Your Federal Quarterly Tax Payments are due April 15th

# Work in Mechanical System by ewghwehws

VIEWS: 3 PAGES: 17

• pg 1
```									    Work in
Mechanical System
Work in Mechanical System
1. What is work?
- exerting a force over a distance
2. When is work done?
- work is only done when a force
causes something to move or change
- the force and the movement MUST
be in the same direction for work to
occur
Work in Mechanical System
3. A man picks up a stack of books.
Does he do work on the books?
- yes; the force and the movement
were in the same direction
4. The man holds the books at his waist.
Is he doing work on the books?
- no; the books are not moving
Work in Mechanical System
5. The same man carries the books
across the room. Is work done on
the books?
- no; the force on the books is not in
the same direction as the books are
moving
Work in Mechanical System
6. How do we calculate work?
-W=FxD
- work = applied force x distance
moved
7. What are the units for work?
- English – foot-pounds (ft-lbs)
- SI – joule (J) – “jewel”
- Newton-meters are also used in SI
Work in Mechanical System
8. Example – A weight lifter applies a
vertical force to raise a 200 lb barbell
5 feet high. How much work is done?
-W=FxD
W = (200 lb)(5 ft)
W = 1000 ft-lb
Work in Mechanical System
9. Example – The weight lifter holds
the weight for 10 minutes. How
much work is done?
- none – the weight is not moving
Work in Mechanical System
10. Example – An electric truck is used
to pull a cart a distance of 100
meters. A force of 900 newtons is
required to keep the cart moving at a
constant speed. How much work is
done on the cart?
-W=FxD
W = (900N)(100m)
W = 90,000 J
Work in Mechanical System
10. What is efficiency?
- the relationship of output work to
input work of a machine
- efficiency is always less than 100%
11. What is input work?
- work done on a machine by a person
12. What is output work?
- work done by the machine
Work in Mechanical System
13. How do we calculate efficiency?
% Efficiency = output work x 100%
input work
Work in Mechanical System
14. Example – A block and tackle
(series of pulleys) is used to lift an
engine. The engine weighs 600 lbs
and is 0.9 ft. The operator pulls with
a force of 100 lbs over a distance of 6
ft. Find the A) input work, B) the
output work, and C) the efficiency of
the machine.
A. Input Work = F x D
Input Work = (100 lbs)(6 ft)
Input Work = 600 ft-lbs
B. Output Work = F x D
Output Work = (600 lbs)(0.9 ft)
Output Work = 540 ft-lbs
C. Efficiency = Output x 100%
Input
Efficiency = 540 ft-lbs x 100%
600 ft-lbs
Efficiency = 90 %
Work in Mechanical System
15. What else can do work in the MS?
- torque does work when it causes an
object to rotate
16. What are some examples of torque
doing work?
- turning wheels on car, a record
Work in Mechanical System
17. How do we calculate work done by
torque?
- Work = Applied Torque x Angle of
Rotation
W=Txq
18. What is q?
- angle of rotation measured in radians;
Greek letter theta
Work in Mechanical System
- another way to measure angles
other than degrees
- one revolution (360o) = 2p radians
- to find the angle of revolution in
revolutions and multiply by 2p
20. Example – An electric motor is used
to drive a water pump. The motor
provides a torque of 150 N-m. How
much work is done while the motor
through 40 revolutions?
- You must
1) Find the q
2) solve for the work
1) q = (# of revolutions)(2p)
q = (40 revolutions)(2p)