Chapter 5 Work and Energy

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Chapter 5 Work and Energy - PowerPoint PPT Presentation

Chapter 5 Work and Energy. Section 5.1 Work. Work is done on an object only when a net force acts on the object to displace it in the direction of a component of the net force. Work = Force x displacement x cos Θ W= fd ( cosΘ ) Work is measured in Nm or Joules. Calculating Work.

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Chapter 5Work and Energy

Section 5.1 Work
• Work is done on an object only when a net force acts on the object to displace it in the direction of a component of the net force.
• Work = Force x displacement x cosΘ
• W=fd(cosΘ)
• Work is measured in Nm or Joules
Calculating Work

When Force and Distance are in the same direction, cosΘ is 1 and W = Fd

Example Problem

A box is dragged across a floor by a 100N force directed 60o above the horizontal. How much work does the force do in pulling the object 8m?

Example Problem #2

Decide if work is done and if so, the sign of the work for each case:

a) A crane lifting a bucket of concrete

b) The force of gravity on the bucket being lifted

c) An athlete holds a weight up in a fixed position

d) An athlete lowers a weight slowly

e) A person pushes a book across the table.

Power
• Power is the rate at which work is done.
• Power = work/elapsed time
• P = W/Δt
• The SI Unit for power is the watt (W) which equals one Joule per second (J/s)
Example Problem
• A 50 kg girl climbs a flight of stairs that is 5.0 m high. Calculate the power output if she takes 10.0 s to do this.
• Find the work done. (Recall that her force is equal to mg)
• 2. Calculate the power.
Energy – Potential and kinetic
• Energy: The ability to do Work
• Potential Energy: Energy of position or stored energy
• ΔPE = mghwhere “h” refers to height.
Other Forms of Stored Energy
• Compressed spring
• Bow pulled back in archery
• Stretched rubber band
Kinetic Energy
• Kinetic Energy: the mechanical energy of motion. It is how much work an object is currently doing.
• KE = ½ mv2
Energy and Work
• The SI unit for energy is the Joule. This is the same unit for work.
• When work is done on an object, energy is transformed from one form to another.
• The sum of the changes in PE, KE and heat energy are equal to the work done on the object.
• Mechanical energy is transformed into heat energy when work is done to overcome friction.
Elastic Potential Energy
• When a string is stretched or compressed, it gains elastic potential energy.
Elastic Potential Energy
• The force that pulls it back and attempts to restore the spring to equilbriumis the restoring force.
• PE = ½ kx2
• Elastic PE = ½ (spring constant)(distance compressed or stretched)2
Example Problem
• A spring with a force constant of 5.2 N/m has a relaxed length of 2.45 m. When a mass is attached to the end of a string, and allowed to come to rest, the vertical length of the spring is 3.57 m. Calculate the elastic potential energy of the spring (page 180 problem #1, Answer: 3.3 J)
Conservation of Energy
• Law of Conservation of Energy: Energy cannot be created or destroyed.
• Total amount of ME in a system remains constant if no work is done by any other force besides gravity.
• Δ KE = ΔPE
• KE and PE before an interaction equals all the KE and PE after the interactionl
• KE0 + PEo = KEf + PEf
Example Problem:
• Bo flings a 0.20 kg pool ball off a 0.68 m high pool table and the ball hits the floor with a speed of 6.0 m/s. How fast was the ball moving when it left the pool table?