Poll

1
Poll
Frope
Suppose a block is being raised by a rope at
constant speed. Which is the correct free-body
diagram of the block?
Frope
Frope
F
F
F
grav
grav
grav
A B C
2
Poll
y
In each situation, choose the answer that
correctly describes the forces on the moving
object in the coordinate frame shown.
car speeding up on level road
x
3
Poll
child going down slide at constant speed
In each situation, choose the answer that
correctly describes the forces on the moving
object in the coordinate frame shown.
y
x
4
Poll
y
car speeding up on hill
In each situation, choose the answer that
correctly describes the forces on the moving
object in the coordinate frame shown.
x
5
Poll
You hold a baseball bat at one end. Your other
hand is 10 cm from the end of the bat. If the bat
has a mass 0.85 kg and the center of mass if 0.6
m from the end of the bat, what upward force must
you apply in order to hold the bat horizontallly?

1. 0.85 N
2. 0.05 N
3. 8.33 N
4. 50 N
5. 5.0 N

6
Chapter 6--Energy
Surroundings
System
7
Accounting Analogy
Surroundings
Your bank accounts
8
Conservation of Energy
9
Kinetic Energy - Energy of Motion
Objects that are moving have energy associated
with their motion, called kinetic energy.
Units are Joules (J). 1 J 1 N m
10
Timeout! -- Einstein
A particle of mass m has a total energy E
It can be broken into two parts energy at rest
(rest energy) and energy of motion (kinetic
energy).
11
Poll
A sprinter starts from rest at the starting line
and reaches a speed of approximately 10 m/s.
During the interval from the starting line to the
finish line, the kinetic energy of the sprinter
in other words, its change in kinetic
energy is .

1. increases positive
2. decreases negative
3. remains constant zero

12
Poll
You drive your car at a constant speed of 45 mph
for 30 s. During this time interval, the change
in your kinetic energy is

1. positive
2. negative
3. zero

13
Poll
While in your car, you double your speed from 20
mph to 40 mph. Your kinetic energy at 40 mph is

1. the same as your kinetic energy at 20 mph
2. 2 times your kinetic energy at 20 mph
3. 4 times your kinetic energy at 20 mph
4. 8 times your kinetic energy at 20 mph

14
Work
A force on the system by the surroundings does
work on the system, which either adds energy to
the system or removes energy from the
system. Positive work transfers energy to the
system from the surroundings. Negative work
transfers energy from from the system to the
surroundings.
15
Simple System
Consider a VERY simple system that only has
kinetic energy. If a force does positive work on
the system, the kinetic energy of the system

1. increases
2. decreases
3. remains constant

Surroundings
System
16
Poll
Consider a VERY simple system that only has
kinetic energy. If a force does negative work on
the system, the kinetic energy of the system

1. increases
2. decreases
3. remains constant

Surroundings
System
17
Calculating work
F
F
?r
Only the component of the force parallel to the
displacement (of the point of application of the
force) does work on the system.
18
Example
During a training run, a person pushes the
bobsled with a force of 100 N at an angle of 20?
downward (from the horizontal) for a distance of
5 m. How much work did she do on the bobsled?
19
Example
Suppose that you exert a force of 20 N to the
left on the handle of a rowing machine as it
moves 0.8 m to the right. How much work did you
do on the system?
20
Total Work
If more than one force does work on the system
then the total work done on the system is the sum
of the work done by each force.
21
Example
Suppose that the guy on the bike exerts a force
of 80 N to the right and the guy in the brown
jacket exerts a force of 70 N to the left on the
girl with the backpack as she skates 4 m to the
right. What is the total work done on her?
22
Example
The girl with the backpack has a mass of 50 kg
and an initial speed of 0.5 m/s. If the others do
40 J of work on her, what will be her speed?