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A-Level Physics - OCR A > M3 Newton's Laws and Momentum > Flashcards

M3 Newton's Laws and Momentum Flashcards

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1
Q

how are acceleration and force related

A

acceleration is directly proportional to force

the bigger the force, the greater the acceleration

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2
Q

how do we represent force

A

use an arrow
the direction of the arrow shows the direction of the net force
the size of the arrow shows the magnitude of the force

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3
Q

define newton

A

the force that will give a 1kg mass an acceleration of 1 ms^-2

units = kgms^-2

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4
Q

what’s the equation for weight

A

mass X gravitational field strength

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5
Q

for objects of different masses what do they have the same?

A

acceleration

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6
Q

what is the difference between mass and weight

A

mass is the same everywhere

weight depends on the gravitational field strength

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7
Q

would a moon buggy be easier to push on the moon? would it be easier to lift?

A

pushing it would be the same because the mass and acceleration are the same and F = ma so the force would be the same regardless of where you were

lifting would be easier of the moon because g is smaller on the moon and W = mg so the weight would be smaller hence being easier to lift

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8
Q

draw a velocity-time graph for someone in free fall

A

increases, reaches terminal velocity, parachute released, decreases, terminal velocity again

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9
Q

What does the magnitude of drag depend on

A

Speed of object
Cross sectional area of object
Roughness or texture of object
Density of fluid the object is travelling through

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10
Q

What is the special theory of relativity

A

As you approach the speed of light, mass is no longer constant
So F=ma doesn’t apply

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11
Q

Define work

A

Force applied X distance moved in the direction of the force

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12
Q

Define joule

A

The energy transferred when a force of 1N is applied over a distance of 1m

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13
Q

What is the equation for work done when there’s an angle and when there isn’t an angle

A

When there’s an angle W = Fcos(x)s

No angle W = Fs

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14
Q

Is work done a scalar or vector

What are its units

A

Scalar

J or Nm

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15
Q

Derive the equation for kinetic energy

A

Ek = Fs

F = ma and s = 0.5(u+v)t and a = (v-u)/t

So Ek = m((v-u)/t) X 0.5(u+v)t
= 0.5m(v-u)(v+u)
=0.5m(v^2 - u^2)
But at the initial velocity there is no kinetic energy so it doesn't need to be there so
Ek=0.5mv^2
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16
Q

What is the conservation of energy

A

Energy cannot be created or destroyed

It can be transferred from one form to another, however the total energy will remain constant (in a closed system)

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17
Q

Derive the equation for gravitational potential energy

A

Ep = Fs
S is the height
F = mg

Ep = mgh

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18
Q

what is the elastic potential energy of something

A

work done by the deforming force

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19
Q

derive an equation for elastic potential energy

A

PE = average force X extension
Hooke’s law: F = kx
Average force = 0.5kx

PE = 0.5kx X x
= 0.5kx^2

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20
Q

in a free fall with no air resistance what happens to the kinetic energy and the potential energy at the start and end

A

START
Ek = 0
E = Ep + Ek
= mgh + 0

END
Ep = 0
Ek = 0.5mv^2
apply v^2 = u^2 + 2as
u = 0
v^2 = 2as
Ek = 0.5m(2as)
=0.5m(2ah)
a=g
Ek = 0.5m(2gh) = mgh

so Ep at start = Ek at end

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21
Q

what is the equation for
Ep?
Ek?

A 
Ep = mgh
Ek = 0.5mv^2
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22
Q

A pendulum has a mass of 5kg and is held at a height of 0.15m
how fast will it move at its lowest point

A 
GPE loss = mgh = 5 X 9.81 X 0.15 = 7.36J
At the bottom the energy is transferred to kinetic energy so
Ek = 7.36 = 0.5mv^2
0.5 X 5X v^2 = 7.36
v = 1.7ms^-1

OR

mgh = 0.5mv^2
gh = 0.5v^2
v = square root of 2gh
= square root of 2 X 9.81 X 0.15
= 1.7 ms^-1
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23
Q

if there is friction what does Ep equal

A

Ep = Ek + Wf

Wf is friction which is the force X displacement

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24
Q

define power

A

the rate at which work is done

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25
Q

what are the base units of power

A

Js^-1

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26
Q

are these scalars or vectors:

power, work done and time

A

all scalars

27
Q

what is the equation for efficiency in terms of power

A

power output / power input X 100

28
Q

what is power when the force and motion are in different directions

A

P = Fvcosx

29
Q

what are the equations for kinetic energy

A

Ek = 0.5mv^2

Ek = mas

Ek = Ep - Wf

30
Q

what do sankey diagrams represent

what are the features of them

A

represent the efficiency of an appliance
useful energy goes to the right and wasted downwards
the size of the arrow represents the fraction of energy transformed

31
Q

what is newton’s first law of motion

A

a body will stay still or move in a straight line at a constant speed unless there is a resultant force acting on it

means when there is constant velocity, there is a zero resultant force acting on you

32
Q

what is Newton’s second law of motion

A

(Resultant) force is (directly) proportional or equal to the rate of change of momentum

F = ma or F = change in p / change in t

33
Q

define stopping distance

what is the equation

A

distance travelled in the time between a driver first spotting an obstacle and vehicle coming to a complete stop

thinking distance + braking distance

34
Q

what is the equation for thinking distance in terms of SUVAT

A

s = ut

35
Q

define thinking distance

what factors affect it

A

the distance travelled during the driver’s reaction time

age, tiredness, alcohol, drugs, illness, speed

36
Q

define braking distance

what factors affect it

A

the distance travelled after the brakes are applied until the car comes to rest

road condition, tyre condition, speed, brake condition, mass of car

37
Q

what is the equation for braking distance in terms of SUVAT

A

s = -(u^2) / 2a

acceleration will be negative also as the car will be decelerating so the displacement overall will be positive

38
Q

derive F = p/t into F = ma

A 
change in p = mv - mu
F = mv - mu / t
F = m(v-u) / t
mass is constant so
F = m (v-u)/t
(v-u)/t = a
F = ma
39
Q

what is Newton’s third law of motion

A

When body A exerts a force on body B, body B exerts a force on body A that is equal, opposite in direction and of the same type

40
Q

how would you describe an object with 300N going to the right and 300N going in the opposite direction

A

balanced

net force = 0N

41
Q

how would you describe an object with 400N going to the right and 300N going to the left

A

unbalanced
object will move the right
400-300 = 100
resultant force in 100N to the right

42
Q

a car which is accelerating then changes forces so 400N goes behind and 300N in front
what happens

A

decelerates
300-400 = -100
resultant force is -100N to the left

43
Q

define resultant force

A

sum of all the forces acting on an object, taking relative directions into considerations

44
Q

what is momentum

what does a higher momentum mean

A

the stop ability of an object

the more momentum an object has, the harder it is to stop

45
Q

what does p=mv mean

what are the units for momentum

A

momentum = mass X velocity

kgms^-1

46
Q

is momentum a vector or scalar quantity

A

vector

47
Q

what is the equation for change in momentum

A

p = mv - mu

48
Q

how can you relate the equation for momentum and newton’s 2nd law

A 
F = p / t
Ft = p
Ft = mv - mu
49
Q

A ball hits a wall at 4ms^-1 and bounces back at 3.5ms^1. the mass of the ball is 2kg, what is the change in momentum?

A

p = mv - mu
p = (2 X 4) - (2 X -3.5)
= 8 - - 7
= 15 kgms^-1

50
Q

what is impulse

A

the product of the force and the time interval over which the forces act

51
Q

what is the equation for impulse

A

F X change in t

52
Q

relate impulse to Newton’s 2nd law

A 
F = ma
= m(v-u)/t
=mv-mu / t
Ft = mv - mu
Ft = p
so impulse = change in momentum
53
Q

on a force-time graph, what represents the impulse

A

area under the graph

54
Q

is impulse a vector or scalar quantity

A

vector

55
Q

how would you reduce the force in a collision

A

increase the time

seat belts
crumple zones

56
Q

what is the conservation of momentum

A

total linear momentum of two objects before they collide equals the total linear momentum after the collision

57
Q

apply the conservation of momentum to an air rifle recoiling

A

the forward momentum gained by the pellet is equal in magnitude to the backward momentum of the rifle

58
Q

what is an elastic collision

A

where momentum is conserved and kinetic energy is conserved

so no energy is dissipated into other forms

59
Q

what does it mean if a collision is inelastic

A

some of the kinetic energy is converted into other forms during the collision eg. sound or heat
(most collisions are inelastic)

60
Q

what are the steps for 2D momentum

A2

A

Step 1: Work out the initial horizontal momentum of ball A.
Step 2: Work out the horizontal and vertical components of A after the collision
Step 3: Use the horizontal component to work out the horizontal velocity of B after the collision
Step 4: Use the vertical component to work out the vertical velocity of B after the collision.
Step 5: Use Pythagoras to find the magnitude of the velocity and calculate the angle below the horizontal.

61
Q

what is the conservation of momentum in terms of 2D momentum (A2)

A

the horizontal momentum before a collision = the horizontal momentum after a collision

the verticle momentum before a collision = the verticle momentum after a collision

62
Q

describe how forces acting on a parachutist change as he falls

A

the skydiver leaves the plane and accelerates until the -air resistance and his weight equal

  • he’s traveling at terminal velocity but this is too fast to land
  • the parachute is opened which increases the air resistance making it larger than weight
  • he slows down until air resistance equals weight again, reaching terminal velocity again but at a safer level to land
63
Q

define terminal velocity

A

this occurs when the frictional forces and the driving force of an object equal

A-Level Physics - OCR A (20 decks)

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