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Physics > Forces > Flashcards

Forces Flashcards

1
Q

What is the unit used for momentum?

A

kg m/s

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2
Q

In a closed system, what can be said about the momentum before and after a collision?

A

The total momentum before is equal to the total momentum afterwards

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3
Q

What quantity is equal to the force experienced in a collision?

A

The rate of change of momentum

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4
Q

If an objects change of momentum is fixed, what is the only way to reduce the force that the object experiences?

A

Increase the length of time over which the change of momentum occurs

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5
Q

Explain how a seatbelt improves a passenger’s safety during a collision

A
  • passenger must decelerate from the vehicle’s velocity at impact to zero, meaning they undergo a fixed change of momentum
  • the force they experience is equal to the rate of change of momentum
  • seatbelts increase the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced
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6
Q

Does a distance quantity require a specific direction? (is it a scalar or vector quantity?)

A

No specific direction is required so it is a scalar quantity

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7
Q

State a typical value for the speed of sound

A

330 m/s

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8
Q

What is a typical value for human walking speed?

A

1.5 m/s

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9
Q

What is a typical value for human running speed?

A

3 m/s

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10
Q

What is a typical value for human cycling speed?

A

6 m/s

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11
Q

Why can an object travelling at a constant speed in a circle not have a constant velocity?

A
  • speed is a scalar quantity
  • velocity is a vector quantity which means it can only be constant if the direction is constant
  • in circular motion, the direction is continuously changing
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12
Q

How can speed be calculated from a distance-time graph?

A

The speed is equal to the gradient of the graph

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13
Q

How can the distance travelled by an object be calculated from a velocity-time graph?

A

It is equal to the area under the graph

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14
Q

What can be said about the resultant force acting on an object when it is falling at terminal velocity?

A
  • the resultant force is zero
  • when at terminal velocity, the object is moving at a constant speed and so isn’t accelerating
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15
Q

State Newton’s first law for a stationary object

A

If the resultant force on a stationary object is zero, the object will remain at rest

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16
Q

State Newton’s first law for a moving object

A

If the resultant force on a moving object is zero, the object will remain at constant velocity

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17
Q

If an object changes direction but remains at a constant speed, is there a resultant force?

A

Since there is a change in direction, there is a change in velocity and so there must be a resultant force

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18
Q

What is inertia?

A

The tendency of an object to continue in its state of rest or uniform motion

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19
Q

State Newton’s second law

A

An object’s acceleration is directly proportional to the resultant force acting on it and is inversely proportional to its mass

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20
Q

What is inertia mass?

A
  • a measure of how difficult it is to change a given object’s velocity
  • the ratio of force over acceleration
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21
Q

State Newton’s third law

A

Whenever two objects interact, the forces that they exert on each other are always equal and opposite

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22
Q

What is the stopping distance of a vehicle equal to?

A

The sum of thinking distance and braking distance

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23
Q

Give a typical range of values for human reaction time

A

0.2 seconds - 0.9 seconds

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24
Q

Give three factors which can affect a driver’s reaction time

A
  1. tiredness
  2. drugs
  3. alcohol
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25
Q

Give two factors which may affect braking distance

A
  1. adverse (wet/icy) road conditions
  2. poor tyre/brake condtions
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26
Q

Describe the energy transfers that take place when a car applies its brakes

A
  • work is done by the friction force between the brakes and wheel
  • kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
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27
Q

State two consequences of a vehicle undergoing very large decelerations

A
  1. kinetic energy converted to heat is very high causing brakes to overheat
  2. loss of control of the vehicle
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28
Q

What is meant by the term fluid?

A

A liquid or a gas

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29
Q

In any fluids, at what angle do the forces due to pressure act on a given surface?

A

At right angles to the surface

30
Q

What happens to the density of the atmosphere with increasing altitude?

A

The atmosphere becomes less dense and altitude increases

31
Q

Explain why atmospheric pressure decreased with an increase in height

A
  • pressure is created by collisions of air molecules
  • the quantity of molecules (and so weight) decreases as the height increases
  • this means atmospheric pressure decreases with an increase of height
32
Q

How does pressure in fluids increase with depth

A

As the depth increases, the mass of liquid above that depth also increases. This means that the force due to the mass increases. Since the force has increased whilst the area has remained constant, the pressure will increase

33
Q

Why does pressure in fluids increase with density?

A

As the density of a fluid increases, the number of particles in a given volume increases. Consequently the weight of the fluid is greater. This means that the force from the fluid above a certain point is larger. Since the force has increased, the pressure also increases

34
Q

What is upthrust always equal to?

A

The weight of the fluid that the object displaces

35
Q

What factors influence whether an object will sink of float?

A
  • upthrust
  • weight
  • density of fluid
36
Q

Explain why an object with a density greater than that of water can never float

A

Upthrust is equal to the weight of the displaced fluid. If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object’s weight. This means that it will sink

37
Q

What is an alternative name for the turning effect of a force?

A

A moment

38
Q

What distance measurement is used when calculating a moment?

A

The perpendicular distance from the pivot to the line of action of the force

39
Q

If an object is in equilibrium, what can be said about the moments acting on the object?

A

The clockwise moments are equal to the anticlockwise moments

40
Q

What three parts make up a lever system?

A
  1. load
  2. effort
  3. pivot
41
Q

How can a lever be used as a force multiplier?

A

If the distance between the effort and pivot is greater than the distance between the pivot and load, the force applied on the load is greater than the effort force. This is since the moment on both the effort and load must be the same

42
Q

Give an example of when a lever may be used to multiply a force

A

A wrench, which has a long handle so that the force applied by the user is multiplied

43
Q

What determines the moment of a gear wheel?

A

The size of the wheel

44
Q

Describe the moment and speed of a larger gear being driven by a smaller gear

A

The larger wheel will rotate more slowly but will also produce a larger moment

45
Q

Explain the relationship between the force applied and the extension of an elastic object

A

The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

46
Q

What is meant by an inelastic deformation?

A
  • A deformation which results in the object being permanently stretched
  • the object doesn’t return to its original shape when the force is removed
47
Q

What type of energy is stored in a spring when it is stretched?

A

Elastic potential energy

48
Q

What can extension be replaced with in the equation for spring force?

A

Compression

49
Q

What does it mean if a force is said to do work?

A

The force causes an object to be displaced through a distance

50
Q

What distance must be used when calculating work done?

A

It must be the distance that is moved along the line of action of the force

51
Q

Under what circumstance is 1 joule of work done?

A

When a force of 1 Newton causes a displacement of 1 metre

52
Q

How mant Newton-metres are equal to 1 joule of energy?

A

1 Nm = 1 J

53
Q

What occurs when work is done against frictional forces?

A
  • a rise in temperature of the object occurs
  • kinetic energy is converted to heat
54
Q

Why does air resistance slow down a projectile?

A
  • the object does work against the air resistance
  • kinetic energy is converted in to heat, slowing down the object
55
Q

What is a scalar quantity?

A

A quantity that only has a magnitude

56
Q

What is a vector quantity?

A

A quantity that has both a megnitude and an associated direction

57
Q

How can a vector quantity be drawn and what does it show?

A
  • as an arrow
  • the length of the arrow represents the magnitude
  • the arrow points in the associated direction
58
Q

What is a force?

A

A push or pull acting on an object due to an interation with another object

59
Q

What are the two categories that all forces can be split into?

A
  1. contact forces
  2. non-contact forces
60
Q

Give three examples of contact forces

A
  1. friction
  2. air resistance
  3. tension
61
Q

Give three examples of non-contact forces

A
  1. gravitational forces
  2. electrostatic forces
  3. magnetic forces
62
Q

Is force a vector or a scalar quantity?

A
  • vector
  • it has both a magnitude and an associated direction
63
Q

Give three examples of vector quantities

A
  1. velocity
  2. displacement
  3. force
64
Q

Give three examples of scalar quantities

A
  1. temperature
  2. time
  3. mass
65
Q

What is weight?

A

The force that acts on an object due to gravity and the object’s mass

66
Q

What quantities does weight depend on?

A
  • the objects mass
  • the gravitational field strength at the given position in the field
67
Q

What is the unit used for weight?

A

The Newton (N)

68
Q

What is the unit used for gravitational field strength?

A

N/kg

69
Q

What is meant by an object’s centre of mass?

A

The single point where an object’s weight can be considered to act through

70
Q

What piece of equipment can be used to measure an object’s weight?

A

A calibrated spring-balance or newton-metre

71
Q

What is the name given to the single force that is equivalent to all the other forces acting on a given object?

A

The resultant force

Physics (8 decks)

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