Chapter 5 Work and Energy. Work Energy Conservation of Energy Power.

Chapter 5

Work and Energy

Work and Energy

• Work• Energy• Conservation of Energy• Power

Everyday vs. Scientific Work

Definition of Work

Work = component of force in the direction of displacement ×

magnitude of displacement

Units of Work

1 Joule (J) = 1 Newton × 1 meter

Definition of Work

𝑊=𝐹 cos(𝜃)Δ𝑥

𝜃

𝐹

Δ𝑥

Which show positive work? Which show negative work?

If the object moves 5 m to the right, and the applied force is 2 N, how much work is done on the object?

60°60°

5 N 10 N

PracticeTwo forces act on the 10-kg block below, causing it to accelerate from rest over the period of 3 seconds. What is the work done on the block if these are the only two forces acting on the block?

Quick Check

Is work being done?• Picking up a book off the floor• Carrying a lunch tray across the cafeteria• Pushing a parked car• Pulling a suitcase across the airport

Energy

Kinetic Potential

Mechanical

?

??

Connecting work to energy…

Start with:

and

Kinetic Energy

the energy of an object due to its motion

Units = ?

𝐾𝐸=12𝑚𝑣2

PracticeCalculate the speed of an 8.0×104 kg airliner with a kinetic energy of 1.1×109 J.

Work-Kinetic Energy Theorem

• Work done by the object or work done on the object?

𝑊 net=Δ𝐾𝐸

PracticeOn a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10?

Ans=2.5 m

Potential Energythe energy associated with an object due to its

position, shape, or condition

*Potential energy is relative

Gravitational Elastic

Energy

Kinetic

Mechanical

Potential

Gravitational Potential Energy

is the height, relative to a zero level

Units = ?

𝑃 𝐸𝑔= h𝑚𝑔

60 m

25 m

500 kgA

B

C

Elastic Potential Energy

is the spring constantis the compressed or stretched distance

Units = ?

𝑃 𝐸elastic=12𝑘𝑥2

Elastic Potential Energy

REVIEW

Where is the maximum KE?

Where is the maximum PE?

Mechanical energy = ?

Conservation

• What does it mean that something is conserved?

• Can energy be created or destroyed?

Conservation of Mechanical Energy

𝑀𝐸 𝑖=𝑀𝐸 𝑓

True only if there is no friction

60 m

25 m

500 kgA

B

C

the rate at which work is done

Units = ?

Another form of the equation?

Power

𝑃=𝑊Δt

PracticeTwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.

Calculate the power delivered by the horses.

How much work is done by the two horses?