Work and Power Chapter 4

What is Work? Work – is done on an object when the object moves in the same direction in which the force is exerted. No work without motion

Force vs. Work Applying a force does not always result in work being done. If the applied force does not result in motion, then work has not been done. Work is done when: 1) The force applied causes an object to move. 2) The direction of the object’s motion is the same as the direction of the force.

Formula: work = force X distance Calculating Work Formula: work = force X distance Units: - Force = Newtons - Distance = meters - Work = Newton - meters (N - m).

James Prescott Joule Studied work in the mid- 1800’s One Newton-meter = one Joules (J) A Joule is the unit for both work and energy.

Power – the amount of work done on an object in a unit of time.

The same amount of work is done sanding by hand and sanding with an electric sander. Why would you use an electric sander? Sanding occurs in less time because the electric sander has more power.

Calculating Power Formula: Power = work/time (P = W/t or P = f x d ) t Units: - Work = Newton-meters or Joules - Time = seconds - Power = Newton-meters/sec., joules/sec., watts, kilowatts, or horsepower.

Power is measured in J/s, this is also known as a watt (W) in honor of James Watt, who made improvements to the steam engine. 1 J/s = 1 W 1kW = 1,000 W

Power Math Practice A tow truck exerts a force of 11,000 Newtons to pull a car out of a ditch. It moves the car a distance of 5.0 meters in 25 seconds. What is the power of the tow truck? Power = force X distance time Power = 11,000 N X 5.0 m 25 s Power = 55,000 N·m 25s Power = 2,200 N·m/s or 2,200 J/s

When people talk about engines for vehicles, they use another power unit called horsepower. 1 horsepower = 746 watts

Let’s practice some work and power math