Evans SPH 4U1

Physics Grade 12

Unit 4: Energy & Momentum

Note 1: Work/Conservation of Energy

Reference: Chapters 4.1 to 4.4 and 6.3

You should supplement these notes with our in-class discussions, proofs and derivations.

Work (scalar, measured in Joules (N.m):

Energy transferred to an object when a force, acting on the object, moves it through a distance. 

The product of the magnitude of the displacement times the component of the force in the direction of the displacement (area under a force displacement graph, dot product of force and displacement).

                                No work:    ~ No work is done on an object if the object doesn't move.

                                                   ~ No work is done if and are perpendicular (i.e. cos 90 degrees is 0)

                                                   ~ No work is done if no force is acting to produce the displacement.

Kinetic Energy: (scalar, measured in Joules)

Work-Energy Theorem:

Total work done on an object equals the change in the object's kinetic energy, provided there's no change in any other form of energy (i.e. gravitational potential energy).  Work = Change in Kinetic Energy

Example 1:

A disc slides along a shuffleboard in 3.0 m.  The mass of the disc is 0.5kg.  The coefficient of kinetic friction between the disc and the shuffleboard is 0.4.  What is the work done? -5.9 Joules

Example 2:

You push a block of mass 2.0 kg across a table by exerting a force of 9.0 N on it.  the magnitude of kinetic friction on the block is 5.0N.  The block starts from rest.  What's the speed of the block after it moves 0.3m? 1.1 m/s

Homework:

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Page 183 # 5,6

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