Unit Two: Mechanical Unit Two: Mechanical EnergyEnergy

• A: Work-– in everyday life, it means many things– In physics, work is “what is

accomplished when a force acts on an object that moves” or the measure of a job done

• Work is the product of an applied force and the displacement of an object in the direction of the applied force

• Equation: W= • where- F= applied force and d=

displacement• Units= N*m which is a.k.a a Joule (J)

ExamplesExamples• 1. When you push a wall, no work is

done because the wall does not move

• 2. When we lift a load against the Earth’s gravity, work is done– The heavier the load the more work that

is done

Situations where work is not Situations where work is not donedone

1. There is a force but no displacement (i.e pushing on a wall)

2. A. The force is perpendicular to the displacement (i.e walking forward while carrying books)

B. Satellites orbiting the earth

3. An object moves without an applied force (skating on ice)

Word ProblemsWord Problems1. If a person pushes a car with a force

of 800N for a displacement of 30m, how much work was done?

2. Find the work required to lift 3.0 x 107 to a height of 5.0m?

Positive workPositive work• is done when the applied force and

the displacement act in the same direction.

• When the weight is raised above his head, the displacement and the applied force are in the same direction.

Example:Example:• If the total mass of the barbell and

the weight is 135 kg, and he raises the barbell a distance of 65 cm above his chest with a constant velocity what is the work done on the barbell by the weight lifter?

Negative workNegative work• is done when the applied force is still

upward; however the displacement is opposite in direction.

• When the weight is lowered, the applied force is the same upward direction; however the displacement is opposite in direction.

Sample ProblemSample Problem• A box that weighs 200 N is lifted a

distance of 0.5 m straight up. How much work is done?

• To lift a box of 200 N upward requires you to apply an upward force of 200 N. If this box must be lifted 0.5 m upward, then the displacement would be 0.5 m [up]. Note: the applied force and the displacement are in the same direction.

• In some cases, the force and displacement moved are not in the same direction:–

– *consider the component of force parallel to the direction in which the object moves

– To find this, use W=Fd*cos Θ

ExampleExample– A child pulls a wagon by exerting a force

of 20N at an angle of 30o to the

horizontal. If the child pulls it 100m along the ground, calculate the work done.

AnswerAnswer