Transcript Work, Power and Energy

Work, Power and Energy
Work
• When a force is applied and an object
moves in the direction of the force, work
has occurred.
• Ex. Picking up a box –you do positive work
on the box and the box does negative
work on you
• Work = Force X distance
• Work is measured in joules.
Example problem
• A weight-lifter picks up a 45 N set of
weights and lifts them a distance of .75 m.
How much work was done?
• W=F X d
• W= 45N X .75m
• W=_________ J
Energy
• Energy is the ability to do work. It is also
measured in joules.
• Kinetic energy is the energy an object has
due to its motion.
• KE = .5 mv 2
• Energy is used to do work.
Work energy theorem
• The change of kinetic energy is a system
is equal to the work done on a system.
• Change in Ke = work
• This is the work energy theorem.
• Ex. If a 2kg object moves at 3m/s, what is
its kinetic energy?
• This is the amount of work that has been
done by the object.
• Through the process of doing work,
energy can move between the
environment and the objects or systems
involved.
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/energy/u5l1c.html
• www.physicsclassroom.com
•
Work when forces are applied at
an angle.
• When finding the amount of work done on
an object where the force is applied at an
angle, you must use the formula below.
• W= F x D x cos theta
Ex.
• A boat is being pulled by a rope that
makes a 30 degree angle with the
horizontal. The force applied at that angle
is 300N. If the boat is pulled a distance of
2 m, how much work was done?
• Work= F X d X cos theta
• W= 300N X 2 m X cos (30)
• 520 J
• http://apps1.eere.energy.gov/education/les
sonplans/grades.cfm/grade=3
Power
• Power is measured in watts. A watt is a
J/sec.
• Power = work/time
• If a machine does 25 J of work in 20
seconds, what is its power?
• A weight lifter lifts 30 kg of weight a
distance of 1.3 meters in 15 seconds.
What is his power?
• P = (F X d) / time
• P= (30kg X 9.8 X1.3m) / 15 sec
• 25.48 watts
Potential Energy
• Potential energy is the energy an object
had due to its position.
• GPE= m X g X h
• The higher the object is from the ground
the more GPE it has.
Example
• How much GPE does a 55kg rock have
sitting on top of a 190 m high cliff?
• GPE= 55kg X 9.8 X 190m
• = 1.02 X 10 5 Joules
Examples
• Use this principle to determine the blanks in the following
diagram. Knowing that the potential energy at the top of
the tall platform is 50 J, what is the potential energy at
the other positions shown on the stair steps and the
incline?
•
•
GPE = mgh
•
•
•
•
•
A=40 J
B=30 J
C=20 J
D=10 J
E and F = 0 J
• The loss of GPE is always equal to the
gain in KE.
• This is an example of the Law of
Conservation of Energy.
• http://www.physicsclassroom.com/Class/e
nergy/u5l2a.cfm
Mechanical Energy
• the mechanical energy of an object can be the result of
its motion (i.e., kinetic energy) and/or the result of its
stored energy of position (i.e., potential energy). The
total amount of mechanical energy is merely the sum of
the potential energy and the kinetic energy. This sum is
simply referred to as the total mechanical energy
(abbreviated TME).
• TME= GPE + KE
Example of TME
• The diagram below depicts the motion of Li Ping
Phar (esteemed Chinese ski jumper) as she
glides down the hill and makes one of her
record-setting jumps.
More…
• A horizontal force can never change the
potential energy of an object. Horizontal forces
cannot cause vertical displacements. The only
means by which an external force can
contribute to a potential energy change is if the
force has a vertical component. Potential energy
changes are the result of height changes and
only a force with a vertical component can cause
a height change.
More Examples
• http://www.physicsclassroom.com/Class/e
nergy/u5l1b.cfm