Group Work

1
Group Work
 

• The 2004 Tour de Frances Alpe dHuez time trial
  stage was a steep climb with its finish 1200 m
  higher than the start. Lance Armstrong won with a
  time Dt of 3941 (2381 s). He and his gear had a
  combined mass of 84 kg.

What was Lances change in gravitational
potential energy DE over the stage?
2
Announcements

• Test 1 next Thursday
• Covers material through today (energy)
• Homework due in class Tuesday
• Drill 4 due next Tuesday night

3
Objective

• Calculate the average power necessary to change
  an objects energy by a given amount in a given
  time.

4
Power

• Rate of doing work

DE change in energy ( work) Dt time interval
5
Units of Power
J/s
W watt
6
Group Work
 

• The 2004 Tour de Frances Alpe dHuez time trial
  stage was a steep climb with its finish 1200 m
  higher than the start. Lance Armstrong won with a
  time Dt of 3941 (2381 s). He and his gear had a
  combined mass of 84 kg.

What was Lances average power during the stage?
7
Power

• A different but equivalent formula

Dd change in position
8
Group Work

• Show that (forcevelocity) gives the same units
  as (work/time).

force units velocity units work units
time units
9
Conservation of Energy
10
Whats the point?

• Nature keeps careful account of energy.

11
Objectives

• Track energy transfers in interactions.

12
Simple Machines

• What do they give you?
• What is the trade-off?

13
Simple Machines

• Input and output forces can be different
• Trade-off is distance traveled
• Work is unchanged
• work input work output

14
CPS Question

• Which is greater?
• A. The force F1 exerted downward on the lever arm
• B. The force F2 exerted upward on the rock
• C. Neither

15
CPS Question

• Which is greater?
• A. The distance d1 traveled by the lever arm
• B. The distance d2 traveled by the rock
• C. Neither

16
CPS Question

• Which is greater?
• A. The work done on the lever arm
• B. The work done on the rock
• C. Neither

17
Simple Machines
18
Convert Potential ? Kinetic

• Gravity exerts force mg as object drops distance
  h.
• work mgh
• PE decreases mgh
• KE increases mgh

Source Griffith, The Physics of Everyday
Phenomena
19
Conservation of Energy

• Energy can be transferred between objects or
  transformed into different forms, but the total
  amount of energy can never change.

20
Conservation of Energy
Source Griffith, The Physics of Everyday
Phenomena
21
CPS Question

• The piglet has a choice of three frictionless
  slides to descend. Which slides path gives the
  piglet the greatest change in potential energy?

A
B
C

• Same for all.

22
CPS Question

• The piglet has a choice of three frictionless
  slides to descend. At the end of which slide
  will the piglet have the greatest kinetic energy?

A
B
C

• Same for all.

23
CPS Question

• The piglet has a choice of three frictionless
  slides to descend. At the end of which slide
  will the piglet have the greatest speed?

A
B
C

• Same for all.

24
CPS Question

• The piglet has a choice of three frictionless
  slides to descend. Which slide will the piglet
  descend the quickest?

A
B
C

• Same for all.

25
Rebound and Stop
26
CPS Question

• Which changes its momentum the most?
• A moving object that stops when it hits a
  barrier.
• A moving object that bounces back from a barrier.

Hint Momentum is a vector.
27
CPS Question

• Which changes its kinetic energy the most?
• A moving object that stops when it hits a
  barrier.
• A moving object that bounces back from a barrier.

Hint kinetic energy depends on speed.
28
Group Work

• A perfectly elastic ball of mass m collides with
  velocity v directly perpendicular into a rigid,
  massive wall.
• What is the direction of the net force on the
  ball as it squishes into the wall?
• What is the direction of the net force on the
  ball as it pushes away from the wall?

29
Group Work

• A perfectly elastic ball of mass m collides with
  velocity v directly perpendicular into a rigid,
  massive wall.
• What is the sign of the work done on the ball as
  it squishes into the wall?
• What is the sign of the work done on the ball as
  it pushes away from the wall?

30
Group Work

• A perfectly elastic ball of mass m collides with
  velocity v directly perpendicular into a rigid,
  massive wall.
• The balls initial and final speeds are equal.
  What is the relation between work done during
  compression and work done during recovery?