an inelastic collision, the kinetic energy at the end is not equal to the kinetic energy at the start - some energy is lost to the surroundings.

OCR A-level Physics A (6 decks)

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