Transcript 5.1 Work and Power

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
Work
Work done on an object
Joule
Kinetic energy
Potential energy
Gravitational Potential Energy
Elastic Potential Energy
Spring constant
Mechanical Energy
5.1 WORK
What is work?
In everyday
language
To do something:
To learn
To kick a ball
To think
To hold a chair
In science
The product of the
magnitude of
the displacement
and the component of
a force parallel to that
displacement
Work is done – when a force causes a
displacement
Work done on a
car
W=F∙d
Constant horizontal force = F
Displacement = Δx = d
NO WORK is done on the object unless the
objects moves (i.e. has a displacement)!!!
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
W=F∙d
Constant horizontal force = F ∙ cos𝜃
Displacement = Δx = d
NO WORK is done on the object if the force is non –
parallel to the direction of the displacement
Example 1
(on a worksheet)
IS WORK DONE ON AN OBJECT?
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
Example 1
(on a worksheet)
IS WORK DONE ON AN OBJECT?
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
Reminder...
If WORK is done against GRAVITY
W=F∙d
W = mg ∙ h
POWER
Rate of Energy Transfer
the rate at which work is done or energy is
transformed
POWER
the rate at which work is done or energy is transformed
work
Power =
time interval
Power = force x speed
UNITS = J/s
= Watt
=W
HORSE POWER (hp)
• Another unit used
• 1 hp = 746 watts
Reminder...
If WORK is done against GRAVITY
W=F∙d
W = mg ∙ h
HOMEWORK
Page: 139 - 141
Problems: all
18
WORKSHEET EXAMPLE
Many 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 EXAMPLE
A light bulb is described as “having 60 watts.” What’s wrong with this
statement?
WORKSHEET EXAMPLE
A 193.0 kg curtain needs to be raised 7.5 m, at constant speed, in as close to
5.0 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 EXAMPLE
A 193.0 kg curtain needs to be raised 7.5 m, at constant speed, in as close to
5.0 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?
m = 193.0 kg
Δt = 5.0 s
d = 7.5 m
UNKNOWN?
P = ? watts
WORKSHEET EXAMPLE
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?
Investigation 5 – 1A
1. 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?
Example 1
(on a worksheet)
IS WORK DONE ON AN OBJECT?
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 EXAMPLE
Many 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
Assuming 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 EXAMPLE
A light bulb is described as having 60 watts. What’s wrong with this statement?
Light bulbs don’t have the
energy stored within them;
energy is transferred to them
in the form of electricity at a
rate of 60 J/s.
WORKSHEET EXAMPLE
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?