# Work and Energy

I. Section 5.1—Work

A. Definition of work

1. Work does not mean the same thing in Physics as it does in the everyday sense of the word. 2. Work is defined as a force causing a displacement.

Work = force x displacement

W = Fd

3. Work is NOT done on an object unless the displacement is greater than zero 4. The only forces that are considered to do work are those that are parallel to the displacement. 5. For this reason we use our trigonometric functions to calculate forces applied at an angle.

Insert Fig 5-2

6. Note that Θ is the angle between the applied force and the displacement. 7. Work is described in Newtons x meters (force x displacement). The unit of work is the Joule (J) 8. 1 Newton meter = 1 Joule

9. Work is a vector with both direction AND magnitude. This means WORK CAN BE NEGATIVE! 10. Negative work is most commonly used to slow an object down or decrease its velocity.

II. Section 5-2: Energy

A. Kinetic Energy

1. Kinetic energy is associated with an object in motion. 2. Kinetic energy depends on speed and mass

Kinetic Energy = ½mv2

3. Kinetic energy is a scalar and will use Joules as its standard unit. 4. It takes some work to change an object’s kinetic energy either by changing the velocity or changing the mass. 5. This fact leads us to the Work-Kinetic Energy Theorem

WORK-KINETIC ENERGY THEOREM

Wnet = ∆KE

Net work = change in kinetic energy

6. Notice that Wnet = ∆KE

and

Wnet = Fnetd(cosΘ) ( (from section 1) therefore,...

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