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Physics IGCSE > Forces and Motion > Flashcards

Forces and Motion Flashcards

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1
Q

How do you find actual (not average) speed?

A

Discover how fast the car moves in the shortest time you can measure.

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2
Q

How do you find out the speed of a racing car?

A

Take an average of two runs (one down the course and one up again) to cancel out the effects of wind.

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3
Q

When does something accelerate?

A

When its velocity is changing. A changing velocity can mean either changing speed or changing direction.

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4
Q

How do you indicate direction in a vector?

A

Use + or -.

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5
Q

What does +10m/s mean?

A

10m/s to the right.

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6
Q

On a distance-time graph, how do you find speed?

A

Find the gradient.

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7
Q

What does it mean when gradient = 0 on a distance-time graph?

A

Gradient = speed, so when the line is horizontal, speed = 0 (the car has stopped).

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8
Q

On a speed-time graph, how do you find acceleration?

A

Find the gradient.

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9
Q

On a speed-time graph, how do you find distance?

A

Find the area under the graph (use the triangle or rectangle formula depending).

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10
Q

How many dots does a typical ticker-tape timer record per second?

A

50 dots per second.

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11
Q

What does it mean when the dots on a ticker-tape timer are far apart?

A

The trolley is moving at high speed.

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12
Q

What happens when you arrange the sections side-by-side?

A

You get a speed-time graph.

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13
Q

Explain why 5 sections of ticker-tape would produce 0.4 seconds worth of time.

A

Each section starts 0.1 seconds after the one before (it takes 0.1 second to get to the 2nd section, 0.1 second to get to the 3rd, 0.1 to the 4th and 0.1 to the 5th = 0.4, not 0.5).

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14
Q

Why does the ticker-tape produce a speed-time graph and not a distance-time graph?

A

Speed increases with length. The distance is given not by length but by area of the strips.

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15
Q

What must you bear in mind when calculating speed and acceleration?

A

You are calculating an average.

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16
Q

Why don’t objects fall with the acceleration of free fall?

A

Air resistance changes depending on the surface area of the object.

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17
Q

What is g?

A
  • The acceleration of free fall, 9.8m/s.
  • Gravitational field strength, 10 N/kg.
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18
Q

Why does the value of the acceleration of free fall vary on the earth’s surface?

A
  • Earth’s gravitational pull varies - it decreases slightly out into space.
  • However, the acceleration of free fall for a body near the earth is constant.
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19
Q

Distinguish between speed and velocity.

A

Velocity is a vector quantity, speed is scalar. (Velocity is speed and diection).

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20
Q

Describe qualitatively the motion of bodies falling in a uniform gravitational field with air resistance.

A
  1. Initially acceleration is g (no/little speed).
  2. As speed increases, air resistance increases.
  3. Eventually air resistance is enough to balance weight.
  4. When this happens, velocity reaches its maximum and stays constant.
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21
Q

Why doesn’t the skydiver stay still, if forces are in balance?

A
  • If there is no speed, there is no air resistance.
  • So, with only her weight acting, she would gain velocity.
  • Follows Newton’s 1st law: when forces are in balance, either straight line motion at steady velocity or no motion.
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22
Q

Describe qualitatively the motion of bodies falling in a uniform gravitational field without air resistance.

A
  1. As ball is thrown upwards, downward (negative) velocity increases by 10m/s each second.
  2. As ball reaches highest point, downward velocity becomes 0m/s.
  3. Downward velocity increases by 10m/s each second still as the ball falls.
  4. Ball eventually has the same speed that it had to begin with, except in the opposite direction.
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23
Q

What does a non-uniform acceleration mean?

A

An object is not gaining velocity at a steady rate.

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24
Q

Why is a car’s acceleration rarely steady?

A

Think - since A = F/m, a change in either mass or force can cause a non-uniform acceleration:

  • For a car that is not very aero-dynamic, as maximum velocity is approached, acceleration becomes less and less until it is 0 (reaches terminal velocity - air resistance balances thrust).
  • The mass of fuel in the car can decrease, causing an increasing acceleration.
  • Alternatively, a gear change can cause a non-steady acceleration.
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25
Q

What is an object’s acceleration when stationary?

A

It is still g - e.g. a stone thrown upwards at its highest point still has an acceleration of 10m/s2.

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26
Q

How do you calculate the distance travelled when acceleration is non-uniform?

A

Make an estimate by counting squares - remember to use axis values and not the actual area on the paper.

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27
Q

What is a force?

A

A push or a pull, exerted by one object on another.

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28
Q

Give four examples of vector quantities.

A
  • Acceleration (because it uses velocity)
  • Force (weight, thrust, friction…)
  • Velocity
  • Momentum (because it uses velocity)
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29
Q

How do you measure small forces?

A

Use a spring balance - a larger force will stretch the spring more and give a higher reading on the scale.

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30
Q

What is upthrust?

A

The upward force from a liquid or gas that is responsible for making things float.

N.B. density is an indicator of whether or not things float - density does not cause floating or sinking by itself, however.

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31
Q

What is tension?

A

The force in a stretched material.

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32
Q

What is friction?

A

The force between two surfaces which impedes motion and results in heating.

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33
Q

What is air resistance?

A

A type of friction - note air resistance is a FORCE!

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34
Q

What is thrust?

A

The force from an engine.

35
Q

What is weight?

A

The gravitational force on an object.

36
Q

What happens to the motion of an object without force?

A
37
Q

What do you require a force to do?

A

Accelerate.

Because acceleration is a vector (because acceleration = change in velocity), with no force, you have no change in speed or direction.

  • Start moving
  • Increase or decrease speed
  • Change direction
38
Q

What happens to an object if forces are in balance?

A

The object acts as if no forces are acting on it.

An object with balanced forces obeys Newton’s 1st law.

39
Q

What happens when a skydiver opens a parachute?

A
  1. Extra material increases surface area.
  2. Increased surface area increases air resistance (more contact between sky and material=more friction).
  3. This means that forces are no longer in balance, because your weight must stay the same, yet air resistance has increased.
  4. This means that your speed has to decrease in order to decrease this air resistance again, so that forces are again in balance.
  5. This lowers terminal velocity.
40
Q

What factors determine terminal velocity reached?

A
  • Mass of skydiver and therefore weight
  • Surface area of skydiver
  • Atmospheric conditions (denser air = more particles to collide with = higher air resistance=lower terminal velocity)
41
Q

Why do all objects resist a change in velocity?

A

Because a force is needed to make an object accelerate (start moving, change direction or speeding up).

42
Q

What is inertia?

A

Resistance to a change in velocity.

43
Q

Does inertia vary?

A

Yes. Objects with more mass have more inertia.

44
Q

What is mass?

A

A property that ‘resists’ change in motion.

45
Q

What does acceleration mean?

A

A change in velocity:

  • Speed
  • Direction
46
Q

What is Newton’s second law of motion?

A

F=ma

47
Q

How do you find experimentally the link between Force, Mass and Acceleration?

A
  1. Apply different forces to a trolley by using different numbers of elastic bands.
  2. Vary the mass of the trolley by adding on multiple trolleys in a stack.
  3. Calculate acceleration from the ticker tape by measuring the length of a section of ticker tape and dividing it by time.
48
Q

What is a Newton?

A

The SI unit of force: the force needed to give an object of 1kg an acceleration of 1m/s2.

49
Q

What can a force change?

A
  • Change in size
  • Change in shape
  • Change in velocity (acceleration)
50
Q

Why is it often hard to detect a force’s effect on shape?

A

Some harder objects do not bend as much.

51
Q

Why is friction disadvantageous?

A
  • Prevents machinery from moving freely and heats up moving parts (which can seize up).
  • Air resistance is a type of friction, and reduces the speed of cars, increasing fuel costs.
52
Q

Why is friction advantageous?

A
  • Provides grip
  • Used in braking systems
  • Produces meteors and fireballs
53
Q

What is the difference between static and dynamic friction?

A
  • Dynamic friction occurs in moving objects, static friction in non-moving > moving objects.
  • Dynamic friction is smaller than static friction.
  • Dynamic friction heats materials up.
54
Q

How do you increase static friction?

A

Increase downward force on the object.

55
Q

Describe measures taken to reduce friction.

A
  • Tyres are mounted on roller bearings with grease (that being said, tyres both use and reduce friction - in braking systems with brake pads).
  • The underside of a surfboard is glossy to reduce friction with waves (that being said, the top is coated with bumps of wax to increase friction with feet for grip).
  • A curling stone slides across an area that is brushed so that an ice layer melts, reducing friction (that being said, to produce the heat in the first place, the brush has friction with the ice and this dynamic friction is what releases thermal energy that melts the ice).
  • Car bodies are designed to have a low frontal area to reduce surface area and therefore air resistance.
56
Q

How does dynamic friction heat up objects?

A
  • Kinetic energy is converted into thermal energy.
57
Q

What is gravitational field?

A

An area in which a mass experiences a force due to gravitational attraction.

58
Q

What are the three features of a gravitational force?

A
  • All masses attract each other
  • Bigger masses exert a stronger force
  • Masses that are closer together exert stronger forces on each other
59
Q

What is weight?

A

The effect of a gravitational field on a mass.

60
Q

How do you calculate weight?

A

W = mg

61
Q

What is the gravitational field strength of the earth?

A

10N/kg

62
Q

Distinguish between weight and mass.

A
  • Weight is the effect of a gravitational field on a mass. Therefore it changes. Mass always stays the same because an object’s resistance to a change in motion stays the same.
63
Q

How do you compare masses?

A
  • Use a beam balance:
    • Put the object of unknown mass in one pan
    • Add standard masses to the other pan until it balances
64
Q

Why can you use a beam balance to find mass, but you can’t use a modern balance to find mass (everywhere)?

A
  • A beam balance compares weight and not masses, but because it uses a balancing effect, if two objects have the same weight they have the same mass.
  • A modern balance only uses Earth’s value of g and ONE OBJECT and therefore can only be used to find mass on earth.
65
Q

What is momentum?

A

mass x velocity

66
Q

Is momentum a vector or scalar quantity?

A

Momentum is a vector.

67
Q

How do you find resultant force from momentum (Newton’s second law)?

A

Resultant force = change in momentum / time

68
Q

Explain the link between F=ma and F=change in momentum/time.

A

F=(mv1-mv2)/t

F=m(v1-v2)/t

(A=v1-v2/t)

F=ma

69
Q

What is an impulse?

A

(A change in momentum:) Resultant force x time

70
Q

What should you bear in mind when making calculations involving force?

A

‘Force’ in an equation is usually just an abbreviation of ‘resultant force’, but they don’t mean the same thing.

71
Q

Why does mass x velocity to left = mass x velocity to right?

A
  • Mass x velocity is the same in both cases:
    • A trolley with a larger mass gains less velocity.
    • A trolley with a smaller mass gains more velocity.
  • Newton’s third law:
    • Forces act for same amount of time.
    • Forces are equal but opposite.
    • Therefore change in momentum (=force x time) should be equal but opposite for each trolley.
    • So the total momentum (because momentum is a vector) should be 0.
72
Q

Why is momentum conserved?

A
  • Because a larger trolley has less velocity (and vice versa), momentum should be EQUAL but OPPOSITE.
  • This means that both momentums should cancel each other out.
  • This means that total momentum = 0.
  • If total momentum stays at 0, it is unchanged by the release of the spring and is conserved.
73
Q

Why is momentum not always practically conserved?

A
  • Moving objects have kinetic energy.
  • In a collision, some kinetic energy is changed into thermal energy and sound energy.
  • This means that total kinetic energy is not conserved.
  • Kinetic energy is measured as velocity, and because velocity is factored into momentum, there may be a discrepancy .
74
Q

What is a vector?

A

Has both:

  • Magnitude
  • Direction
75
Q

What happens when a force is no longer able to maintain circular motion?

A
  • Travels at a steady speed
  • In a straight line (tangential to the point at which the string breaks)
76
Q

When do you need more centripetal force?

A
  • Mass increases
  • Speed increases
  • Radius decreases
77
Q

What is centripetal force?

A

Any force needed to maintain a circular motion.

78
Q

What can function as centripetal force?

A
  • Tension in a string
  • Friction between car tyres and ground
  • Gravitational force between planet and sun
  • etc.
79
Q

Why does an object moving centripetally accelerate?

A
  • Acceleration = change in velocity / time.
  • Change in velocity = change in speed or direction of motion.
  • With circular motion, speed may not be changing but direction always is.
80
Q

In which direction does an object experiencing centripetal force accelerate?

A

Centripetal = centri + petere = centre-seeking.

Accelerates towards the centre of the circle without getting closer to it.

81
Q

Why do objects accelerate down slopes?

A

Friction is overcome because the object is more in line with its centre of mass. Eventually the only friction left opposing motion is air resistance itself.

82
Q

Even with an engine working at full power, a car loses acceleration as it goes faster. Why is this? [3]

A
  • It is approaching terminal velocity.
  • As speed increases, so does air resistance.
  • This means that the forces begin to balance, so less resultant force is produced. A resultant force is required for acceleration.
83
Q

Describe how it is possible how changing the direction of one of these forces (4N and 3N) can give a resultant of: 1. 7N 2. 1N

A
  • Draw vector diagram.
  • 7N resultant given when both in same direction (3+4=7).
  • 1N resultant given when in opposite directions (4-3=1).

Physics IGCSE (9 decks)

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