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Chapter 5 – WORK and ENERGY
TODAY’s OBJECTIVES
• Recognize the difference between the “scientific” and “ordinary”
work• Define work by relating it to force
and displacement
5.1 – 5.3 Work and Energy Vocabulary
WorkWork done on an object
JouleKinetic energy
Potential energyGravitational Potential Energy
Elastic Potential EnergySpring constant
Mechanical Energy
5.1 WORK
What is work?
In everyday language
In science
To do somethin
g:To learnTo kick a
ballTo thinkTo hold a
chair
The product of the
magnitude ofthe
displacementand the
component of a force
parallel to that
displacement
Work is done – when a force causes a displacement
Work done on a car
Constant horizontal force
= FDisplacement =
Δx = d
NO WORK is done on the object unless the objects moves (i.e. has a
displacement)!!!
W = F ∙ d
Work is done – when a force acts on an object AND the object must move in the direction
of that force
Work done on a crate
Constant horizontal force = F ∙ cos
Displacement = Δx = d
NO WORK is done on the object if the force is non – parallel to the direction of the
displacement
W = F ∙ d
IS WORK DONE ON AN OBJECT?
Example 1 (on a worksheet)
A teacher holds a chair at arm’s
length for several
minutes.
A person carries a bucket of water
along a horizontal path while walking
at constant velocity
IS WORK DONE ON AN OBJECT?
Example 1 (on a worksheet)
No, chair does not move = no
displacement in the direction of force applied
No, the upward force that holds the buckets is
perpendicular to the displacement
of the bucket
1.1 x 102 J
1.72 ×104 J, or 17.2 kJ
W = F × d = 300.0 x 0 = 0 J
Reminder...If WORK is done against GRAVITY
W = F ∙ dW = mg ∙ h
POWERRate of Energy
Transfer
the rate at which work is done or energy is
transformed
POWER
Power =
the rate at which work is done or energy is transformed
Power = force x speed
WHY? UNITS = J/s =
Watt = W
73 W
5.4 ×105 J/h
Useable work: 2.7 ×1011J
Heat: 8.1×1011 J
HORSE POWER (hp)• Another unit
used• 1 hp = 746
watts
Convert to horsepower.
Reminder...If WORK is done against GRAVITY
W = F ∙ dW = mg ∙ h
HOMEWORKPage: 139 - 141Problems: all
18
WORKSHEET EXAMPLEMany mountain roads are built so that they
zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of
such a design from the viewpoint of energy conservation and power
WORKSHEET EXAMPLEA light bulb is described as “having 60 watts.”
What’s wrong with this statement?
WORKSHEET EXAMPLEA 193.0 kg curtain needs to be raised 7.5 m, at
constant speed, in as close to5.0 s as possible. The power ratings for three
motors are listed as 1.0 kW,3.5 kW, and 5.5 kW.
Which motor is best for the job?
Worksheet EXAMPLEA 193.0 kg curtain needs to be raised 7.5 m, at
constant speed, in as close to5.0 s as possible. The power ratings for three
motors are listed as 1.0 kW,3.5 kW, and 5.5 kW.
Which motor is best for the job?GIVEN
?UNKNOWN
?m = 193.0 kgΔt = 5.0 s
d = 7.5 m
P = ? watts
WORKSHEET EXAMPLETwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the
horses.b. How much work is done by the two
horses?
Investigation 5 – 1A1. Read the lab procedure
2. Answer all the Concluding Questions
Investigation 5 – 1A and 5 – 1B Two horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the
horses.b. How much work is done by the two
horses?
ANSWERS TO
WORKSHEET EXAMPLES
IS WORK DONE ON AN OBJECT?
Example 1 (on a worksheet)
No, chair does not move = no
displacement in the direction of force applied
No, the upward force that holds the buckets is
perpendicular to the displacement
of the bucket
WORKSHEET EXAMPLEMany mountain roads are built so that they
zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of
such a design from the viewpoint of energy conservation and powerAssuming mechanical energy is
conserved, the same amount of energy is needed to reach the top in
both cases. Because the same amount of work must be done, the path with a longer distance takes more time and
hence requires less power.
WORKSHEET EXAMPLEA light bulb is described as having 60 watts.
What’s wrong with this statement?
Light bulbs don’t have theenergy stored within them;
energy is transferred to them
in the form of electricity at arate of 60 J/s.
WORKSHEET EXAMPLETwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the
horses.b. How much work is done by the two
horses?
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