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GCSE AQA Physics > P5: Forces > Flashcards

P5: Forces Flashcards

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1
Q

What do vector quantities have?

A

Magnitude and direction.

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2
Q

What do scalar quantities have?

A

Just magnitude.

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3
Q

Give 5 examples of vector quantities.

A

Force, volocity, displacement, acceleration, momentum.

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4
Q

Give 5 examples of scalar quantities.

A

Speed, distance, mass, temperature, time.

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5
Q

What are the two categories of forces?

A

Contact and non-contact.

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6
Q

What is gravity?

A

The force of attraction between masses.

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7
Q

How are weight and mass different?

A

Mass is a measure of the amount of matter in an object, so remains the same everywhere. Weight is a force which acts on an object due to gravity. A measurement of weight could change with location.

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8
Q

What can be used to measure mass?

A

A mass balance (balancing scales).

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9
Q

What can be used to measure weight?

A

A calibrated spring balance (newtonmeter).

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10
Q

Mass and weight are directly proportional. What formula links them?

A

Weight = mass x gravitational field strength

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11
Q

What is the unit for weight?

A

Newtons (N).

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12
Q

What is the unit for mass?

A

Kg.

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13
Q

What is the unit for gravitational field strength?

A

N/kg.

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14
Q

When a force moves an object through a distance…

A

…work is done on the object.

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15
Q

Work done =

A

energy transferred (and force x distance).

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16
Q

What equation can be used to find work done?

A

Work done = force x distance

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17
Q

If all the forces acting on an object combine to give a resultant force of 0, it is in __.

A

Equilibrium.

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18
Q

What is elastic deformation?

A

Where an object returns to its prior shape after the force deforming it is removed.

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19
Q

What is inelastic deformation?

A

Where an object doesn’t return to its prior shape after the force deforming it is removed.

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20
Q

What is meant by the limit of proportionality (in the context of springs)?

A

The point where a spring’s extension stops being proportional to the force applied to it. A plotted graph would become non-linear at this point.

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21
Q

What equation is used to find the force applied to a spring when stretching/compressing it?

A

F = ke (force = spring constant [N/m] x extension [m])

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22
Q

How can you work out the energy in a spring’s elastic potential store from a force-extension graph?

A

Use the given equation E = 1/2ke2.

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23
Q

Practical:

Plan an investigation to show the relationship between force and extension for a spring.

A
  • Use a stand to suspend a spring. Clamp a mm ruler in line with the spring.
  • Record the natural length of the spring, taking the reading at eye level.
  • Hang a mass on the spring and measure its length. Calculate the extension by substracting the initial length.
  • Repeat this process, adding one identical mass each time, until you have 6 measurements.
  • If one extension is larger than the previous, the limit of proportionality has been succeeded. You’ll need to repeat the experiment, with a new spring and lighter masses.
  • Calculate the force exerted on the spring per mass (weight = mass x gravitational field strength).
  • Plot force exerted on the spring (x axis) against spring extension. You should see a directly proportional / linear relationship.
  • You could find the spring constant using the equation F = ke.
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24
Q

What is the moment (or torque) of a force?

A

Known as the turning effect of a force, it is the rotational equivalent of linear force, i.e. a twisting force.

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25
Q

What equation is used to find the moment of a force?

A

M = Fd (moment [Nm] = force x perpendicular distance from pivot to line of action of force)

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26
Q

What is the unit for moment?

A

Newtonmeters (Nm)

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27
Q

How would you maximise the moment of a force?

A

1) Making the force’s direction perpendicular to the lever (assuming it’s straight). 2) Increasing the length of the lever.

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28
Q

When is an object on a pivot balanced (doesn’t turn)?

A

When the total anticlockwise moment = total clockwise moment.

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29
Q

What are gears used for and how do they work?

A
  • Used to transmit the rotational effect of a force.
  • Turning one causes another to turn in the opposite direction, changing the rotational direction.
  • Transmitting a force to a larger gear causes a bigger moment, but this gear will turn slower.
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30
Q

What is pressure?

A

Force per unit area.

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31
Q

The pressure of a fluid causes a force to be exerted __ to to any surface in contact with the fluid.

A

Normal (perpendicular).

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32
Q

What equation is used to calculate the pressure a fluid exerts on a surface it touches?

A

Pressure (Pa) = force normal to surface/area of surface

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33
Q

What is the unit for pressure?

A

Pascals (Pa).

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34
Q

Pressure in a liquid depends on what?

A

Depth, density and gravitational field strength.

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35
Q

What is upthrust?

A

An upward resultant force on a submerged object which is equivalent to the weight of fluid it displaces.

36
Q

What causes upthrust?

A
  • A fluid will exert pressure on a submerged object from all directions.
  • Since pressure increases with depth, the force exerted on the bottom of the object is larger than on the top.
  • This causes an upward resultant force: upthrust.
37
Q

What would cause an object to float?

A
  • The object would be as or less dense than the fluid.
  • So the volume of fluid it displaced would weigh the same as or more than the object.
  • So upthrust ≥ weight, so it would float.
38
Q

What would cause an object to not float?

A
  • The object would be more dense than the fluid.
  • So the volume of fluid it displaced would weigh less than the object.
  • It could never displace a volume of fluid equal to its weight.
  • Upthrust < weight, so it would sink.
39
Q

If an apple was floating in water, the amount of water it displaced would weigh […] that apple.

A

As much as or more than.

40
Q

What is velocity?

A

Speed in a given direction.

41
Q

At what speed does a person usually walk?

A

1.5m/s

42
Q

At what speed does a person usually run?

A

3m/s

43
Q

At what speed does a person usually cycle?

A

6m/s

44
Q

At what speed does a car usually travel?

A

25m/s

45
Q

At what speed does a train usually travel?

A

55m/s

46
Q

At what speed does a plane usually travel?

A

250m/s

47
Q

What factors can affect wind speed?

A

Temperature, atmospheric pressure and the presence of large structures, e.g. trees or buildings.

48
Q

What is acceleration?

A

Rate of change of velocity.

49
Q

What equation is used to calculate acceleration?

A

acceleration = change in velocity / time

50
Q

Constant acceleration can be referred to as…

A

Uniform acceleration.

51
Q

On a speed-time/velocity time graph, how would you estimate the gradient of a point on a curve?

A

Calculating the gradient of a tangent to the curve.

52
Q

On a speed-time/velocity time graph, how would you work out the distance travelled?

A

By calculating the area under the graph.

53
Q

Friction always acts in what direction relative to motion?

A

The opposite direction.

54
Q

What is drag? Name one type of drag.

A

1) The friction when something moves in a fluid (liquid/gas). 2) Air resistance

55
Q

What is the most important factor in reducing drag?

A

Streamlining.

56
Q

Friction in fluids (drag) increases with __.

A

Speed.

57
Q

Explain how falling objects reach terminal velocity.

A
  • When an object first falls, the downward force of gravity is much more than the upward frictional force, so it accelerates.
  • As speed increases, friction increases, which gradually reduces the object’s acceleration until the frictional force = the accelerating force (resultant force = 0).
  • It has therefore reached its terminal velocity and falls at a steady speed.
58
Q

What would happen if you dropped a hammer and a feather, both from the same height, on the moon?

A

They would land at the same time because both objects share the same accelerating force acting on them: gravity. There is no air on the moon, which means there is no air resistance to change the speeds at which they fall.

59
Q

Why do things fall at different rates on earth?

A

Air resistance, which depends on an object’s shape and area.

60
Q

What does the frictional force of air resistance depend on?

A

An object’s shape and area.

61
Q

Why does an object’s terminal velocity depend on both its drag (air resistance) and its weight?

A

1) Gravity causes an object to experience a downward force: weight. Different masses have different weights. 2) Drag depends on an object’s shape and area, so an object which has equal weight to, but more area than, another experiences more frictional force. Although both objects have the same downward force acting on them, the object with more drag falls at a slower rate and has a lower terminal velocity.

62
Q

What is Newton’s 1st law?

A

An object remains at its current velocity unless a resultant force acts on it (if, so, it accelerates in the direction of the resultant force and keeps accelerating until the force is removed).

63
Q

What is Newton’s 2nd law?

A

An object’s acceleration is directly proportional to the resultant force acting on it, and inversely proportional to its mass (F = ma).

64
Q

What is Newton’s 3rd law?

A

When 2 objects interact, they exert equal and opposite forces on each other.

65
Q

What is inertia?

A

The tendency for objects to remain in the same state of motion. (I.e. Newton’s 1st law).

66
Q

Newton’s 3rd law states that, when 2 objects interact, they exert equal and opposite forces on each other. Why does anything move in relation to anything else?

A

1) The forces act in opposite directions. 2) Objects have varying masses, so move varying amounts when experiencing the same force.

67
Q

What does an object’s inertial mass measure?

A

How difficult it is to change the object’s velocity.

68
Q

How would you find an object’s inertial mass?

A

Rearranging F = ma to give m = F/a. So inertial mass is the ratio of force over acceleration.

69
Q

What is stopping distance?

A

The combined thinking and braking distance.

70
Q

What 2 factors affect thinking distance?

A

Speed and reaction time.

71
Q

What 4 factors affect reaction time?

A

Tiredness, drugs, alcohol and distractions.

72
Q

What 5 factors affect braking distance?

A

Speed, weather, tyre condition, condition of brakes, and road conditions (e.g. leaves).

73
Q

How do brakes work?

A
  • Force is applied to the brakes of a vehicle
  • Work is done by the friction force between the brakes and the wheel
  • This reduces the kinetic energy of the vehicle
  • The temperature of the brakes increases
74
Q

Why is applying the brake too hard/quickly dangerous?

A

Very large decelerations may cause brakes to overheat or cause the wheels to skid.

75
Q

A car travelling at a typical speed makes an emergency stop to avoid hitting a hazard 25m ahead.

Estimate the braking force needed to produce this deceleration.

This formula is given: v12 - v22 = 2as (initial velocity2 - final velocity2 = 2 x acceleration x distance)

A
  1. Assume: v ≈ 25m/s, m ≈ 1000kg.
  2. Assume uniform deceleration.
  3. Rearrange v12 - v22 = 2as to find deceleration: a = (v12 - v22) / 2s= (02 - 252) / (2 x 25) = -12.5
  4. Use F = ma (force = mass x acceleration): F = 100 x 12.5 = 12,500N so force ≈ 12,500N
76
Q

What would be the shape of a graph with speed plotted against thinking distance?

A

Linear, because thinking time remains constant, but the thinking distance increases proportionally to speed.

77
Q

What would be the shape of a graph with speed plotted against braking distance?

A

Exponential, because braking distance increases exponentially with speed.

78
Q

What is momentum?

A

A vector quantity which is the product of both an object’s mass and velocity - it can be defined as “mass in motion”.

79
Q

What equation is used to find an object’s momentum?

A

p = mv (momentum = mass x velocity)

80
Q

What is the unit for momentum?

A

kg m/s

81
Q

What is the law of conservation of momentum?

A

In a closed system, total momentum before an event = total momentum after.

82
Q

What equation links force, change in momentum and time?

A

F = Δm / Δt

Force = change in momentum / time

83
Q

The force causing a change in momentum is equal to…

A

…the rate of change of momentum (bc force = change in momentum / time)

84
Q

Why are cars designed to slow down over a longer time when they crash?

A

The longer it takes for the momentum to decrease, the smaller the rate of change of momentum, so the smaller the force of the crash. A smaller force means injuries are likely to be less severe.

85
Q

Give 3 safety features of cars. What do all of these have in common?

A

Crumple zones, seat belts and air bags. They all slow the time taken for either the car or the passenger to stop in the event of a crash, decreasing the force of the impact.

GCSE AQA Physics (8 decks)

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