Transcript Work in Mechanical Systems Unit 2 Subunit 1

Objectives:

Learning Intentions –
 Students
will be able to:
 Describe
work in the scientific mean
 Solve for work when given force and distance

Success Criteria –
 We
will know we are successful when we can accurately
determine if work is done in a scenario
 Accurately solve for work when given the force and
distance
Think about it…


If you are sitting at home pushing buttons on
the remote control, how much work are you
doing?
In which instance am I doing more work:
 A)
I carry 10 books across the room all at once
 B)
I carry the 10 books across the room one at a
time
What it Means to
Do Work
The word work is used in many ways:
You work on science problems
 You go to work
 Your toaster doesn’t work
 Taking out the trash is too much work

Work
Amount of force applied to
an object over a distance.

In order for you to do work the
object must MOVE in the same
direction as the force

What’s work?





A scientist delivers a speech.
A body builder lifts 350 pounds above his
head.
A mother carries her baby from room to
room.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag to
her car?
What’s work?





A scientist delivers a speech. No
A body builder lifts 350 pounds above his
head. Yes
A mother carries her baby from room to
room. No
A father pushes a baby in a carriage. Yes
A woman carries a 20 km grocery bag to
her car? No
Work = Force X Distance
W = work done by the force
F = force on object along the
object’s line of motion
D = distance object moves while
force is acting
Units for Work
newtonXmeter
= joule (J)
Work is done only while the
force is applied.
Effects of Work
Change the position of
an object.
Change the velocity of
an object.

EXAMPLE:
How much work does it take to move a 500 N
car a distance of 20 m?
F = 500 N
d = 20 m
W = Force X Distance
W = 500 N X 20 m = 10,000 J
Independent Practice:

Complete the back side of the work sheet on your
own or with a partner

Use your notes from class

If you have questions raise your hand

We will review some of the answers before the end
of class
Do Now: Thursday, Jan. 30

In each of the following situations determine
whether or not work was done:
 An
ice skater’s partner lifts her up a distance of 1 m.
 The ice skater’s partner carries her across the ice a
distance of 3 m.
 After practice, the skater lifts her 20-N gym bag up
0.5m.

You pull your sled through the snow a distance of
500 m with a force of 200 N. How much work did
you do?
Objectives:

Learning Intentions –
 Students
will be able to:
 Describe
power in a scientific way
 Solve for power given work and time

Success Criteria –
 We
will know we are successful when we can accurately
determine the power of a scenario given work and time
POWER
POWER IS THE RATE AT
WHICH WORK IS DONE
OR
THE AMOUNT OF
WORK PER UNIT OF
TIME.
UNITS
Force = N
•Distance = m
•Time = sec
•Work = J
•Power = J/s = watts = W
•
Units for Power
Power

Given: A
motor does
6000 J of
work in 40
seconds.

Determine the
power of the
motor in
watts.
How much power does a person
weighing 750 newtons need if he
takes 20 seconds to walk up a
staircase 10 meters high?
Check for Understanding
Two physics students, Ben and Bonnie, are in the
weightlifting room. Bonnie lifts the 50 kg barbell
over her head (approximately .60 m) 10 times in
one minute; Ben lifts the 50 kg barbell the same
distance over his head 10 times in 10 seconds.
Which student does the most work?
Which student delivers the most power?
25
Ben and Bonnie do
the same amount of
work; they apply the
same force to lift the
same barbell the
same distance above
their heads.
Yet, Ben is the most
powerful since he
does the same work
in less time.
Review of Equations:

Work = force X distance
= work ÷ distance
Distance = work ÷ force
Force

Power = work ÷ time
Work
= power X time
Time = work ÷ power
Independent Practice:




Complete the practice problems for work and
power
Any questions let me know
Put the completed sheet in your class tray. If you do
not complete it DO SO FOR HOMEWORK
We will review some problems before the end of
class
Do Now: Fri. Jan. 31 Fill in the chart.
Objectives:

Learning Intentions –
 Students
will be able to:
 Explain
the concept of efficiency
 Solve for efficiency of a machine or scenario

Success Criteria –
 We
will know we are successful when we can accurately
solve for the efficiency of a machine
Review of Equations:

Work = force X distance
= work ÷ distance
Distance = work ÷ force
Force

Power = work ÷ time
Work
= power X time
Time = work ÷ power
Recall: Work

Work is the amount of force applied to an
object over a distance.

Work = Force X Distance

Unit: Joule
Efficiency and Power

1.
2.
Every process that is done by machines can be
simplified in terms of work:
work input: the work or energy supplied to the process
(or machine).
work output: the work or energy that comes out of the
process (or machine).
Efficiency


The efficiency of a machine is the ratio of usable
output work divided by total input work.
Efficiency is usually expressed in percent.
Output work (J)
efficiency = Wo
Wi
Input work (J)
x 100%
Independent Practice:



Complete the following practice problems on work,
power and efficiency
Questions, let me know
Should be completed before the activity on
Monday