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## Work and Energy

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### Work and Energy Outcomes Upon completion of this unit you will be able to: Analyze force problems in terms of energy. Define the term – PowerPoint PPT presentation

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Title: Work and Energy

1
Work and Energy
2
Outcomes
• Upon completion of this unit you will be able to
• Analyze force problems in terms of energy.
• Define the term "work" as it relates to physics.
• Calculate work problems using the various
equations.
• Identify which forces are at work in a given
problem situation.
• Interpret a force displacement curve in relation
to work.
• Calculate work as done by a spring.
• Define power and efficiency.
• Calculate power and efficiency using the
appropriate formulas.
• Demonstrate an understanding that power is a
product of force and velocity.

3
Objectives Cont
• Demonstrate an understanding of kinetic energy as
a concept and as an equation.
• Calculate the change in kinetic energy using the
work-energy theorem formula.
• Explain potential energy.
• Calculate gravitational potential energy.
• Define spring force.
• Calculate spring force using the formula for
Hooke's Law.
• Describe conservative forces.
• Write the conservative energy statement using the
correct mathematical forms.
• Solve energy problems using the appropriate
energy equations.

4
Work
• Work is done if a force is applied for a
particular distance.
• In Physics there is one more stipulation the
force and displacement vectors must point in the
same direction.
• W Fxdx
• Fx is the applied force in the direction of the
object's displacement anddx is the object's
displacement.
• Why the x-subscript? This shows that in order for
work to be done, the force and displacement must
be in the same direction.

5
Extreme Work
• This formula for work leads us to three possible
extreme cases
• Force and Displacement in the Same Direction
(results in max. positive work)
• Example?

6
• Force Perpendicular to Displacement
• No work is done in this case
• Example?

7
• Force and Displacement in Opposite Directions
• this results in max. negative work
• Example?

8
Identify the Force at Work
• It is extremely important to identify what force
is doing the work in question. Look at the case
of lifting then lowering a box.

9
Lifting a box
• As you lift the box, you exert a force in the
same direction as the displacement, so you do
positive work. Gravity (weight) always acts
straight down and here that means it is in the
opposite direction to the displacement, so it
does negative work.

10
Lowering the box
• As the box is lowered, you still exert an upward
force to keep the box from simply falling. Now
your force and the displacement are at 180
degrees, so you do negative work. Gravity is now
doing positive work.

11
Units for Work
• The unit of work is the joule (J).
• lifting a good sized apple at a constant
velocity straight up for a distance of 1 m
requires about 1 J of work.
• While work can be positive or negative and its
calculation depends on direction, it is not a
vector. Work is a scalar. Direction is important
for calculating work, but work itself has no
direction.

12
Try It
• You exert a force of 20 N in order to slide a
textbook across a table a a constant speed. If
the textbook has a mass of 20 kg and you slide it
a distance of 50 cm, How much work do you perform?

13
Try it Again
• A 1500 kg car is brought to a complete stop over
a distance of 43 m. If the coefficient of
friction between the car and the road is 0.32,
how much work is done by friction in bringing the
car to a stop?
• Start by drawing a rough FBD of the situation

14
Force Displacement Graph
• Another useful fact is that the area under a
force displacement curve is equal to the work
done. Of course, the force that is plotted will
have to be the component that is in the same
direction as the displacement.

15
Work done on a Spring
• The work done on a spring when it is compressed
or stretched is given by the formula shown below
• where k is the spring constant (N/m) and x is
the amount of compression (m).
• You can not use the basic work formula W Fx for
a spring. The formula W  Fx assumes that the
force is constant. As you well know, the force a
spring exerts (F) changes as the displacement (x)
changes. You must use the special formula above
if you need to calculate the work done by a
spring.

16
Power and Efficiency
• Power is simply the rate at which you do work.
The formula for average power is shown below.
• ?W change in work (J)
• ?t change in time (s)
• The unit of power is the joule/second or the watt
(W). When you see a light bulb rated at 60 W, it
means that it consumes 60 J of electrical energy
every second.

17
Example
• An elevator motor lifts a mass of 1000 kg over a
distance of 20 m in 15 seconds. What power must
it develop?

18
• A winch rated at 1.5 kW pulls a heavy box along a
horizontal floor. It takes the winch 1.00 min to
pull the box over a distance of 250 m. What Force
is it exerting?

19
Power as a product of F and V
• It is possible to show that power is equal to the
force times the velocity. The formula for this is
shown below
• P Fv
• F applied force (N)v object's velocity (m/s)

20
Efficiency
• Efficiency is the ratio of energy output to
energy input
• Efficiency Eo /Ei x 100
• Eo energy output
• Ei energy input
• It can also be written as a ratio of work output
to work input
• Efficiency Wo /Wi x 100
• Wo work output
• Wi work input

21
Example
• A toaster transforms 1200 J of electrical energy
into 560 J of thermal energy to make a piece of
toast. What is the efficiency of the toaster?
• We know thatEi 1200 J   Eo 560 J
• So our efficiency is