9: Momentum, Force and Energy

Question 1

What is Newton’s first law?

Answer 1

the law of inertia. This states that a body at rest will continue at rest, or a body in motion will continue in constant motion, unless acted upon by an external force.

Question 2

What is Newton’s second law?

Answer 2

the acceleration of an object is directly proportional to the force applied (F = ma)
(The rate of change of momentum of an object is directly proportional to the force which acts on the object)

Question 3

Do all objects accelerate towards the ground at the same rate?

Answer 3

Yes in theory however air resistance (drag) means they don’t

Question 4

What is Newton’s third law?

Answer 4

every action has a complete and opposite reaction

Question 5

Where does Newton’s third law come from?

Answer 5

It is a consequence of the conservation of momentum. The resultant force acting means a change in mass or acceleration – which means a change in momentum.
Momentum is always conserved when no external forces act, so whenever one object exerts a force on another, the second object must exert equal size force back on the first object so that the overall change in momentum is zero

Question 6

What is a perfectly elastic collision?

Answer 6

One where momentum is conserved and kinetic energy is conserved

Question 7

What is an inelastic collision ?

Answer 7

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

Question 8

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

Answer 8

Contact friction between solid surfaces

Fluid friction, drag or fluid resistance or air resistance

Question 9

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 9

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 10

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

Answer 10

A driving force that stays the same all the time

A fictional or drag force that increases with speed

Question 11

When does something reach terminal velocity?

Answer 11

When the frictional force equals the driving force

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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 14

Describe the graph of the velocity against time for the parachutist

Answer 14

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 15

When is work done?

Answer 15

Whenever energy is transferred

Question 16

What is power?

Answer 16

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

Question 17

State the principle of conservation of energy

Answer 17

Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change

Question 18

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

Answer 18

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 its in the air for
3. use horizontal component to find distance travelled horizontally while in air

Question 19

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

Answer 19

if the graph is symmetrical, at the halfway point vertical velocity is 0 for an instantaneous time, so the value of Vv you get the first time 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

Question 20

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 20

there is no acceleration horizontally so 
a = g = 0
this means that u = v (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
s = u(v)  *  t

Question 21

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

Answer 21

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 22

how to find impulse from graph of force against time

Answer 22

the area of the graph is the impulse

Question 23

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

Answer 23

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

Question 24

work done equation

Answer 24

work done (J) = force (N) * distance moved (m)

Question 25

key points to remember for work done

Answer 25

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

gives you definition of joule
work done when 1N of force moves an object 1 metre

Question 26

equation for work done at an angle

Answer 26

work done = force * distance * cos theta

Question 27

definition of power

equation

Answer 27

power is the rate of energy transfer
P = change in energy (work done) / time
W = J s^-1

Question 28

equation linking power, force and velocity

equation linking power, force at an angle and velocity

Answer 28

power = force * velocity
p = fv

power = force cos theta * velocity
p = f cos theta * velocity

Question 29

how to link work done and gpe

Answer 29

work done = f * s
f = mg s = h
work done = mgh = gpe