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Physics IGCSE > Forces and Pressure > Flashcards

Forces and Pressure Flashcards

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1
Q

What is a moment?

A

The turning effect of a force.

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2
Q

How do you increase the moment from a spanner?

A
  • Use a longer spanner
  • Apply more force
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3
Q

What is a moment?

A

Force x perpendicular distance from point

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4
Q

What are the two conditions of equilibrium?

A
  1. Sum of clockwise moments about any point = sum of anticlockwise moments about same point.
  2. Sum of forces in one direction = sum of forces in other direction (no resultant force).
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5
Q

What is the centre of mass?

A

Where all the gravitational forces of an object have a resultant.

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6
Q

How do you find centre of mass?

A
  1. Hang the object from a pin (make sure it can swing freely).
  2. When released, the centre of mass will be vertically under the pin.
  3. Mark a plumb line.
  4. Suspend the card at different points.
  5. Find where the plumb lines intersect.
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7
Q

What is a stable object?

A

An object that, when pushed a small amount and released, resumes its original position.

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8
Q

When does an object topple?

A
  • When its centre of mass passes over the edge of the base.
  • This gives the forces a turning effect.
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9
Q

How do you make an object more stable?

A
  • Wider base
  • Lower centre of mass
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10
Q

What is an unstable equilibrium?

A

Where an object is balanced but only briefly, because centre of mass immediately passes over a small base.

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11
Q

What is a neutral equilibrium?

A

An object that, when moved, stays in its new position (like a ball). Has a centre of mass that always lies directly above ‘base’.

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12
Q

Give everyday examples of a moment.

A
  1. Spanner
  2. Wheels
  3. Door handles
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13
Q

Perform and describe an experiment (involving vertical forces) to show that there is no net moment on a body in equilibrium.

A
  1. Measure the weight of a rule.
  2. Suspend a metre rule from a spring balance.
  3. Hand weights at different positions of the rule.
  4. Adjust so that the rule is in equilibrium.
  5. Test the principle of moments at multiple points using calculations (moment = f x d)
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14
Q

What is an elastic material?

A

A material that, when bent, resumes its original shape.

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15
Q

What happens when an elastic material is stretched too far?

A
  • Break
  • Permanently deformed
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16
Q

What is a plastic material?

A

Materials that, when bent, keep their new shape.

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17
Q

What is load of a spring?

A

The force applied to a spring.

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18
Q

What is extension of a spring?

A

Difference between stretched and unstretched lengths.

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19
Q

What are the characteristics of a proportional graph?

A
  • Straight line through the origin.
  • If one variable doubles, other variable doubles.
  • One / other variable gives the same value.
  • Every increase in variable by 1N gives the same increase in the other variable.
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20
Q

What is not true about proportional graphs?

A

One variable multiplied by the other variable does NOT always give the same value.

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21
Q

What is the point at which the graph curves upwards?

A
  • The limit of proportionality, or elastic limit.
  • The point at which the object stops behaving elastically and stops obeying Hooke’s Law.
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22
Q

What is Hooke’s Law?

A

A material behaves Hooke’s Law if, beneath its elastic limit, extension is proportional to load.

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23
Q

Give examples of objects that do and do not obey Hooke’s Law.

A
  1. Obey:
    • Steel wires
    • Steel springs
    • Glass
    • Wood
  2. Disobey:
    • Rubber
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24
Q

What is the formula for Hooke’s Law?

A

Load = Spring Constant x Extension

F=kx

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25
Q

What is a Pascal?

A

1N/m2, unit of pressure

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26
Q

What is pressure?

A

Pressure = Force / Area

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27
Q

Give examples of objects that increase pressure by decreasing area.

A
  • Studs on a football boot so that the boot sinks into the ground and has grip
  • Knife to slice through material
  • Drawing pin to push into a surface
28
Q

Give examples of objects that decrease pressure by increasing area.

A
  • Skis so that the person does not sink into the snow
  • Wall foundations have a large horizontal area to stop sinking into ground
  • Load-spreading washer ensures nut is not pulled into wood when tightened up
29
Q

What is the typical pressure of a flat boot?

A

20kPa

30
Q

What is a typical pressure of a high heel?

A

500kPa

31
Q

What is the typical pressure of a drawing pin?

A

5 million kPa

32
Q

What are the characteristics of pressure in liquids?

A
  1. Acts in all directions
  2. Increases with depth (greater weight = force of liquid on top)
  3. Depends on the density of the liquid
  4. Does not depend on the shape of the container
33
Q

Give two practical applications of pressure.

A
  1. Dams have thicker bases than tops, to withstand crushing effect of deep water.
  2. Deep sea vessels have thick sides all around because pressure acts in all directions.
34
Q

What is the formula for pressure in a liquid?

A

Pressure = pgh

35
Q

Why are liquids used to transmit pressure in hydraulic systems?

A
  1. Virtually incompressible
  2. If one part is put under pressure, pressure is transmitted to all parts of the liquid
36
Q

How does a hydraulic brake system work?

A
  1. Brake pedal is pressed
  2. Piston forces brake fluid from one cylinder along connecting pipe to another cylinder
  3. Fluid pushes on another piston
  4. Piston presses brake pad onto rotating wheel of car
  5. Friction slows the wheel
  6. Car slows down
37
Q

Why is a hydraulic jack a force multiplier?

A
  • Get a larger force on the putput piston than input piston by a factor.
  • Because output piston has a smaller area by the same factor.
38
Q

What is a practical problem associated with hydraulic systems?

A

Friction reduces output force.

39
Q

Give four practical uses of pressure in liquids.

A
  1. Hydraulic jack
  2. Hydraulic press (gap closes as piston moves upwards)
  3. Car braking system
  4. Hydraulic motors, e.g.in diggers to drive caterpillar tracks (high pressure oil comes from a pump driven by a diesel engine)
40
Q
A
41
Q

What are the characteristics of pressure in a gas?

A
  1. Acts in all directions
  2. Pressure is increases with depth of air (lower pressure as you rise up through it)
  3. Can be compressed (which also makes pressure lower as you rise up through it)
42
Q

What is atmospheric pressure?

A

100kPa

43
Q

Why aren’t you crushed by atmospheric pressure?

A

It is matched by the pressure in your lungs and blood system.

44
Q

Why is a can crushed when there is no air (a vacuum) inside it?

A
  1. There is nothing to resist atmospeheric pressure.
  2. Atmospheric pressure acts in all directions, so can is crushed in all directions.
45
Q

How does drinking through a straw work?

A
  1. Expand lungs to increase volume and decrease pressure in lungs.
  2. Subsequently pressure decreases in the straw.
  3. Atmospheric pressure pushes liquid up the straw.
46
Q

How does a vacuum cleaner work?

A
  1. Fan lowers air pressure just beyond bag.
  2. Atmosphere rushes in carrying dust and dirt with it.
  3. Bag acts as filter and stops dust and dirt but not air.
47
Q

What is a barometer?

A

An instrument that measures atmospheric pressure.

48
Q

How does a barometer work?

A
  1. Barometer contains vacuum at top.
  2. Due to vacuum, atmospheric pressure can push mercury up the tube.
  3. The higher the atmospheric pressure, the higher the mercury is pushed up the tube.
49
Q

What height of mercury is supported at sea level by atmopheric pressure?

A

760mm of mercury

50
Q

What factors change atmospheric pressure?

A
  1. Altitude
  2. Rain clouds form large areas of lower pressure due to rising air
51
Q

What is an altimeter?

A

An instrument fitted in aircraft to measure altitude.

52
Q

What is a millibar?

A

Unit of pressure used in weather forecasting.

1mb = 100Pa

53
Q

What does a manometer do?

A

Measure pressure difference in two different liquids or gases.

54
Q

How do you use a manometer?

A
  1. Connect one end of manometer to gas supply and the other to atmosphere.
  2. Find height difference in mm of mercury.
  3. Add 760mm of mercury (atmospheric pressure) to this height difference.
  4. Convert this total to Pascals by using pressure = pgh

If the manometer is closed (i.e. in a vacuum), ignore step 3.

55
Q

Give two examples of when pressure, volume and temperature all change.

A
  1. Hot air balloon rising through atmosphere
  2. Gases in the cylinders of a car engine
56
Q

State Boyle’s Law.

A

For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.

P1 x V1 = P2 x V2 (at constant temperature, PxV is a constant)

57
Q

Describe an experiment to show the relationship between pressure and volume.

A
  1. Trap a fixed mass of dry air in a glass tube with volume scale.
  2. Connect to reservoir with oil.
  3. Reduce volume in stages by pumping air into reservoir.
  4. OIl is pushed further up the tube.
  5. Use a gauge to measure pressure at each stage volume is decreased (measures pressure in oil reservoir but same as in tube because oil transmits pressure).
  6. After each reading, wait for air to return to original temperature.
58
Q

What are the features of an inversely proportional graph?

A
  1. If pressure halves, volume doubles.
  2. Pressure x volume gives same result.
  3. Pressure v 1/volume gives a straight line through the origin.
59
Q

What does a graph of pressure vs 1/volume look like?

A

A proportional graph.

60
Q

Explain Boyle’s Law.

A
  • If volume halves, there are twice as many molecules per cubic metre.
  • This means that there are twice as many collisions with the container / second.
  • Pressure doubles.
61
Q

What must you keep constant when investigating Boyle’s Law?

A
  1. Ideal gas (to limit attractions between molecules):
    • low density
    • temperature well above liquefying point
    • dry gas not full of water vapour
  2. Fixed mass
  3. Constant temperature
62
Q

Does pressure at the bottom of a container of water = atmospheric pressure?

A

No, because you have the added pressure of water as well and pressure increases with depth.

63
Q

If at atmospheric pressure a column of water 10m high is supported, what is the height of water supported at a depth of 20m?

A

30m of water

64
Q

What should you remember about drawing a curve after limit of proportion is reached?

A
  1. Either upwards or downwards
  2. NO NEGATIVE GRADIENT
65
Q

Unit of k?

A

N/m

Physics IGCSE (9 decks)

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