Unit 2: Chapter 3 - Work, Energy and Momentum

Question 1

Energy and Work

What is work done?

Give an example.

Answer 1

ENERGY TRANSFERRED = WORK DONE

WORK DONE = FORCE APPLIED X DISTANCE MOVED IN
DIRECTION OF FORCE

W = F x d

For example, if you rub your hands together, they become warm. Your muscles do work to overcome friction between your hands. The work you do is transferred into energy that warms your hands.

Question 2

Gravitational Potential Energy

What are the 3 factors that affect GPE?

What is power (W)?

What is Joule?

Answer 2

Gravitational energy is energy stored in an object because of the position in the Earth’s gravitational field.

In other words, there are two factors that we can change to affect the gravitational potential energy. This is the CHANGE IN HEIGHT and WEIGHT.

Because:
Change is GPE = WEIGHT X CHANGE IN HEIGHT

There is also a third factor: Gravitational Field Strength

This is because the GFS is smaller on the moon than the Earth. This is because the moon is smaller than the Earth.

So to conclude:

Change in GPE (J) = Weight (Kg) X GFS (N/Kg) X Change in height (m)

Power is the rate of transfer of energy. Joules is energy/s.

So P = E/t

Question 3

Kinetic Energy

Kinetic energy? Explain it.

Explain how elastic energy is stored.

Answer 3

KINETIC ENERGY = 1/2 (MASS) X (SPEED)2

The height dropped is directly proportional to the (speed)2. Since the height drop is a measure of the ball’s kinetic energy, we can say that the ball’s kinetic energy is directly proportional to the square of its speed.

ELASTIC POTENTIAL ENERGY IS THE ENERGY STORED IN AN ELASTIC OBJECT WHEN WORK IS DONE ON IT TO CHANGE ITS SHAPE.

Question 4

Momentum

Momentum equation?

What do these variables show about momentum?

What is the Law of Conservation of Momentum?

Answer 4

MOMENTUM = MASS X VELOCITY (Kg m/s)

p = m x v

Momentum has size and direction.

LAW OF CONSERVATION OF MOMENTUM

In a closed system, the total momentum before an event is equal to the total momentum after the event.

Total Momentum Before = Total Momentum After

Question 5

Explosions

What are explosions? Give and example.

Answer 5

(Mass(A) X Velocity(A)) = - (Mass(B) X Velocity(B))

MOMENTUM IN ACTION

What a shell is fired from an artillery gun, the gun barrel recoils backwards. The recoil of the gun is slowed down by a spring. This lessens the backwards motion of the gun.

WORKED EXAMPLE:
An artillery gun of mass 2000Kg fires a shell of mass 20kg at velocity 120m/s. Calculate recoil of the gun.

(Mass(A) X Velocity(A)) = - (Mass(B) X Velocity(B))

A = Gun
B = Shell

2000kg X V(A) = -(20Kg x 120m/s)

V = 2400kg m/s/2000kg = -1.2m/s

Question 6

Impact Forces

How do we calculate impact forces?

Answer 6

Crumple zones of car lessens impact forces to make it more safe. Hence the rule:

The longer the impact time, the more the impact force is reduced.

WORKED EXAMPLE
A bullet of mass 0.004kg moving at velocity 90m/s by a bulletproof vest in 0.0003s.

Calculate:

a) The deceleration
b) Impact force

a) Initial velocity of bullet = 90m/s
Final velocity of bullet = 0m/s

Change in velocity = 90 - 0 = -90m/s

Therefore -90m/s / 0.0003s = -300,000m/s2

b) Force = mass X acceleration

Therefore, 0.004kg x -300,000m/s2 = 1,200N

NOTE: Usually in a test, a truck will crash into a stationary car. Assuming that the truck is heavier than the car, it will stop and the car will move. It will often ask you to work out the acceleration of the car.

Question 7

Car Safety

Answer 7

There are many failsafes to keep you safe:

• SEAT BELT - Wide SURFACE AREA and is the time take taken to stop someone in a car is longer because it will reduce impact force.
• AIR BAGS - Increases duration of impact so it will decrease size of force.
• CHILD CAR SEATS - Baby seats must be facing backwards and should be in the back seat.