Work, Potential Work, Potential Energy, Kinetic Energy, Kinetic Energy & PowerEnergy & Power

FThe force, F,

pushes the box for a short distance. This causes the

box to start moving!!!!!!

I just don’t have any energy Whoa!!! Now I do!!!

The force gives the object energy, it does work!!!!!Energy is the ability to use a force and move

something a distance. Energy is the ability to do work.

WorkWorkIf a force, F, is used to move an object a distance, d, then the force does work on the

object.

F

d

W = Fd

Units for WorkUnits for Work The units for force are Newtons (N) and the The units for force are Newtons (N) and the

units for distance are meters (m).units for distance are meters (m). Since Work is equal to force times distance, we Since Work is equal to force times distance, we

could use a Newton-meter (Ncould use a Newton-meter (N··m).m). But the more common unit is the Joule (J)But the more common unit is the Joule (J) 1 J = 1 N1 J = 1 N··mm 1 J is defined as the amount of work for 1- N 1 J is defined as the amount of work for 1- N

of force to move an object 1-mof force to move an object 1-m

Watch Video

The weight lifter in the video lifts 280-kg a distance of 0.75-m. How much work does he do during one lift?

If he lifts it twice, how much work would he do?

The force needed to lift the barbell is equal to its weight!!!!F = w = mg = (280)(9.8) = 2744-Nd = 0.75-m W = Fd = (2744)(0.75) =2058-J

F = 2744-N

d = 2(0.75-m) = 1.5-mW = Fd = (2744)(1.5) =4116-J

WorkWork

Work is the product of force and Work is the product of force and distance.distance.

W = FdW = Fd If work is done on an object its If work is done on an object its

energy is changedenergy is changed

Lifting a BallLifting a BallThis ball has a mass of 2-kg.What is its weight?w = mg = (2)(9.8) = 19.6-NWhat force must be used to lift this ball?

Its weight: F = w = 19.6-N

How much work will be done to lift this ball to a height of 5-m?F = 19.6-Nd = 5-m

W = Fd = (19.6)(5) = 98-J

19.6-N

5-m

Since 98-J of work was done on the ball its energy was increased by 98-J. This was added

to a form of energy called Potential Energy.

Potential EnergyPotential EnergyPotential Energy (PE) is energy stored in an object due to its position, and is stored

by doing work.For our ball, its position is its height,

5-m, above the ground.

This form of potential energy is technically called Gravitational Potential Energy

Gravitational Potential Energy is found by multiplying the mass (m) of the object by

gravity (g) and height (h).

5-m

PE = mgh

98-J of work was done lifting this

ball.

It now has 98-J of potential energy

stored in it.

It now has the ability to do 98-J

of work.

PE = (2)(9.8)(9) = 98-J

Pile Driver

A pile driver is a machine that uses the conversion of potential energy into work to drive beams and poles into the ground.

The block at the top of the pile driver has a mass of 275-kg

and is 9-m above the pole it is driving into the ground.

What is the potential energy of the block?m = 275-kgh = 9-m

PE = mgh = (275)(9.8)(9) = 24,255-J

This means the pile driver (block) can do 24,255-J of work on the pole to drive it into the ground!!!!!!

A 2-kg ball is rolled up a ramp until it reaches a height of 5-m. What is the potential energy of the ball?

5-m

m = 2-kgh = 5-m

PE = mgh = (2)(9.8)(5) = 98-J

The same as if we lifted it straight up!!!!

A 2-kg ball is rolled up a ramp until it reaches a height of 5-m. What is the potential energy of the ball?

5-m

m = 2-kgh = 5-m

PE = mgh = (2)(9.8)(5) = 98-J

The same as if we lifted it straight up!!!!

When you are calculating the potential energy of an object, all

you need to know is the mass and height of the object.

How the object gets to the height doesn’t matter!!!!! 5-m

No matter what path the 2-kg ball takes to get to a height of

5-m, it still has a potential energy of 98-J!!!

Lifting a BallLifting a Ball When you lift a ball to a When you lift a ball to a

certain height you do certain height you do work on it.work on it.

This work (W) is equal to This work (W) is equal to the weight of the ball the weight of the ball (w=mg) times the height (w=mg) times the height (h).(h).

The work done to lift the The work done to lift the ball is stored as potential ball is stored as potential energy.energy.

h

W = Fd =wh = mgh

PE = W = mgh

Lifting a BallLifting a Ball The ball now has potential The ball now has potential

energy stored in it.energy stored in it. This means the ball has the This means the ball has the

potential to do work if you potential to do work if you drop it on something.drop it on something.

But before it can do that But before it can do that work it must convert the work it must convert the PE to another form of PE to another form of energy called kinetic energy called kinetic energy – energy of motionenergy – energy of motion

h

PE = mgh

Kinetic EnergyEnergy of Motion

Kinetic EnergyKinetic Energy

Kinetic Energy (KE) Kinetic Energy (KE) is energy of motion.is energy of motion.

Kinetic Energy Kinetic Energy depends on the depends on the moving object’s mass moving object’s mass (m) and the square of (m) and the square of its speed (v).its speed (v).

2mv21

=KE

Kinetic Energy ExampleFind the kinetic energy of a 5000-kg Asian elephant running at 4-m/s.

sm4=v

kg5000=m

J000,40=)16(2500=

)4)(5000(21

=mv21

=KE 22

How could the elephant increase its kinetic energy?

The elephant could increase its mass, increase its speed or both.But changing its speed would be better since KE depends more on speed.---Speed is squared!!!!!!

Kinetic Energy ExampleThe kinetic energy of a 5000-kg Asian elephant running at 4-m/s

was calculated to be 40,000-J.

What would its kinetic energy become if its mass doubled?

2mv21KE

J000,80000,402KE2m2

What would its kinetic energy become if its speed doubled?

2mv21KE

J000,160000,404KE4KE2v2 2