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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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section 5 1 work
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
Calculating Work

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

example problem
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
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
  • 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 problem1
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 – 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
Other Forms of Stored Energy
  • Compressed spring
  • Bow pulled back in archery
  • Stretched rubber band
kinetic energy
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
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
Elastic Potential Energy
  • When a string is stretched or compressed, it gains elastic potential energy.
elastic potential energy1
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 problem2
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
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 problem3
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?
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