Chapter 9: Momentum, Work, Power Flashcards

1
Q

*Momentum equation

A

momentum = mass x velocity

units: kgms-1

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2
Q

What would happen when a ball(1) of velocity v with mass m hits a stationary ball(2) of the same mass

A

Ball(1) would stop on impact, ball (2) would move off with the same velocity v, conserving momentum

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3
Q

What would happen when a ball(1) of velocity v with mass m hits a stationary ball(2) that is much heavier

A

Ball(1) would rebound off of it, while ball(2) would move only a bit slowly as a response

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4
Q

How is momentum conserved in collisions and explosions

A

The momentum before is the same as the momentum after.

mass before * velocity before = mass after * velocity after

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5
Q

Is momentum a vector or scalar

A

Vector, the equation for momentum contains velocity, which is a vector

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6
Q

use the equation ‘mv’ to show conservation of momentum between a ball of velocity v hitting a stationary ball, both have same mass

A

mv + 0 = mv

=> 0 + mv = mv

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7
Q

State Newton’s First Law - the Law of Inertia

A

A body at rest will stay at rest (and a body in constant motion will stay in constant motion) unless a resultant external force acts on it.

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8
Q

State Newton’s Second Law.

A

The acceleration of an object is directly proportional to the resultant force applied to it (F = ma).

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9
Q

How does Newton’s second law apply to momentum?

A

The rate of change of momentum is directly proportional to the force F = Δp / Δt = ma

Δp = mΔv

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10
Q

What is impulse of a force?

A

The force multiplied by the time the force is applied for, so impulse is equal to the change in momentum Δp = FΔt

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11
Q

How do you increase impulse?

A

Increase the force, or the time it acts for (or both)

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12
Q

How are crumple zones in cars used for improved driver safety

A

They reduce the force on the driver as momentum is constant, and Δp = FΔt crumple zones increase the duration of the crash, so change in time increases, but to keep change in momentum constant, the force decreases so the force on the passenger decreases, improving their safety

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13
Q

How do seat belts airbags improve safety?

A

They reduce the force on the driver as momentum is constant, and Δp = FΔt seat belts stretch slowly and air bags deflate gradually, increasing how long it takes for a person to come to a halt if the car comes to a halt

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14
Q

How do you find impulse from a force-time graph?

A

The area under a force time graph between 2 time points on the X-axis give the impulse

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15
Q

What does area under a graph of force-time give?

A

Impulse (change in momentum)

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16
Q

What does the peak of the force-time graph give?

A

Maximum deformation of an object

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17
Q

State Newton’s third law

A

If body A exerts a force on body B then body B exerts an equal and opposite force on body A

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18
Q

Equation of force for free falling object near earth’s surface?

A

F = m g

m is mass in kg

g is acceleration usually 9.81 ms-2

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19
Q

What is the equation for work done or change in energy?

A

Work done ΔE (J) = Force x displacement in direction of force

Force in N

Displacement in m

20
Q

What is the conservation of energy?

A

Energy is never created or destroyed, only transferred between different forms and stores of energy

21
Q

How do you calculate kinetic energy gained?

A

KE =½ mv2

22
Q

How do you calculate gravitational potential energy?

A

GPE = mgh

23
Q

How do you combine kinetic energy with GPE?

A

kinetic energy gained = change in gravitational potential energy

24
Q

Define projectile and give example

A

Object projected outwards such as a cannonball

25
Q

How do you do calculations for a force acting at angles?

How would you do it for a weight, mg, acting at angle theta

A
  • You need to resolve the weight into vertical and horizontal components
  • Use the component acting in the direction of the motion i.e horizontal component mg cos theta = mg * cos theta
26
Q

Define power

A

Rate at which work is done

Rate at which energy is transferred

27
Q

Equations for power

A

P = work done / time

P = ΔE / t

P = Force * velocity

28
Q

What are the five rules for Newton’s Third Law Pairs of Forces?

A
  1. Same type of force
  2. Same magnitude
  3. Act along same line of action
  4. Act in opposite directions
  5. Act on two different bodies
29
Q

What is a perfectly elastic collision?

A

One where momentum is conserved and kinetic energy is conserved

30
Q

What is an inelastic collision ?

A

One where some of the kinetic energy is converted into other forms during the collision. But momentum is always conserved

31
Q

What are the two types of friction? What are the differences?

A

Contact friction between solid surfaces
Fluid friction, drag or fluid resistance or air resistance

32
Q

Three things you need to know about or frictional forces:
Their direction?
Their affect on the speed of the object?
Which types of energy do they convert?

A

They always act in the opposite direction to the motion of the object
They can never speed things up or start something moving
They convert kinetic energy into heat

33
Q

You will reach your terminal velocity at some point, if you have…

A

A driving force that stays the same all the time e.g. weight
A fictional or drag force that increases with speed

34
Q

When does something reach terminal velocity?

A

When the frictional force equals the driving force e.g. when weight = drag due to air resistance

35
Q

Describe the graph of velocity against time for an object reaching terminal velocity

A

Curve, where the gradient decreases, eventually turning into a horizontal line. Graph starts at the origin and increases

36
Q

Describe the graph of acceleration against time for an object reaching terminal velocity

A

Acceleration starts off high then decreases. The rate of decrease of acceleration starts slow then speeds up then slows again. The acceleration finally reaches zero. The graph looks like an unexaggerated S shape

37
Q

Describe the graph of the velocity against time for the parachutist

A

First half of the graph looks like the normal velocity against time for an object reaching terminal velocity. Then the graph suddenly drops to a lower velocity and flattens out, where it reaches a new terminal velocity

38
Q

When is work done?

A

Whenever energy is transferred

39
Q

What is power?

A

The rate of doing work – it’s the amount of energy transferred from one form to another per second

40
Q

How do you answer a question where somethings projected at an angle

A
  1. resolve initial velocity into horizontal and vertical components
  2. use vertical component to find how high it goes and and to work out how long it’s in the air for
  3. use horizontal component to find distance travelled horizontally while in air
41
Q

How do you use vertical component to find highest point of projectile’s motion and how long it’s in the air for?

A

If the graph is symmetrical: at the halfway point, vertical velocity is 0 for an instantaneous time, so the value of vv must be multiplied by 2

To get the second time vv = 0 which is when it will hit the ground
so vv = 0, uv = resolve into vertical and horizontal to find out
a = g = -9.81 (upwards as neg)
t = ?
use v = u + at to get time
This is time taken to get to highest point; multiply by 2 to get the time to reach ground again

42
Q

How do you use horizontal component to find distance travelled after you have found time before it hits the ground again using vertical component

A

There is no acceleration horizontally so
a = g = 0
this means that uH = vH (speed)
time for it to hit ground found from vertical component = t
use this to find s, distance travelled
you can use speed = distance / time
distance = speed * time

43
Q

how to answer question where projectile is thrown horizontally, you want how long it takes to hit ground and how far it travelled

A

break it into vertical and horizontal

vertical: thrown horizontally with speed Vh so
for vertical u = o
a = g = (-) 9.81
if done above ground level s = xm
so use s = ut + 1/2 at^2
usually just goes to s = 1/2 at^2
use this to find time

horizontal: Vh isnt affected by gravity so Uv = Vv and a = N/A
use time t from vertical component
usually can use speed = distance / time
distance = speed * time
s = Vv (Uv) * t

44
Q

What are the forces acting on a person falling at constant speed

A

if falling at constant speed, forces are equal, drag = mg

45
Q

Key points to remember for work done

A

Work done is not necessarily the total energy. If you move an object higher up you have increased its potential energy, but it already had some potential energy to start with

Force, F, will usually be fixed.
Equation assumes force is in same direction as direction of movement

46
Q

Equation for work done at an angle

A

Work done = force * distance * cos theta

47
Q

Equation linking power, force and velocity
Equation linking power, force at an angle and velocity

A
Power = Force \* velocity 
P = Fv 
Power = Force cos theta \* velocity 
P = F cos theta \* velocity