Chapter 5
Energy
Energy defined and Work
 Energy- is defined as the ability to do work. Work is done
when a force causes something to move
 https://www.youtube.com/watch?v=xBnS23U_ao4
 Energy can also be defined as the ability to cause change.
 Energy moves from place to place and changes forms.
 Work can be calculated by using the formula:
 Work=Force x distance
 Work is labeled in Joules (J)
Work Calculations
 A 10-N force is applied to push a block across a friction free
surface for a displacement of 5.0 m to the right
Work Calculations
 An approximately 2-kg object is sliding at constant
acceleration of 2 m/s/s across a friction free surface for a
displacement of 5 m to the right.
Forms of Energy
 Mechanical
 Radiant/ Light
 Sound
 Chemical
 Heat/ Thermal
 Electrical
 Nuclear
What forms of Energy are in
these?
Kinetic Energy….let’s go!
 https://www.youtube.com/watch?v=BGmUVoX5s58
 Kinetic energy is the energy a moving object has because of
its motion. It’s kinetic energy depends on mass and speed.
 Kinetic energy:
 Kinetic energy is labeled in Joules (J)
Calculations of K.E.
 A jogger with a mass of 60 kg is moving at 2 m/s. What is
the kinetic energy of the jogger?
 A ball with a mass of 2 kg is moving at 3 m/s. What is the
balls kinetic energy?
Potential Energy
 https://www.youtube.com/watch?v=Tji6PDBck_8
 Stored energy due to position is called Potential Energy.
This means that motionless objects can have energy! Energy
is stored in the object.
 http://www.pinterest.com/pin/75998312437909650/
 Elastic potential energy-is energy stored by something that
can stretch or compress.
 Examples?
 Chemical potential energy- is energy stored in chemical bonds.
 Examples?
Calculate PE or GPE
 Anything that can fall has gravitational potential energy, or
GPE.
 GPE is energy stored by objects due to their position above
Earth’s surface.
 The GPE depends on the objects mass and height above
ground
Practice!
 What is the GPE of a book that has a mass of 1kg and is
sitting on a shelf that is 2 m above the ground?
 What is the GPE of a ball that has a mass of 5 kg and is
stored 4 m above ground?
Energy Bar Charts
Energy Skatepark Sim
 http://phet.colorado.edu/en/simulation/energy-skate-parkbasics
Draw a Bar Energy Chart
 A ball is dropped from rest from a bridge. As the ball falls
through the air, it encounters a small amount of air
resistance. The final state of the ball is the instant before it
strikes the water.
What could this explain?
 A baseball is caught by a catcher after passing over home
plate. The initial state is the baseball moving at high speed
just prior to hitting the catcher's mitt. The final state is the
baseball just after the catcher has applied the force to stop
the ball. Assume that the ball does not change height as the
catcher is catching it.
Explanation
 initially:

PEgrav - present (ball is elevated a little)

PEspring - absent (no springs)

KE - present (ball is moving)

 Finally:

PEgrav - present (ball is elevated a little)

PEspring - absent (no springs)

KE - absent (ball is stopped)

 Wext - present and negative (the catcher applies a force in a direction opposite the ball's motion)
Work and Energy Webquest!