Transcript What are the units of power?

Work, Energy & Power
 Work
 Kinetic Energy
 Work/Kinetic Energy Theorem
 Potential Energy
 Power
 Work can only take place when an
object is moved as a result of a
force.
 Work is defined as a force that
causes displacement

W = F.d

What are the units of work?

Work has ONLY occurred when an object has
moved!

A tugboat pulls a ship with a horizontal force of 500 N
and causes the ship to move through the harbor. How
much work is done on the ship if it moves a distance
of 3 meters? 32 km?

Brandon lifts a set of weights a vertical distance
of 2.0m. If a constant net force of 350N is
exerted on the weights, what is the net work
done on the weights?

Take, for example, the idea of Avery pulling her rolling
suitcase in the airport with a force of 25 N… If the angle
of the handle with the ground is 35o, and the suitcase is
pulled 100 m, what is the work done on the suitcase?

Deyjah pushes a shopping cart with a force of 40N
directed at an angle of 40o down from the horizontal.
Find the work done by Deyjah on the cart as the
shopper walks 65m across the parking lot.

If 2.0 Joules of work is done by Josiah raising a
180 gram apple, how far is it lifted?

The formula for kinetic energy is:
KE = ½

2
mv
The units of kinetic energy are also Joules…

Ricardo rolls a 7.0 kg bowling ball at 3.0 m/s. How
much KE does the bowling ball have? How fast
should he roll a 2.45 gram ping-pong ball in order to
have the same amount of KE?

Calculate the speed of an 8000 kg car with a
kinetic energy of 1.1 x 109J.

A car has a kinetic energy of 4.32 x 105 J when
traveling at a speed of 23 m/s. What is its mass?

What is the speed of a .145 kg baseball if its
kinetic energy is 109 J?
W = ΔKE

Derive the ‘Work-Energy Theorem’ using:
 W=F.x
vf2 = vi2 + 2ax

Jorge pushes Manuel in a bobsled a distance of 13 m
starting from rest. If the total mass of the bobsled (with
Manuel) is 125 kg, and it reaches a speed of 5.5 m/s,
what is the net force of the bobsled?

Sam’s truck experiences a forward force of 240 N provided by the
traction between the wheels and the road. How far must Sam’s
truck travel to reach a speed of 3m/s from rest?

The formula for gravitational potential energy
is:
 U = mgh
▪ (the abbreviation for potential energy is U)
▪ m = mass
▪ g = acceleration due to gravity
▪ h = height

Ryan raises a 1kg bag of sugar 2m above the ground.
What is its gravitational potential energy associated
with the bag of sugar relative to the surface of the
earth?

What is the height of a 35 gram object with a
potential energy of 41.2 Joules?

Calculate the change in potential energy of a 15 kg
object that moves 20 m down an inclined plane of 25o.
50m

The mechanical energy of an object is defined
as the kinetic energy + the potential energy
 ME = KE + U

Moreover, the mechanical energy of a system is
conserved (if there is no frictional or retarding forces
involved)
 MEi = MEf

In other words, energy is neither created nor destroyed!
So – if…. ME = KE + U
And…
MEi = MEf
Then…
doesn’t it stand to reason that:
KEi + Ui = KEf + Uf
2
 ½ mvi
+ mghi = ½
2
mvf
+ mghf

Jariana steps off a diving board 10.0 m above the water’s
surface. Find Jariana’s speed just before striking the water.
First use kinematics, then using conservation of energy.

Noah shoots a cannon ball straight up in the air with an
initial velocity of 40m/s. Determine the ball’s max height.

Ti’Ara starts from rest and zooms down a frictionless
slide from an initial height of 3.0 m. What is her
speed at the bottom of the slide?

Zabbiel releases a pendulum bob from some initial
height such that the speed of the bob at the bottom of
the swing is 1.9 m/s. What is the initial height of the
bob?

Remember the formula for conservation of
energy…

A 50N child falls off a 10m diving board into the water.
What is the child’s velocity just before he hits the water?
What is his velocity when he is half-way down? What is his
velocity when he is 2m above the surface of the water?

Two people of equal mass jump off a 15m diving board.
Describe the kinetic energy of each right before they hit the surface of the water…
•The 1st person jumps straight up
with an initial velocity of 5m/s.
15 m
•The 2nd person jumps straight
down with initial velocity of 5m/s.
15 m
Power
•
The rate at which work is done
P = Work / time
•
So… What are the units of power?

How long does it take a 19kW steam engine
to do 6.8 x 107 Joules of work?

A 1.5 x 103 kg car accelerates uniformly from rest
to 10.0 m/s in 3.00 s.
 A. What is the work done on the car?
 B. What is the power delivered by the engine in this
time interval?

A rain cloud contains 2.66 x 107 kg of water vapor. How
long would it take for a 2.00kW pump to raise the same
amount of water to the cloud’s altitude a distance of
2.00km?