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Physics 2 > P5 Forces > Flashcards

P5 Forces Flashcards

1
Q

What happens to braking distance if speed doubles when braking force is constant?

A

If speed doubles, kinetic energy increases by a factor of 4. Therefore, work done increases and increases by a factor of 4. As Wd = F x s and braking force is constant the braking distance must also increase by a factor of 4 (they are proportional)

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2
Q

Explain in terms of moments why the maximum load must not be exceeded

A

Danger that object will topple over. This is because line of action of the weight and centre of gravity will lie outside the base. Therefore there will be a resultant moment.

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3
Q

Explain the word stable.

A

Will not (easily) topple over. Centre of mass will remain within the base area if object is pushed (a measure of how likely object to topple when pushed/moved)

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4
Q

What must an object have in order to be stable?

A
  • a low centre of gravity
  • a wide base
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5
Q

Explain why an object may topple over if it is tilted too far back.

A

Centre of mass has changed so the line of action of the weight is outside the base there is a resultant moment.

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6
Q

Explain what will happen on a balanced see saw when one person moves closer to the pivot on the right side.

A

Persons (clockwise) moment will decrease (as distance decreases) so moments are no longer balanced. Therefore, the person will move downwards.

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7
Q

Explain why increasing the distance makes it easier to turn a screw.

A

Force is applied further from the pivot so there is a greater moment

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8
Q

Explain why a liquid is virtually incompressible.

A

Particles are in an irregular arrangement but are close together so there is no room for more movement.

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9
Q

Explain another advantage of packing a case with the heavier items closer to the wheels.

A

Centre of mass will be closer to the wheels so more stable and less likely to topple over. Less effort is needed to hold the case (her arm is further from the load) acting as a force multiplier

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10
Q

Suggest and explain the purpose of the large concrete blocks on the other side of a crane.

A

To keep the crane stable so it doesn’t topple over as keeps centre of mass acting within the base. Total anticlockwise moment=total clockwise moment.

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11
Q

What is centre of mass/gravtiy?

A

Point where mass/gravity acts through an object.

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12
Q

RPA 6: how could you improve?

A

Attach a horizontal pointer to the spring to go across the ruler from the bottom of the spring.

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13
Q

RPA 6: why is it important for the ruler to be straight when taking measurement of the extension?

A

Avoid parallax error for measuring the extension. If ruler is at an angle the extension would appear shorter.

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14
Q

RPA 6: what can you say about the deformation of a spring past a certain weight?

A

Spring becomes inelastically deformed as it exceeds its limit of proportionality (so it doesn’t return to its original shape and length once weight is removed)

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15
Q

RPA 6: how can you tell a spring has exceeded its limit of proportionality on a graph?

A

Past a point it is no longer directly proportional.

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16
Q

RPA 6: two observations when a weight is applied to a spring

A
  1. length increased
  2. coils more spaced out
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17
Q

RPA 6: how can you get more results?

A

Use more masses. Smaller masses to not exceed limit of proportionality.

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18
Q

Using the idea of forces, explain why the parachutist reaches a terminal velocity and why opening the parachute reduces the terminal velocity

A

Initially once leaving plane the only force is gravity/weight. As parachutist falls air resistance acts upwards but gravity/weight is greater than air resistance. There is a resultant force downwards so the parachutist accelerates and as velocity increases so does air resistance. Terminal (maximum) velocity reached once forces are balanced.
Opening the parachute increases surface area and so air resistance. Air resistance is now greater than gravity so there is a resultant force upwards, meaning parachutist decelerates and so air resistance decreases so forces balance but at a lower velocity.

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19
Q

How can you tell on a free body diagram that there is a constant velocity?

A

Equal length arrows show velocity is not changing and so the resultant force is 0.

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20
Q

Explain why atmospheric pressure decreases with increasing altitude

A

Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases as the atmosphere gets more dense with increasing altitude. Less air molecules means less collisions so less force meaning less pressure.

21
Q

Explain why an aircraft window has been designed to have this shape

A

Force from air pressure acting from inside to outside bigger than force acting inwards so keeps the window in position

22
Q

How can a vehicle accelerate at a constant speed?

A

Changing direction changes velocity as velocity is a vector and so has direction. Acceleration is the change in velocity.

23
Q

As the aircraft moves along the runway to take off, its acceleration decreases even though the force from the engines is constant.
Explain why

A

As speed increases air resistance increases so resultant force is reduced.

24
Q

Explain elastic deformation.

A

The object will return to its original shape/length once the force is removed.

25
Q

Explain why cars have a top speed.

A

Air resistance increases with speed until force is equal to the maximum forwards force then acceleration is 0 - more safe.

26
Q

What factor affects both thinking and braking distance?

A

Speed

27
Q

Explain why temperature of the brakes increases when the brake pedal is pressed.

A

Work is done on the brakes which causes friction between the brakes and the wheels. This reduces kinetic energy in the wheels which is transferred to increase the thermal energy in the brakes

28
Q

Why does pressure increase in a liquid with the height of the column of liquid above that point.

A

The fluid particles at the bottom of a container have the added weight of the particles on top of them, which creates more pressure on the particles on the bottom as there is a greater force.

29
Q

Why must more than one force be applied to change the shape of a stationary object by stretching, bending or compressing?

A

a single force would simply cause the object to move in the direction in which the force were to be applied

30
Q

Describe the difference between elastic deformation and inelastic deformation caused by stretching forces

A

Elastic deformation: object returns to original shape and length when the object is removed
Inelastic deformation: object doesn’t return to original shape and length when the object is removed and is permanently stretched

31
Q

Why, in a liquid, does pressure at a point increase with the height of the column of liquid above that point and with the density of the liquid?

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.

32
Q

What is upthrust and when does it happen with objects submerged?

A

A resultant force upwards. When a partially (or totally) submerged object experiences a greater pressure on the bottom surface upwards than on the top surface downwards, creating a resultant force

33
Q

What factors affect floating/sinking?

A
  • If weight of liquid displacement = weight of object, the upthrust equals the weight of the object so the object now floats
  • If an object is more dense than the liquid, it will not be able to displace it to equal its weight so the weight of the object is now greater than upthrust and sinks
34
Q

What is displacement?

A

Includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. A vector quantity.

35
Q

RPA: what can be used to prevent the string from rubbing?

A

Use a pulley

36
Q

RPA: explain steps, including a risk assessment for one hazard

A
  • set up clamp and stand and attach a horizontal ruler to the clamp
  • hang a spring on the clamp and measure and record its original length
  • add (1N) weight on the spring and measure the springs new length then use it to measure and record the extension
  • repeat with weights 1-5N and record extension for each weight
  • plot a graph with the results
  • hazard: weights fall on feet if stand falls over
  • risk assessment: use a counter weight on the stand
37
Q

Describe how you know variables are directly proportional

A

There is a straight line going through the origin

38
Q

Explain why a drivers reaction time affects the thinking distance of the car

A

Greater reaction time means they will travel further before reacting to press the brakes, this means the thinking time increases.

39
Q

Explain how protective pads help reduce injury of ice hockey players

A

Increases time before collision/impact so a decrease in the rate of change of momentum so a decreased force on the players meaning it is more safe

40
Q

Explain why the forces on a brick at the bottom of the pool cause the brick to be stationary

A

Upthrust acts (upwards on the brick)
Normal contact force acts upwards (on the brick)
Weight is equal to upthrust plus normal contact force (so the resultant force is 0N)

41
Q

Explain why a car has a maximum speed

A
  • there is a maximum forward force (provided by the motor)
  • as the speed of the car increases air resistance increases until air resistance is equal in size to forward force so resultant force is 0
  • the car can no longer accelerate
42
Q

What is the result of a driven gear made smaller than the driving gear

A

it will rotate more quickly but with a smaller moment, both gears have the same force acting on them

43
Q

How are levers force multipliers?

A

They allow a larger force to act upon the load than is supplied by the effort, so it is easier to move large or heavy objects

44
Q

Explain why acceleration can decrease whilst force supplied by object is constant

A

Opposite force of air resistance as velocity increases, this reduces the resultant force

45
Q

How will streamlining a go kart impact its speed?

A

More streamlined so air resistance is smaller meaning speed increases more

46
Q

Explain why a man hasn’t toppled over on a ladder

A

The centre of mass of the man and the ladder and so the line of action of weight are within the base area so there is no resultant moment

47
Q

What are the range of typical reaction time values?

A

0.2-0.9 seconds

48
Q

What are factors for why reaction times vary? (4)

A
  • alcohol
  • tiredness
  • drugs
  • distractions

Physics 2 (4 decks)

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