Force(N)

Compression (cm)

By using Hooke’s Law and the graph shown, work out the spring’s Spring Constant.

A seagull (mass = 20 kg) is flying in a small circle 9 m above the ground.

a) Calculate its gravitational potential energy.

An angry fisherman shoots at the seagull. He misses, but the seagull, petrified and unable to move, falls straight onto the ground. The air friction is negligible. The fisherman is sent to jail.

b) What is the kinetic energy of the seagull the moment before it hits the ground?

c) Calculate the speed of the seagull the moment before it hits the ground.

Question

How many different forms of energy do you know?

Energy• Energy is what makes things

‘happen’.• Examples - turning the lights on,

driving a car, using a Bunsen, etc.• Energy is measured in _____.

Question

What is the ‘Law of Conservation of Energy’?

Law of Conservation of Energy• Energy can neither be created nor

destroyed.

• Energy can only be transferred (from one place to another) or;

• be transformed (from one form to another)

Energy Transformations

The three forms of Energy in Physics1. Kinetic Energy

2. Gravitational Potential Energy

3. Elastic Potential Energy

Kinetic Energy

m = mass of the moving object

v = speed of the moving object

Gravitational Potential EnergyDefinition - energy stored in an object which is raised against the gravitational field

m = mass of the moving object

g = acceleration due to gravity (9.8 ms-2)

h = height of the object

KE & GPE TransformationA 24 kg rock is dropped on someone’s head. The speed of the rock the moment before it comes into contact with the person’s skull is 19.6 ms-1.

a) Calculate the kinetic energy of the rock the moment before it hits the person

b) Assuming that no energy is lost due to friction, calculate the height (above the head) that the rock was dropped from.

Hooke’s Law

F = k xF = force applied on the stretchy object

x = length of stretch (or compression)

k = spring constant (measured in ____ )

Examplea) A mass of 0.5 kg hung from the end

of a spring extends the spring by 25 cm. Calculate the spring constant.

b) Another mass of 0.5 kg is added to the first mass. What is the new extension?

Elastic Potential Energy

k = spring constant

x = length of stretch (or compression)

ExampleA 80 kg child stands on a trampoline and causes the trampoline to sag by 1 m.a) What is the child’s weight?b) What is the trampoline’s spring

constant?c) How much elastic potential energy is

stored in the trampoline?

Gravitational Potential Energy

Kinetic Energy

Elastic Potential Energy

Work

W = Fd• Work is the process that transfers

energy from one form to another.• The amount of work depends on the

forces involved and the distance through which those forces act.

• Work is measured in _________.

Examples• If a man pushes his car (mass = 1600 kg)

with a force of 500 N and causes it to roll 2 m, how much work has he done?

• If the man now lifts the car above his head, which is 2 m high above the ground, how much work has he done?

Chocofish QuestionIf someone holds a 10 kg mass on his hands and walk 5 m across the floor, how much work has he done?

CAUTION!!!If someone holds a 10 kg mass on his hands and walk 5 m across the floor, how much work has he done? – NONE!!!

Why? - The direction of the force applied on the mass (downward, vertical) and the direction of its movement (forward, horizontal) are perpendicular. Since vertical force cannot affect horizontal movement, the force applied on the mass (holding) is not accountable for its movement (across the floor) at all.

ExampleA man is pushing his lawnmower with a force of 100 N, at an angle of 30o from the ground. While he is pushing, the lawnmower moves 10 m across the lawn.

a) Calculate the vertical component of the force.

b) Calculate the horizontal component of the force.

c) Calculate the amount of work done by the man on his lawnmower.

Power

• Power is how much work is done “per second”.

• It’s not just the amount of work being done, but HOW FAST it’s being done!

• Power is measured in _______

ExampleTwo men are lifting weights. Each man lifts a weight (mass = 120 kg) 1.4 m above the ground.

a) Calculate the work done by each man.

One of the men, named ‘Andy’, takes 2 seconds to lift the weight. The other man, named ‘Billy’, takes 2.5 seconds to lift the weight.

b) Calculate Andy’s power output.

c) Calculate Billy’s power output.