Energy in a Mechanical System

5.1

Definitions:

Property—quality or trait belonging to the system

Energy—the ability to do work

Kinetic energy—energy of motion

Internal energy—sum of the microscopic kinetic and potential energies of all the atoms and molecules that make up a system

Total energy—the sum of kinetic, potential, and internal energies of an object or system

Law of conservation of energy—energy cannot be created nor destroyed; it can change form

I. Kinetic Energy

A.

B. Kinetic energy = ½ mass x speed2

C. Unit for energy the same as for work—Joules, foot-pounds

KE 1

2mv2

II. Kinetic Energy in Rotational Systems

A. Moment of inertia 1. A measure of a body's resistance to angular acceleration, equal to: the product of the mass of a particle and the square of its distance from a reference

2. I

3.

4. Kinetic energy = ½ (moment of inertia) (angular speed)2

KE 1

2mr22

KE 1

2I2

5. center of mass—measure from that point

6. The moment of inertia of an object is the sum of the moments of all the particles that make up the object.

7. p. 235—formulas for moments of inertia for different-shaped objects.

III. The Work-Energy Theorem A. Work—a measure of the energy that is

transmitted by a force, such as a contact, gravitational, electrical, or magnetic force.

B. Example: a skater striking a hockey puck with a stick

C. W = ΔKE