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:
Page 181 #7
Page 183 # 5,6
Page 186 #5, 7