# Work Energy Power Work Kinetic Energy WorkKinetic Energy

• Slides: 36

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

Work = Force x Distance W = F. d What are the units of work? Work has ONLY occurred when an object has moved!

Calculate the work: 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. 0 m. If a constant net force of 350 N is exerted on the weights, what is the net work done on the weights?

Work is done only in the direction of the movement… 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 35 o, and the suitcase is pulled 100 m, what is the work done on the suitcase?

Deyjah pushes a shopping cart with a force of 40 N directed at an angle of 40 o down from the horizontal. Find the work done by Deyjah on the cart as the shopper walks 65 m across the parking lot.

If 2. 0 Joules of work is done by Josiah raising a 180 gram apple, how far is it lifted?

Kinetic Energy 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 109 J.

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?

Work – Energy Theorem W = ΔKE Derive the ‘Work-Energy Theorem’ using: W = F. x vf 2 = vi 2 + 2 ax

Tayvionia is wearing ice-skates, and is pushed by Jennifer with a force of 45 N (from rest) friction-free ice rink. How far must Tayvionia be pushed so her final KE is 352 Joules? .

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 3 m/s from rest?

Potential Energy 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 1 kg bag of sugar 2 m 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 25 o. 50 m

What did you learn from the previous slide?

Mechanical Energy 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

KEi + Ui = KEf + Uf ½ 2 mvi + mghi = ½ mvf + mghf 2

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 40 m/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?

Conservation of Energy Continued… Remember the formula for conservation of energy…

A 50 N child falls off a 10 m 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 2 m above the surface of the water?

Two people of equal mass jump off a 15 m diving board. Describe the kinetic energy of each right before they hit the surface of the water… • The 1 st person jumps straight up with an initial velocity of 5 m/s. 15 m • The 2 nd person jumps straight down with initial velocity of 5 m/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 19 k. W 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. 00 k. W pump to raise the same amount of water to the cloud’s altitude a distance of 2. 00 km?