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AQA Physics GCSE > Forces - More on speed, Newton Laws and Momentum > Flashcards

Forces - More on speed, Newton Laws and Momentum Flashcards

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1
Q

what is terminal velocity

A

when an objects velocity remains constant and they are no longer accelerating or decelerating

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2
Q

what factors can affect how many collisions there are between a person and air particles

A
  1. Surface area - the larger the surface area, the larger the area over which collisions can take place
  2. Velocity - the faster the person is moving, the more particles they can collide with
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3
Q

what happens when a person falls off an airplane (with parachute)

A
  1. The skydiver accelerates as they begin to fall due to the force of gravity acting on it
  2. As the skydiver falls, he experiences an upward force of friction with air particles called air resistance
  3. At terminal velocity, the air resistance force and weight are equal so speed is constant
  4. The parachute opens which increases the air resistance and causes diver to slow down
  5. The skydiver continues to slow down until the new air resistance force and weight are equal again
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4
Q

what does the size of air resistance depend on when a person is falling

A

how many collisions there are between the person and air particles

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5
Q

what is air resistance due to

A

the collisions between the person and all the tiny air particles that make up the air

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6
Q

why would a diver accelerate downwards immediately after leaving an aircraft

A

because there is a resultant force acting downwards

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7
Q

what happens as a skydiver gains speed

A

their weight stays the same but the air resistance increases

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8
Q

what is Newton’s first law

A
  • an object will stay stationary or continue moving at the same speed or direction, unless a resultant force is applied
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9
Q

what happens if the resultant force of a stationary object is 0

A

it remains stationary

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10
Q

what happens if the resultant force on a moving object is 0

A

it will carry on moving at the same velocity

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11
Q

what is Newton’s second law of motion

A
  • the size of the resultant force is proportional to the acceleration it causes and inversely proportional to the mass of the object
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12
Q

what are the 5 things that could happen to an object if a resultant force is applied

A
  • it may start moving
  • it might speed up
  • it might slow down
  • it might stop moving
  • it might change directions
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13
Q

why does a change in direction cause a change in acceleration

A

because acceleration is defined as the change in velocity/change in time, and since velocity is determined by speed and direction, any change in direction changes the velocity

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14
Q

why do we say the moon is accelerating around the earth

A

although speed of the moon remains constant, its direction is always changing, as the Earths mass exerts a gravitational pull on the moon, which acts perpendicular to the moons motion

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15
Q

what would happen to the acceleration if we doubled the resultant force

A

the acceleration will double

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16
Q

what is Newton’s second law as an equation

A

F=ma - mass x acceleration

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17
Q

what is inertia

A

the tendency for an object to remain unchanged (basically just newtons first law)

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18
Q

what is an objects inertial mass

A

how difficult it is to change an objects velocity

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19
Q

how to find an objects inertial mass

A

by dividing force by acceleration (newton second law)

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20
Q

what is Newton’s third law

A

that when two objects interact, the forces they exert on each other are equal and opposite

21
Q

what do we call the force that an object is exerting

A

the normal contact force

22
Q

in order for an object to be accelerated and moved, what needs to happen?

A

you need either a high force or smaller mass

23
Q

what is stopping distance

A

the minimum distance required to stop a vehicle in an emergency

24
Q

stopping distance equation

A

stopping distance = thinking distance + braking distance

25
Q

what is thinking distance

A

how far the car travels during the drivers reaction time

26
Q

what things affect thinking distance

A
  • speed of vehicle
  • reaction time
  • anything that decreases alertness (being tired, drunk, high, or just distracted)
27
Q

what is braking distance

A

the distance taken to stop under the braking force

28
Q

what affects braking distance

A
  • the speed of vehicle
  • the mass of vehicle
  • conditions and quality of the brakes
  • conditions of the road (eg whether or not the road is wet, icy) as they reduce friction betweeen the tyres and the road
  • conditions of tires, bald tires unable to grip road as well
29
Q

why might the quality of a cars brakes affect braking distance

A

if they are worn or faulty, they wont be able to apply as much pressure against the wheel, meaning they wont slow the car down as quickly

30
Q

why does the speed an d the mass of a vehicle affect braking distance

A

because they both affect kinetic energy, which will have to be reduced to 0 in order for the vehicle to stop

31
Q

what is momentum equation

A

mass x velocity

32
Q

what is the conservation of momentum

A

in a closed system, the total momentum before an event = the total momentum after an event

33
Q

when do objects have no momentum

A

when it is stationary

34
Q

change in momentum equation

A

force = change in momentum (kgm/s)/ time

35
Q

what would happen in a car crash in terms of momentum

A
  • momentum will decrease from a large amount to 0 almost instantly
  • this massive change in monentum means that the people inside the car will experience a huge force which may cause injury like head trauma
36
Q

how to reduce the chance of damage in a car crash

A

spread the change in momentum over a longer period of time, as this will reduce the force they experience

  • in order to help them acheive this, cars have to have safety features such as crumple zones, seatbelts, air bags
37
Q

what do seat belts do

A
  • stop you flying out of the car
  • slightly stretchy so you slow down a bit more slowly
38
Q

what are forces like friction called

A

resistive forces

39
Q

Forces acting on a skydiver

A
  • as soon as the skydiver jumps out, the only force acting is weight. this is due to gravity
  • because of weight, the skydiver experiences a resultant force acting downward, so they accelerate towards the ground
  • as he falls, he experiences friction with air molecules, air resistance increases
  • weight is still greater than air resistance, so the skydiver continues to accelerate towards the ground
  • as the skydivers velocity increases, air resistance also increases. at a certain point the air resistance balances the weight
  • this means there is no resultant force and velocity stays constant. the skydiver reaches terminal velocity
40
Q

what happens when a skydiver opens their parachute

A
  • surface area now increases, causing air resistance to massively increase
  • at this point, air resistance is greater than the weight, so there’s a resultant force acting upwards
  • this causes the river to decelerate (velocity decreases)
  • because velocity has decreased, air resistance also decreases
  • at some point, the air resistance will balance the weight and the resultant force will be 0, so velocity will stay constant
  • skydiver falls at a lower terminal velocity, safe for them to hit the ground
41
Q

what is reaction time

A

the time taken for the driver to spot the obstruction, make a decision, then move their foot to the brake

42
Q

the greater the speed of the vehicle…

A

the greater the stopping distance (assuming that the same braking force is applied)

43
Q

typical ranges of reaction times

A

0.2 to 0.9

44
Q

what happens in terms of energy stores, when a car brakes

A
  • during braking, the brake presses against the wheel
  • the force of friction now acts between the brake and the wheel
  • the kinetic energy of the car is now converted into thermal energy in the brakes
  • this causes the temperature of the brakes to increase
  • at the same time, the car slows down as it loses kinetic energy
45
Q

if the car is travelling at a high speed, what can we say about the size of the braking force they have to apply to stop

A

it has to be large

46
Q

how can brakes overheat

A
  • a large braking force will cause a car to decelerate very quickly
  • at the same time, a large amount of kinetic energy is transferred to the thermal energy in the brakes
  • this can cause the brakes to overheat and can also cause the driver to lose control of the vehicle
47
Q

momentum in a canon example

A
  • before a canon fires, both the cannon and the cannonball are not moving, so the total momentum before firing equals 0
  • when a cannon fires, the cannonball is moving at a very large velocity, so the cannonball has momentum in the forward direction
  • at the same time, the cannon recoils (moves backwards), meaning the canon has momentum acting in the backwards direction
  • the backwards momentum of the canon equals the forward momentum of the cannonball, meaning the total momentum after firing is 0
48
Q

what are crash tests used for

A

to design safety features such as seat belts

49
Q

examples of safety devices that slow down momentum change

A
  • airbags
  • seatbelts
  • crash mat
  • bike helmet
  • cushioned surface

AQA Physics GCSE (38 decks)

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