Chapter 9: Momentum, Work, Power

Question 1

*Momentum equation

Answer 1

momentum = mass x velocity

units: kgms^-1

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

How is momentum conserved in collisions and explosions

Answer 4

The momentum before is the same as the momentum after.

mass before * velocity before = mass after * velocity after

Question 5

Is momentum a vector or scalar

Answer 5

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

Question 6

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

Answer 6

mv + 0 = mv

=> 0 + mv = mv

Question 7

State Newton’s First Law - the Law of Inertia

Answer 7

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.

Question 8

State Newton’s Second Law.

Answer 8

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

Question 9

How does Newton’s second law apply to momentum?

Answer 9

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

Δp = mΔv

Question 10

What is impulse of a force?

Answer 10

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

Question 11

How do you increase impulse?

Answer 11

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

Question 12

How are crumple zones in cars used for improved driver safety

Answer 12

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

Question 13

How do seat belts airbags improve safety?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

Impulse (change in momentum)

Question 16

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

Answer 16

Maximum deformation of an object

Question 17

State Newton’s third law

Answer 17

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

Question 18

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

Answer 18

F = m g

m is mass in kg

g is acceleration usually 9.81 ms^-2

Question 19

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

Answer 19

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

Force in N

Displacement in m

Question 20

What is the conservation of energy?

Answer 20

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

Question 21

How do you calculate kinetic energy gained?

Answer 21

KE =½ mv²

Question 22

How do you calculate gravitational potential energy?

Answer 22

GPE = mgh

Question 23

How do you combine kinetic energy with GPE?

Answer 23

kinetic energy gained = change in gravitational potential energy

Question 24

Define projectile and give example

Answer 24

Object projected outwards such as a cannonball

Question 25

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

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

Answer 25

• 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

Question 26

Define power

Answer 26

Rate at which work is done

Rate at which energy is transferred

Question 27

Equations for power

Answer 27

P = work done / time

P = ΔE / t

P = Force * velocity

Question 28

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

Answer 28

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

Question 29

What is a perfectly elastic collision?

Answer 29

One where momentum is conserved and kinetic energy is conserved

Question 30

What is an inelastic collision ?

Answer 30

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

Question 31

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

Answer 31

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

Question 32

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?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

When does something reach terminal velocity?

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

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

Question 37

Describe the graph of the velocity against time for the parachutist

Answer 37

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

Question 38

When is work done?

Answer 38

Whenever energy is transferred

Question 39

What is power?

Answer 39

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

Question 40

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

Answer 40

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

Question 41

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

Answer 41

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

To get the second time v_v = 0 which is when it will hit the ground
so v_v = 0, u_v = 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

Question 42

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

Answer 42

There is no acceleration horizontally so
a = g = 0
this means that u_H = v_H (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

Question 43

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

Answer 43

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

Question 44

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

Answer 44

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

Question 45

Key points to remember for work done

Answer 45

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

Question 46

Equation for work done at an angle

Answer 46

Work done = force * distance * cos theta

Question 47

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

Answer 47

Power = Force \* velocity 
P = Fv 

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