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Work and Energy


Work and Energy Outcomes Upon completion of this unit you will be able to: Analyze force problems in terms of energy. Define the term – PowerPoint PPT presentation

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Title: Work and Energy

Work and Energy
  • Upon completion of this unit you will be able to
  • Analyze force problems in terms of energy.
  • Define the term "work" as it relates to physics.
  • Calculate work problems using the various
  • Identify which forces are at work in a given
    problem situation.
  • Interpret a force displacement curve in relation
    to work.
  • Calculate work as done by a spring.
  • Define power and efficiency.
  • Calculate power and efficiency using the
    appropriate formulas.
  • Demonstrate an understanding that power is a
    product of force and velocity.

Objectives Cont
  • Demonstrate an understanding of kinetic energy as
    a concept and as an equation.
  • Calculate the change in kinetic energy using the
    work-energy theorem formula.
  • Explain potential energy.
  • Calculate gravitational potential energy.
  • Define spring force.
  • Calculate spring force using the formula for
    Hooke's Law.
  • Describe conservative forces.
  • Write the conservative energy statement using the
    correct mathematical forms.
  • Solve energy problems using the appropriate
    energy equations.

  • Work is done if a force is applied for a
    particular distance.
  • In Physics there is one more stipulation the
    force and displacement vectors must point in the
    same direction.
  • W Fxdx
  • Fx is the applied force in the direction of the
    object's displacement anddx is the object's
  • Why the x-subscript? This shows that in order for
    work to be done, the force and displacement must
    be in the same direction.

Extreme Work
  • This formula for work leads us to three possible
    extreme cases
  • Force and Displacement in the Same Direction
    (results in max. positive work)
  • Example?

  • Force Perpendicular to Displacement
  • No work is done in this case
  • Example?

  • Force and Displacement in Opposite Directions
  • this results in max. negative work
  • Example?

Identify the Force at Work
  • It is extremely important to identify what force
    is doing the work in question. Look at the case
    of lifting then lowering a box.

Lifting a box
  • As you lift the box, you exert a force in the
    same direction as the displacement, so you do
    positive work. Gravity (weight) always acts
    straight down and here that means it is in the
    opposite direction to the displacement, so it
    does negative work.

Lowering the box
  • As the box is lowered, you still exert an upward
    force to keep the box from simply falling. Now
    your force and the displacement are at 180
    degrees, so you do negative work. Gravity is now
    doing positive work.

Units for Work
  • The unit of work is the joule (J).
  • lifting a good sized apple at a constant
    velocity straight up for a distance of 1 m
    requires about 1 J of work.
  • While work can be positive or negative and its
    calculation depends on direction, it is not a
    vector. Work is a scalar. Direction is important
    for calculating work, but work itself has no

Try It
  • You exert a force of 20 N in order to slide a
    textbook across a table a a constant speed. If
    the textbook has a mass of 20 kg and you slide it
    a distance of 50 cm, How much work do you perform?

Try it Again
  • A 1500 kg car is brought to a complete stop over
    a distance of 43 m. If the coefficient of
    friction between the car and the road is 0.32,
    how much work is done by friction in bringing the
    car to a stop?
  • Start by drawing a rough FBD of the situation

Force Displacement Graph
  • Another useful fact is that the area under a
    force displacement curve is equal to the work
    done. Of course, the force that is plotted will
    have to be the component that is in the same
    direction as the displacement.

Work done on a Spring
  • The work done on a spring when it is compressed
    or stretched is given by the formula shown below
  • where k is the spring constant (N/m) and x is
    the amount of compression (m).
  • You can not use the basic work formula W Fx for
    a spring. The formula W  Fx assumes that the
    force is constant. As you well know, the force a
    spring exerts (F) changes as the displacement (x)
    changes. You must use the special formula above
    if you need to calculate the work done by a

Power and Efficiency
  • Power is simply the rate at which you do work.
    The formula for average power is shown below.
  • ?W change in work (J)
  • ?t change in time (s)
  • The unit of power is the joule/second or the watt
    (W). When you see a light bulb rated at 60 W, it
    means that it consumes 60 J of electrical energy
    every second.

  • An elevator motor lifts a mass of 1000 kg over a
    distance of 20 m in 15 seconds. What power must
    it develop?

  • A winch rated at 1.5 kW pulls a heavy box along a
    horizontal floor. It takes the winch 1.00 min to
    pull the box over a distance of 250 m. What Force
    is it exerting?

Power as a product of F and V
  • It is possible to show that power is equal to the
    force times the velocity. The formula for this is
    shown below
  • P Fv
  • F applied force (N)v object's velocity (m/s)

  • Efficiency is the ratio of energy output to
    energy input  
  • Efficiency Eo /Ei x 100
  • Eo energy output
  • Ei energy input
  • It can also be written as a ratio of work output
    to work input  
  • Efficiency Wo /Wi x 100
  • Wo work output
  • Wi work input

  • A toaster transforms 1200 J of electrical energy
    into 560 J of thermal energy to make a piece of
    toast. What is the efficiency of the toaster?
  • We know thatEi 1200 J   Eo 560 J
  • So our efficiency is