7.4

1
7.4   Work

• Work, W is energy transferred to or from an
  object by means of a force acting on the object.
• Energy transferred to the object is positive
  work, and energy transferred from the object is
  negative work.
• Work is a scalar. Unit for work joule (J)

2
7.5   Work and Kinetic Energy
Finding an Expression for Work
Work done by a constant force
Units for Work
3
WorkKinetic Energy Theorem
Sample Problem 7-2
4
7.6   Work Done by the Gravitational Force
Consider a tomato of mass m, thrown upward
The minus sign tells us that during the objects
rise, the gravitational force acting on the
object transfers energy in the amount mgd from
the kinetic energy of the object. This is
consistent with the slowing of the object as it
rises. During the fall of the tomato
The plus sign tells us that the gravitational
force now transfers energy in the amount mgd to
the kinetic energy of the object. This is
consistent with the speeding up of the object as
it falls.
5
Work Done in Lifting and Lowering an Object
Now suppose we lift a particle-like object by
applying a vertical force to it. During the
upward displacement, our applied force does
positive work on the object while the
gravitational force does negative work on it.
If the object is stationary before and after the
lift,
6
Sample Problem 7-5

• An initially stationary 15.0 kg crate of cheese
  wheels is pulled, via a cable, a distance, d
  5.70 m, up a frictionless ramp to a height h of
  2.50 m, where it stops.
• How much work is done on the crate by the
  gravitational force during the lift?
• How much work is done on the crate by the force
  from the cable during the lift?