Page 1 of 6
Physics I Notes: Ch 5 Work and Energy
“Work” has a variety of meanings in every day language…BUT in physics, its meaning is VERY specific.
I. Work
– Work is energy transferred by a force acting through a distance. Work is a scalar quantity so it has no
direction associated with it.
• Work is done only if a force acts over some distance, so Work = Force x distance (W=Fd) BUT the component of
the force used must be parallel to the displacement of the object. The units for work are N • m = J (joule). One joule
is about the amount of work you do in lifting your calculator to a height of one meter.
Questions:
1. You lift a barbell and do some work…How much work is done if you lift a barbell that is twice as heavy the same
distance?
2. How much if you lift that twice as heavy barbell twice as high?
3. How much if you hold the barbell over your head for 10.0 minutes?
4. How much work is done when you carr y a 50.0 N stack of books in your arms across the 12.0 m long room?
Work can be done against another force. Ex: against gravity or friction and,
Work can be done to change the motion of an object. Ex: stopping a car
Work can be done ON an object…Work can be done BY an object…BUT objects CANNOT Possess Work!
Objects can – and definitely DO possess or contain Energy!
When work is done on or by an object, it changes the energy that object possesses…energy is the ability to do work!
Calculating Work (W): W = Fd cos θ
θ θ
θ
• θ is the angle between the direction of the force and the objects displacement; when F is parallel to the
displacement, θ = 0, and cos 0 = 1, and W = Fd
• Since W = Fd; if an object at rest has zero displacement, therefore W = 0
• When the Force is parallel to the motion, maximum work will be done. If the Force is exerted at some angle
between θ = 0
o
and θ = 90
o
to the motion, then less work is done depending on how large the parallel component of
the Force is. When F is perpendicular to motion, W = 0
• The sign of work is important. Work can either be positive or negative, depending on whether the parallel component
of force is in the same direction (+) or opposite direction (-) of the displacement
• Kinetic friction does negative work. It is negative because the force involved in producing the work is OPPOSITE
to the direction of motion.
• Example 1: How much work is done on a car being pushed 1550.0 m by a force of 3750.0 N? How about if the force
is applied by a tow truck cable at an angle of 42
o
from the horizontal?
θ
Fcosθ
Fsinθ
F
d
