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Physics Paper 2 > Forces > Flashcards

Forces Flashcards

1
Q

Why do crisp packets swell up in aeroplanes?

A
  • atmosphere is most dense at the surface of the earth
  • atmosphere gets less dense as we increase in altitude
  • as a surface increases height against ground level, the number of air molecules above the surface decreases
  • atmospheric pressure decreases with height

At ground level, the air pressure inside the packet is the same as the air pressure outside the bag

  • when aeroplanes are flying, the air pressure in cabin is kept at a slightly lower level than the air pressure on the ground
  • so the air pressure inside the the packet is greater than in the cabin
  • which causes the bag to inflte
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2
Q

Describe the pressure of water in a container.

A

-water at the bottom of the container is at a higher pressure than the water at the top

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3
Q

Why does the pressure of liquids increase with depth?

A

-because as depth increases, there is a greater weight of liquid acting downwards

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4
Q

Why does the pressure of a liquid increase with the density of a liquid?

A

-liquids with a greater density have a greater weight acting downwards

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5
Q

Describe an object immersed in a liquid.

A
  • the bottom of the object is at a greater depth than the top
  • the bottom experiences a larger pressure than the top
  • there is a larger force acting on the bottom of the object than at the top
  • so there is a resultant force acting upwards (upthrust)
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6
Q

For an object to sink:

A

Upthrust is less than the object’s weight

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7
Q

The size of of upthrust acting on an object is the same as…

A

the weight of water displaced by the object.

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8
Q

For an object to float:

A
  • upthrust must be equal to the object’s weight
  • so if an object can displace its own weight of water then the upthrust will equal the object’s weight
  • object can float
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9
Q

If the object is less dense than the water:

A
  • the object only has to displace a small volume of water before the weight of the water equals the weight of the object
  • now the upthrust equals to the weight of the object and then it floats
  • floats high in the water
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10
Q

If object has the same density as water:

A
  • the object would have to displace its own volume of water in order for the weight of water displaced to equal the weight of the object
  • upthrust then equals the weight of the object and object floats
  • the surface of the object is at the surface of the water
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11
Q

Objects more dense that water:

A
  • the object cannot displace a volume of water equal to its own weight
  • therefore the weight of the object is greater than upthrust so the object sinks
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12
Q

Define displacement.

A

Tells us the distance an object moves in a straight line from the start point to the finish point
-must include direction of the line

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13
Q

For objects moving in a circle at a constant speed?

A
  • direction is constantly changing
  • which means velocity is constantly changing too
  • so it’s technically accelerating
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14
Q

State Newton’s First Law

A
  • if the resultant force acting on a stationary object is zero, the then the object will remain stationary.
  • if the resultant force acting on a moving object is zero, then the object will keep moving in the same direction at the same speed (velocity)
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15
Q

State Newton’s Second Law.

A

-the acceleration of an object is proportional to the resultant force acting on an object and inversely proportional to the mass of the object

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16
Q

What is inertia?

A

-an object will stay stationary, or continue moving at the same speed and direction, unless a resultant force is applied.

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17
Q

What is inertial mass?

A
  • a measure of how difficult it is to change the velocity of an object
  • it is the ratio of the force needed to accelerate an object over the acceleration provided
18
Q

State Newton’s Third Law

A

-whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction

19
Q

Stopping distance

A

Total distance travelled from when the driver first spots the obstruction to when the car stops

20
Q

Thinking distance

A

-the distance travelled by the car during the driver’s reaction time

21
Q

Braking distance

A

-the distance the car travels from when the driver applies the brakes to when the car stops.

22
Q

How to measure reaction time?

A
  • one person holds the ruler and a volunteer places their fingers on either side
  • ruler is dropped and volunteer has to catch it
  • the further the ruler falls before it is caught, the longer the reaction time
23
Q

What is acceleration?

A

The change in velocity over a given time.

24
Q

What does the gradient of a velocity-time graph tell us?

A

The acceleration of an object

25
Q

What does un upward/downward sloping sloping line show us?

A

Accelerating/decelerating

26
Q

What does the total area under the graph tell us?

A

The distance travelled in a specific direction (displacement).

27
Q

What does it mean the lines are curved?

A

The acceleration/deceleration is not constant.

So to calculate the total area, count squares.

28
Q

What equation do we use when an object is accelerating at a constant rate?

A

v^2 - u^2 = 2as

s being distance

29
Q

What do objects initially accelerate at when falling toward the surface of the earth? Why?

A

9.8m/s^2

This is due to the force of gravity acting on the object.

30
Q

Describe how terminal velocity works on a skydiver.

A

When they jump out the plane, they initially accelerate towards the surface of the earth at 9.8m/s^2.
This is due to the force of gravity acting on the object.
As they fall, they experience an upward force of friction with the air particles. (Air resistance)
After some time the force of air resistance balances the force due to gravity.
The object stops accelerating and moves at a constant velocity.
This is terminal velocity.

31
Q

If an object experiences a greater force of friction, how is terminal velocity affected?

A

It will have a lower terminal velocity.

32
Q

Forces acting on a skydiver.

A

When the skydiver jumps out, the only force acting is weight.
This is due to gravity, and the force does not change during the journey.
They experience a resultant force acting downwards.
So they accelerate towards the ground.
As they fall, they experience friction with air molecules.
This is air resistance which acts upwards.
However the weight is still greater than air resistance so the skydiver continues to accelerate to the ground.
As their velocity increases, the air resistance also increases.
Eventually the air resistance balances the weight.
Now there is no resultant force so velocity stays constant- terminal velocity.
This velocity is extremely great and the skydiver would die if they hit the ground.
So they open their parachute.
The surface area increases which causes air resistance to massively increase.
The air resistance is now greater than the weight, so there is a resultant force acting upwards.
So they decelerate.
Because velocity has decreased, the air resistance also decreases.
So air resistance balances the weight and resultant force will be zero.
So velocity stays constant.
The sky diver is now falling at a lower terminal velocity.
Safe for them to hit the ground.

33
Q

What happens when a car brakes?

A
  • the brake presses against the wheel
  • so now a force of friction acts between the brake and the wheel
  • the kinetic energy store of the car is transferred to the thermal energy store of the brakes
  • the temperature of the brakes increases
  • the car slows down as it loses kinetic energy
34
Q

Describe the forces on elastic materials?

A
  • stretching/squeezing forces are equal in magnitude but opposite in direction so they are balanced
  • causes objects to stretch
  • when forces are released, the elastic object returns to its original length
  • elastic deformation
35
Q

In order to change an object’s shape or length..

A
  • must apply more than one force
  • if we only applied one force to a stationary object, the forces would be unbalanced
  • so it would simply move
36
Q

What happens to inelastic materials when stretched?

A
  • they do not return to their original length when forces are removed
  • inelastic deformation
37
Q

What happens when we stretch or compress an elastic object?

A
  • we are using a force to do work
  • elastic potential energy is stored in the object
  • work done is equal to the EPG
  • only true if the object is not inelastically deformed
38
Q

Equation for force needed to stretch an elastic object:

A

force = spring constant (N/m) x extension/compression (m)

39
Q

RP6: Stretching a spring

A

We start with a clamp stand, two bosses and two clamps.

  1. Place a heavy weight on the clamp stand to stop it from falling over
  2. Attach a metre rule and a spring
  3. The top of the spring should be at the zero point on the metre rule
  4. Metre rule should be vertical for accurate readings
  5. The bottom of the spring has a wooden splint attached as a pointer.
  6. Point must be horizontal or the readings will be inaccurate
  7. Read the position of this pointer on the metre rule-this is the unstretched length of the spring.
  8. Hang a 1N weight on the spring
  9. Read the new position of this pointer on the metre rule
  10. Continue adding 1N weights to the spring and reading the position of the pointer
  11. Calculate the extension produced by adding each weight.
40
Q

Describe centre of mass on a block

A
  • the weight of the block acts directly from the centre of mass
  • if we tilt the block, the line of action of the weight lies outside the base of the object
  • there is a resultant moment acting on the block
  • topples

Physics Paper 2 (5 decks)

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