Chapter 5.2

What do you think? What is meant when scientists say a quantity is conserved? Describe examples of quantities that are conserved. Are they always conserved? If not, why?

Mechanical Energy (ME) ME = KE + PE g + PE elastic Does not include the many other types of energy, such as thermal energy, chemical potential energy, and others ME is not a new form of energy. Just a combination of KE and PE

Classroom Practice Problems Suppose a 1.00 kg book is dropped from a height of 2.00 m. Assume no air resistance. Calculate the PE and the KE at the instant the book is released. Answer: PE = 19.6 J, KE = 0 J What do you think the PE and the KE just as the book reaches the floor will be? Answer: PE = 0 J, KE = 19.6 J

Table of Values for the Falling Book h (m)PE(J)KE(J)ME(J)

Conservation of Mechanical Energy The sum of KE and PE remains constant. One type of energy changes into another type. For the falling book, the PE of the book changed into KE as it fell. As a ball rolls up a hill, KE is changed into PE.

Law of Conservation of Energy Energy may change from one form to another, but the total amount of energy never changes. Aka “energy cannot be created or destroyed” – just converted

Law of Conservation of Energy She should swing forever right? Friction converts some mechanical energy into thermal energy. motion Air resistance friction Some mechanical energy is converted to thermal energy because of friction and air resistance

Conservation of Energy in your Body Chemical potential energy from food is Used to Fuel metabolic reactions in your body Make your heart beat Move your arm Think about what we are talking about Given off as heat to surroundings

What is the mechanical energy of a 500 kg roller coaster car moving with a speed of 3 m/s at the top of a hill that is 30 m high? Given: M = 500 kg V = 3 m/s H = 30 m Find: ME ME = PE + KE ME = mgh + ½ mv 2 ME = [(500)(9.8)(30)] + [(1/2)(500)(3) 2 ] ME = ME = 149,250 J

A boulder with a mass of 2500 kg on a ledge 200 m above the ground falls. If the boulder’s mechanical energy is conserved, what is the speed of the boulder just before it hits the ground? Given: M = 2,500 kg H = 200 m Find: V PE= mgh KE = ½ mv 2 KE = PE PE = 2500 x 200 x 9.8 PE = 4,900,000 J = KE KE = ½ mv 2 4,900,000 = ½ (2500) v 2 4,900,000 = 1250v = v = v

Page 148 in your book

If friction can be ignored, the ___ of a system doesn’t change Mechanical energy

The energy stored in a compressed spring is Elastic potential energy

The muscles of a runner transform chemical potential energy into Kinetic energy

According to the ____, the amount of energy in the universe doesn’t change. Law of conservation of energy

For which of the following is kinetic energy converted into potential? a. A boulder rolls down a hill b. A ball is thrown upward c. A swing comes to a stop d. A bowling ball rolls horizontally

When an energy transformation occurs, which of the following is true? a. Mechanical energy doesn’t change b. Mechanical energy is lost c. The total energy doesn’t change d. Mass is converted into energy

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