Chapter 5 Work and Energy

Work and Energy Work Energy Conservation of Energy Power

Everyday vs. Scientific Work

1 Joule (J) = 1 Newton × 1 meter 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=𝐹cos(𝜃)Δ𝑥 𝐹 𝜃 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°

Practice Two 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? 5 N 10 N

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 Mechanical ? Kinetic Potential

Connecting work to energy… Start with: 𝑊 net =𝐹Δ𝑥 and 𝑣 𝑓 2 = 𝑣 𝑖 2 +2𝑎Δ𝑥 𝑊 net = ?

the energy of an object due to its motion Kinetic Energy the energy of an object due to its motion Units = ? 𝐾𝐸= 1 2 𝑚 𝑣 2

Practice Calculate the speed of an 8 Practice Calculate 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

Practice On a frozen pond, a person kicks a 10 Practice On 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 Energy the energy associated with an object due to its position, shape, or condition *Potential energy is relative

Energy Mechanical Kinetic Potential Gravitational Elastic

Gravitational Potential Energy ℎ is the height, relative to a zero level Units = ? 𝑃 𝐸 𝑔 =𝑚𝑔ℎ

500 kg A 60 m C 25 m B

Elastic Potential Energy 𝑘 is the spring constant 𝑥 is the compressed or stretched distance Units = ? 𝑃 𝐸 elastic = 1 2 𝑘 𝑥 2

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 True only if there is no friction 𝑀 𝐸 𝑖 =𝑀 𝐸 𝑓

500 kg A 60 m C 25 m B

the rate at which work is done Units = ? Another form of the equation? Power the rate at which work is done Units = ? Another form of the equation? 𝑃= 𝑊 Δt

Practice Two horses pull a cart. Each exerts a force of 250 Practice Two 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?