3.1-3.3 & 3.5 Forces Flashcards

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 force experienced by a cable, rope, or string when pulled or hung. It acts away from the mass of the object.

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27
Q

Define normal contact force.

A

The reaction force between an object and a surface.

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.

29
Q

Define friction.

A

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

30
Q

Define drag.

A

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

31
Q

What direction do drag forces such as friction act in?

A

The opposite direction to the movement causing it.

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
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.
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?

35
Q

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

A

Drag and weight.

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
37
Q

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

A

Moment = force x perpendicular distance

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 upward force exerted on an object immersed in a fluid is equal to the weight of the fluid displaced by the object

51
Q

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

A

p = h ρ g
where p is the pressure, h is the depth of the liquid, ρ is the density of the liquid, and g is the acceleration due to gravity.

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?

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

State Newton’s first law.

A

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

61
Q

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

62
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.

63
Q

What is impulse?

A

The change in momentum:

64
Q

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

A

It is equal to the area underneath the graph.

65
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.