Transcript Energy and workx

ENERGY
AND WORK
What is energy?
WHAT IS ENERGY?
“The ability to cause change
or to do work”
“Energy can not be created
nor destroyed, only
converted from one form to
another form.”
MECHANICAL ENERGY
Associated with Motion.
• Kinetic Energy is the energy of motion.
• Potential Energy is energy due to position or stored energy.
WHAT IS ENERGY?
“The ability to cause change
or to do work”
WORK
Work- transfer of energy that occurs when a force makes an
object move
• Force must result in movement
• Movement must be same direction as force
Holding an object- is not work!
• A force is applied but movement does not occur
WORK
Walking across the room while holding an object- is NOT
work!
• Movement is not in the same direction as force, so arms do
no work
When work is done: energy is transferred from one object
to another
WORK
A system uses
(loses) energy
when doing work.
A system gains
energy when
work is done to
it.
WORK
Energy- the ability to cause change or
do work
Energy is transferred from the object
that is doing the work to the object on
which work is done
Amount of work depends on amount
of force and distance over which
force is applied
W = Fd
WORK DONE = FORCE X DISTANCE
Another physics formula to learn!
Copy it down!!
WORK
Units
• Force - Newtons N
• Distance – meters m
• Work – Joules- J
When calculating work, use the distance
over which force is applied
GROUP ACTIVITY:
Why am I
doing this?
READ THE STATEMENTS BELOW
WHICH ONES ARE EXAMPLES OF WORK BEING DONE?
1. I apply a force to a wall and become exhausted.
2. A book falls off a table and free falls to the ground.
3. A waiter carries a tray full of meals above his head
straight across the room at constant speed.
4. A rocket accelerates through space.
LIFTING OBJECTS
When we lift objects,
we are doing work because a
force is moving.
Force
Distance
moved
LIFTING OBJECTS
Our lifting force is equal to the weight of the object.
Lifting force
weight
Let’s look at
some examples
GROUP CHALLENGE
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A woman pushes a car with a force of 400 N for a distance of
15m. How much work has she done?
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A woman pushes a car with a force of 400 N for a distance of
15m. How much work has she done?
Work = force x distance = 400 x 15 = 6000 J
GROUP CHALLENGE
WORK DONE (J) = FORCE (N) X DISTANCE (M)
A man lifts a mass of 120 kg to a height of 2.5m. How much
work did he do?
WORK DONE (J) = FORCE (N) X DISTANCE
(M)
A man lifts a mass of 120 kg to a height of 2.5m. How much
work did he do?
Force = weight = 1200N
Work = F x d = 1200 x 2.5
Work = 3000 J
WORK AGAINST
GRAVITY.
As energy can not be created or destroyed, when you do
work against gravity it must be transferred.
If you lift an object you increase its gravitational potential
energy (g.p.e).
The total amount of
g.p.e gained must be
equal to the amount of
energy transferred, or
work done against
gravity.
Lets derive
So work done:
W = Fxd
What would the Force on the sack of
potatoes (mass = m) be?????
h
m
Can you derive an equation for the
work done against gravity when I lift
the sack of potatoes?
So work done:
W = Fxd
If I do work against gravity I raise an object a
certain height (h) increasing its g.p.e. So:
W=Fxh
In order to lift the object I have to overcome the
objects weight (m x g), so the force has to be at
least equal to the weight. So:
W=mxgxh
But since the total amount of g.p.e gained must be equal to
the amount of energy transferred, or work done against
gravity therefore:
Work done against gravity = g.p.e
it follows that:
Easy
Write the equation to calculate work
done.
Medium
What is the work done if we apply a
1.2N force and we move 4 m in the
direction of force?
Hard
Thinking about energy transfers:
When you rub your hands together
what are the energy transfers?
Rearrange this equation for force
applied.
What is the work done if we apply a
7N force and we move 8 m in the
direction of the force?
When you boil a kettle what energy
transfers are happening?
Rearrange this equation for distance What distance is moved if we have
moved in the direction of force.
a 8N force and the work done is 90
J?
Walking up a flight of stairs, what
are the energy transfers?
What is the unit of force?
What is the distance moved if we
have a 70N force and work done is
8 J?
When you roll a ball down a hill what
are the energy transfers?
What is the unit of work done?
What force is required to move 7 m
if the work done is 9 J?
What is the work done when a force
of 5 N is applied to a ball and it
moves 80 m?
What is the unit of distance?
What work is done when we apply a
force of 5N and move in the
direction of the force 2 m?
What force is required to move 19 m What is the work done to a car if a
if the work done is 9 J?
force of 9 N is applied and it moves
7 km?
What force is required to move 7 m
if the work done is 21 J?
What is the work done to a person if
a force of 1.3N is applied and the
person moves 6m?
Easy
Write the equation to calculate work
done.
Medium
What is the work done if we apply a
1.2N force and we move 4 m in the
direction of force?
Hard
Thinking about energy transfers:
When you rub your hands together
what are the energy transfers?
Rearrange this equation for force
applied.
What is the work done if we apply a
7N force and we move 8 m in the
direction of the force?
When you boil a kettle what energy
transfers are happening?
Rearrange this equation for distance What distance is moved if we have
moved in the direction of force.
a 8N force and the work done is 90
J?
Walking up a flight of stairs, what
are the energy transfers?
What is the unit of force?
What is the distance moved if we
have a 70N force and work done is
8 J?
When you roll a ball down a hill what
are the energy transfers?
What is the unit of work done?
What force is required to move 7 m
if the work done is 9 J?
What is the work done when a force
of 5 N is applied to a ball and it
moves 80 m?
What is the unit of distance?
What work is done when we apply a
force of 5N and move in the
direction of the force 2 m?
What force is required to move 19 m What is the work done to a car if a
if the work done is 9 J?
force of 9 N is applied and it moves
7 km?
What force is required to move 7 m
if the work done is 21 J?
What is the work done to a person if
a force of 1.3N is applied and the
person moves 6m?
AIM: TO INVESTIGATE HOW
HARD WE CAN WORK AS A
CLASS!
Name
Mass
(kg)
Force
(N)
Distance
(m)
Work of
one lift
(J)
# of lifts in
1 min
Total work (J)
Mean Total Work for the group :
J
ARM CURLS
Force required = weight of object = mass (kg) x 10
distance
CLASS RESULTS FOR EACH
GROUP
Group
Mean Total
work (J)
Class Mean:
1
2
3
4
5
Conclusion:
Are we a
hardworking class?
J
LET’S SUMMARISE!
Discuss with the
person next to you
what you have
learned today then as
a pair write down what
you feel is the most
important thing you
learned.