Lecture 17: Work and Kinetic Energy

Lecture 17: Work and Kinetic Energy Objectives 1. Relate the work done by a constant force to the change in kinetic energy of a system. 2. Apply the work-kinetic energy theorem to systems with constant forces.

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If an object is moving, then it is capable of doing work.

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Work is related to the change in speed of an object.

If an object is moving, then it is capable of doing work. It has energy of motion, or kinetic energy (KE).

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Kinetic energy is related to the motion of an object.

 

Energy is a property of objects, transferable among them via fundamental interactions, which can be converted in form but not created or destroyed.

If an object is moving, then it is capable of doing work. It has energy of motion, or kinetic energy (KE).

An infrared camera reveals the heated tire track on the floor. (image: Conceptual Physics, JP Hewitt) 5

Kinetic energy is related to the motion of an object.

 

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Work-energy theorem: when work is done, energy changes

http://zonalandeducation.com/mstm/physics/mechanics/energy/work/work.html

Kinetic energy carries no information about the direction of motion.

Valid only to inertial frame of reference.

If an object is moving, then it is capable of doing work.

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Work is related to the change in speed of an object.

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Work-energy theorem: when work is done, energy changes

Work is a transfer of energy!

http://zonalandeducation.com/mstm/physics/mechanics/energy/work/work.html

Golfer does work on the club

Club does work on the

ballEnergy in the golfer

Energy in the club

Energy in the ball

Sample Problem: (Sled again) A 14700N sled is being pulled by a tractor with the total work done on the sled as 10KJ. If the sled has an initial speed v1 of 2.0m/s, what is the speed of the sled after it moves 20m?

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Solve for v1 and substitute the given values:

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Solve for h and substitute the given values:

Same work-kinetic energy equations will be used to solve for h:

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Seatwork

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A box (mass 2.00 kg) slides along a frictionless irregular road. It starts from rest at point A 2.00 m above the ground and reached point B at 6.26m/s.

1. What is the total work done when it reaches point B?

2.00 m

A

B

Seatwork

Seatwork

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2. Which iceboat crosses the finish line with greater kinetic energy?

3. Which iceboat crosses the finish line first?

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Seatwork answers

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A box (mass 2.00 kg) slides along a frictionless irregular road. It starts from rest at point A 2.00 m above the ground and reached point B at 6.26m/s.

1. What is the total work done when it reaches point B? W = (1/2)(2.00kg)(6.26m/s)2 – 0 = 39.2 J

2.00 m

A

B

Seatwork

Seatwork

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2. Which iceboat crosses the finish line with greater kinetic energy? BOTH 3. Which iceboat crosses the finish line first? Blue or red? RED