Frope

Fgrav Fgrav F grav

Suppose a block is being raised by a rope at constant speed. Which is the correct free-body diagram of the block?

A B C

Frope

Frope

Poll

Poll

In each situation, choose the answer that correctly describes the forces on the moving object in the coordinate frame shown.

car speeding upon level road

x

y

Fy = 0A.Fx = 0,

Fy ≠ 0B.Fx = 0,

Fy = 0C.Fx ≠ 0,

Fy ≠ 0D.Fx ≠ 0,

Poll

In each situation, choose the answer that correctly describes the forces on the moving object in the coordinate frame shown.

Fy = 0A.Fx = 0,

Fy ≠ 0B.Fx = 0,

Fy = 0C.Fx ≠ 0,

Fy ≠ 0D.Fx ≠ 0,

child going downslide at constant speed

x

y• •

Poll

In each situation, choose the answer that correctly describes the forces on the moving object in the coordinate frame shown.

Fy = 0A.Fx = 0,

Fy ≠ 0B.Fx = 0,

Fy = 0C.Fx ≠ 0,

Fy ≠ 0D.Fx ≠ 0,

car speeding upon hill

x

y

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

Chapter 6--Energy

System

Surroundings

Accounting Analogy

Your bank accounts

Surroundings

Conservation of Energy

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

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).

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

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

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

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.

Simple System

System

Surroundings

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

Poll

System

Surroundings

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

Calculating workF 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.

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?

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?

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.

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?

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?