Unit 10 Work, Power, & Energy

Work Work is the transfer of energy that occurs when a force makes an object move

Work To do work: A force must be applied The motion must be in the same direction as the applied force

Work The SI unit for work is Joules or J. The work done on a system only depends on forces and distances

Calculating Work Work occurs when a force moves an object some distance (in the same direction!) W = F x d W= Work (Joules) F= Force (Newtons) d= distance (meters) What variables go where in the triangle? W d F

Calculating Work Jessica picks up a book on the floor. Pushing up against gravity, she applies a force of 2 N and moves the book 1.5 m higher. How much work did Jessica do on the book?

Calculating Work While the book is still resting on Jessica’s hand, she moves it sideways with a force of 0.5 N to a position 0.5 m away. How much work did Jessica do on the book?

Power Power is the rate work is done . Power is measured in Watts (W) or kilowatts (kW).

Power W P t Power is the rate of Work done Power is the rate of Joules used per second 1 Joule = 1Watt/1 second P = W/t P= power (watts) W= work (Joules) t= time (seconds) What variables go where in the triangle? W P t

Calculating Power Watch me do one! How long will it take a runner to do 1,000 J of work if his power is 125 W? P = W/t

Try it on your own Jessica moves a book with a force of 2 N. The book moves 1.5 m in the direction of the force. How much work has Jessica done? If it takes Jessica 2 s to move the book, calculate Jessica’s power.

Energy What do you think energy is?

What is Energy? Energy is the ability to cause change Energy can change the object itself, or its surroundings Energy is measured in Joules

Forms of Energy Some forms of energy are electrical, chemical, radiant, thermal, and mechanical Energy can change between these forms

Kinetic Energy Kinetic energy is the energy of motion KE = ½ mv2 m= mass (kg) v=velocity (m/s) (Can this formula be put in a triangle?)

Kinetic Energy Find the kinetic energy of a 10 kg bowling ball that is rolling at 3 m/s. KE = ½ mv2 KE = (½)(10kg)(3 m/s) 2 KE = 45 J

Practice Problems A ball rolls down a hill and ends up moving 4.5 m/s. If the mass of the ball is 3kg, what is the ball’s kinetic energy?

Practice Problems If a ball is moving 5 m/s and is recorded as having 55J of energy, what is the boxes mass?

Potential Energy Potential Energy is stored in an object “Can or could that object change or do something?” Potential Energy comes from an object’s position Some types are elastic PE, electrical PE, and gravitational PE

Gravitational Potential Energy Gravitational Potential Energy (GPE) is energy stored in objects that can fall. Depends on an object’s height from the ground. GPE = mgh M= mass (kg) h= height (meters) Remember: g = 9.8 m/s2

Gravitational Potential Energy My slinky sits on my desk, 1 m above the floor. The mass of the slinky is 0.5 kg. Find its GPE. I trip, hit my desk, and knocks the slinky on the floor. Find its GPE.

Conservation of Energy

Law of Conservation of Energy Energy is never created or destroyed, only transferred or transformed * The total amount of energy in a closed system never changes

Transfer of Energy

Transforming Energy Mechanical Energy is the total amount of Kinetic Energy and Potential Energy in a system A system is a group of objects that work together

How to Calculate Mechanical Energy Add the kinetics energy and the potential energy together! Mechanical Energy = Kinetic Energy + Potential Energy ME = KE + PE ME = ½ mv2 + mgh

Mechanical Energy For a falling object: GPE decreases because height decreases KE increases because gravity increases the velocity

Example problem What is the mechanical energy of a box that has a potential energy of 15J and a kinetic energy of 32J?

Mechanical Energy What is the mechanical energy of a 75 kg man on a diving board 15 m above the ground?

Mechanical Energy The man jumps. After he has fallen 5 m, he is traveling about 9.9 m/s. Find the mechanical energy of the man.

Conservation of Energy Why do we lose mechanical energy? Energy can be taken out of a system if WORK is done, for example, by friction Friction produces heat that takes energy!