Energy and its Conservation Physics Part I Mechanical

Energy and its Conservation Physics

Part I • Mechanical Energy – Potential – Kinetic • Work Energy Theorem

Energy and Work • Energy is the ability to do work. • Work is the energy transferred to or from a system by a force that acts on it.

Energy • Symbol: E • Scalar • Units: – J, Joule – cal, calorie – kcal, kilocalorie (Cal)

Mechanical Energy • Potential Energy • Kinetic Energy

Kinetic Energy, KE= 1 mv 2 2 Energy of Motion Coyle, Greece, 2005

Problem 1 A bird of mass 2 kg is traveling at a speed of 6 m/s. What is its kinetic energy? (A: 36 J) If the speed of the bird doubles what happens to its kinetic energy?

Question • Can Kinetic Energy be Negative?

Work-Energy Theorem W= KE W=KEf-KEi The net work done on an object is equal to the change in its kinetic energy.

Problem 2 If Robin Hood applies a 15 N force to pull his bow back by 25 cm, with what speed will his 0. 150 kg arrow leave the bow? A: 7. 1 m/s

Problem 3 What is the work done by gravity as an 8 kg object falls from rest at 15 m to 5 m? What is the object’s speed at 5 m? A: 800 J, 14 m/s

Work Done by the Gravitational Force System is an object and the earth. As an object falls: W= mgh As an object is thrown up: W= -mgh

Potential Energy, PE=mgh • Energy of position • Stored energy • Symbol: U or PE • Unit: Joule • Compared to a Reference Point (base level)

Problem 4 A flower pot of mass 5 kg is located 15 m above the ground. A)What is its potential energy with respect to the ground? B) What work was done to raise it to its position? A: 750 J

Part II • Conservation of Energy

Conservation of Energy • Energy can neither be created nor destroyed. It can only change from form to form. • Mechanical Energy, PE and KE • Conservation of Mechanical Energy PE 1 + KE 1 = PE 2 + KE 2

What happens when friction is present? • When friction is present, the work done by the frictional force W=fd is transferred to heat energy. • Friction is a “non-conservative” force.

A Force is “Non-Conservative” if: • “the work it does on an object that moves between two points depends on the path taken. ” • “the work it does on an object that moves through a round trip is non-zero. ” • Example: friction, tension, normal force, propulsion forces.

A Force is “Conservative” if: • “ the work it does on an object that moves between two points depends only on the position of these two points and not on the path. ” • “the work it does on an object that moves through a round trip is zero. ” • Example: gravity, force of a spring.

Problem 5 A 3 kg watermelon sits on a table 1. 2 m above the ground. a) What is its PE at the top compared to the ground? b) What is its KE at the top? c) With what speed will it hit the ground? A: 36 J, 0 J, 4. 9 m/s

Problem 6 A penny is at the top of the Empire State Building 381 m above the ground. It is then released. With what speed will it hit the ground? A: 87. 3 m/s

Problem 7 -Roller Coaster A roller coaster car started from point A at a height of 100 m. What is its speed at point B? A: 44. 7 m/s