Bell Ringer 3-1-10

1
Bell Ringer 3-1-10

• Define work in your own words.
• What are two examples of work?
• Define power in your own words.
• What is an example of something that has power?

2
Chapter 12 Section 1What Is Work?
3
Work

• Occurs when a force causes an object to move in
  the same direction in which the force is exerted.
• Work involves motion, not just effort.

4

• Work is done only when the force you exert on an
  object is in the same direction as the objects
  motion.
• Lifting a clothes basket is work, but carrying
  it while walking is not work.
• When a force is exerted at an angle, only the
  part of the force that is in the same direction
  as the motion does work.

5

• Work can be calculated using the formula
• Work Force x Distance
• The SI unit for work is joule (J).
• Problem If you push a box 20m forward by
  applying a force of 15N in the forward direction,
  what is the work you have done on the box?
• Answer 15N x 20m 300 Joules

6
Lets try these

• If a model airplane exerts 0.25 Newtons over a
  distance of 10 meters, what is the work done?
• A runner impacts the ground with a force of 1083
  N. If he runs a distance of 3000 m, how much work
  does the runner do?
• A runner impacts the ground with a force on her
  heel of 2000 N. If she runs a distance of 3000 m,
  how much work does the runner do?
• An average golfer hits a golf ball with a force
  of 3800 N. If the club contacts the ball for a
  distance of 0.01 m, how much work is done?

7

• 5. A strong golfer hits a golf ball with a force
  of 18,000 N. If the club contacts the ball for a
  distance of 0.01 m, how much work is done?
• 6. An average golfer uses a putter on a golf ball
  with a force of 1200 N. If the club contacts the
  ball for a distance of 0.01 m, how much work is
  done?
• 7. An experimental jet engine flies a distance of
  3000 m in a test flight with a force of 3700 N,
  how much work is done?
• 8. A jet engine moves a distance of 4,000 m with
  a force of 4400 N, how much work does the jet
  engine do?

8

• 9. A basket of apples weighs 12 N. If the basket
  is lifted by a grocer a distance of 0.8 m, how
  much work does the grocer do?
• 10. A sack of hamburgers weighs 6 N. If the sack
  is lifted a distance of 1 m, how much work is
  done?
• 11. A dozen bagels weighs 11 N. If the bag is
  lifted a distance of 1.5 m, how much work is
  done?

9
Answers

• 2.5 J
• 3,249,000 J
• 6,000,000 J
• 38 J
• 180 J
• 12 J
• 11,100,000 J
• 17,600,000 J
• 9.6 J
• 6 J
• 16.5 J

10
Power

• The rate at which work is done or how quickly
  work is done.
• Power can be calculated using the formula
• Power work or force x distance
• time time
• The SI unit for power is the watt (W).

11
Examples of Power

• If a swimmer does 3654 joules of work in 65
  seconds, what is the swimmer's power output?
• Answer P  3654J   56.2 watts
•           65s
• A set of pulleys lifts a piano and does 3,356
  joules of work.  If the piano is lifted in 75
  seconds, how much power is used?
• Answer 44.7 watts

12
Calculate Power

• 1.  How much work is done in order to cook a bag
  of popcorn in a 500 watt microwave oven for 5.5
  minutes?
• 2.  What is the power of an electric toothbrush
  if it can do 755.8 joules of work in 75 seconds?
• 3.  Sara and Josh do the exact same amount of
  work.  Sara does the work in 2.3 hours and Josh
  does it in 2.5 hours.  Who is more powerful?
   Explain.
•  

13

• 4. A dock worker lifts a 375N crate a distance of
  0.5m over his head in 2.3 seconds.  What is his
  power output?
• Hint  You must do a calculation before you can
  begin to solve for power.  
• 5.  An athlete is using the row machine in the
  gym.  She does 3245 joules of work on the oars in
  72 seconds.  What is her power output?
• 6.  John is walking to class up the front
  staircase.  He weighs 595N and the stairs are
  3.15 meters long.  Calculate his power output if
  he climbs the stairs in 15.2 seconds.

14
(No Transcript)
15
(No Transcript)