Download Work and Kinetic Energy

Work and Kinetic Energy
What is kinetic energy?

If an object is moving, it has energy. You can think of
kinetic energy as the energy of motion, and it is one
type of energy. Energy is the ability to do work.

Another type of energy is potential, which is energy based
on position.
The math behind kinetic energy

An object’s kinetic energy depends on:
 mass
 velocity
or
How do you measure kinetic energy?

Kinetic energy is a scalar quantity.

Unit of kinetic energy: Joule
 An
object with mass of 1 kg, moving at 1 m/s, has a
kinetic energy of 0.5 Joule.
 SI

base unit: Kg • m2 • s-2
Why Joule?
Because
this guy
(James Prescott Joule)
Example problems
Solve the following problems involving kinetic energy:
a)
What is the kinetic energy of a car with a mass of 1400kg
moving at 22ms-1?
b)
What is the kinetic energy of a 150g ball moving at 2ms-1
c)
A 200kg go-cart starts at rest and then accelerates to
18ms-1 after 50m. What is the KE at that point?
Example problems (solutions)
Solve the following problems involving kinetic energy:
a)
b)
a)
What is the kinetic energy of a car with a mass of 1400kg
moving at 22ms-1? 338800J
What is the kinetic energy of a 150g ball moving at 2ms-1?
300J
A 200kg go-cart starts at rest and then accelerates to
18ms-1 after 50m. What is the KE at that point?
32400J
Thinking about kinetic energy
Let’s say you are going to shoot a person out of cannon. At
any point as they are flying through the air they have kinetic
energy. When they land they no longer have kinetic energy.
What are two ways you can increase the amount of kinetic
energy they have?
https://www.youtube.com/watch?v=0Y14kEY-6fk
What is work?

Work = force x distance moved in direction of the force
or W = Fs or W = Fd
This is for work done by a constant force.

Unit of work: Joule

Work is a scalar quantity.
Understanding the idea of work

Work is always done by a force

Work is only done when there is movement

A force only does work when it has a component in the
same direction the object is moving.
Has work been done?

A man running from here to Guangzhou.

A woman stepping up onto a chair.

A woman stepping up onto a chair and then jumping back
down.

A student holding a bowling ball.

A student holding a bowling ball and walking up stairs.

A student swinging a bowling ball back and forth.

A man running to Guangzhou and then back here.
Work and friction
When there is friction then energy is dissipated as heat or
sound.

Your shoes on the ground

A gun being shot
Friction is a dissipative force
Gravity on the other hand is a conservative force, and the
energy is not lost. It is stored as potential energy. For
example a book placed on an upper shelf has potential
energy equal to the work done against gravity to place it
there.
Positive and negative work

The man does positive work lifting
the box and negative work
lowering the box.

Gravity does positive work when
the man lowers the box and
negative when he lifts it.
Gravity is a conservative force. We
will discuss more about gravitational
potential energy in the next lesson.
Example problems
Solve the following problems involving Work:
a)
Find the work done by pushing a box 45m using a force of 50N.
b)
What is the work done by pushing a 100kg box 70m across a
rough floor with 150N resistance? The box starts and stops at
rest.
c)
How does the answer to (b) change if the box is still moving at
2ms-1 when it reaches 70m?
d)
A car with a mass of 1200kg is driving at 10ms-1 and over a
period of 20 seconds increases the speed to 20ms-1. Assuming
constant acceleration, what is the work done?
Example problems (solutions)
Solve the following problems involving Work:
a)
Find the work done by pushing a box 45m using a force of 50N. 2250J
b)
What is the work done by pushing a 100kg box 70m across a rough
floor with 150N resistance? The box starts and stops at rest. 10500J
c)
You will add the KE to the total. 10700J
d)
A car with a mass of 1200kg is driving at 10ms-1 and over a period of
20 seconds increases the speed to 20ms-1. Assuming constant
acceleration, what is the work done? 180000J
Work and Kinetic Energy
Kinetic energy is a type or energy and its units are Joules
just like the equation for Work. How are they connected?
Let’s start with the formula for Work:
W = Fs
Work-energy principle
The total work done by the forces acting on a
body is equal to the increase in the kinetic
energy of the body.
When to use each formula.
Type of problem where you use W = Fs
If you have a problem where an object is moved from one
location to the other against a resistive force and starts
and ends at rest.
Problems where you use W = Kef – Kei
If you have a problem with a change in velocity.
Problems where you use might use both
If you have a problem where an object is moving against
a resistive force and has a change in velocity.