Work and Power Chapter 3, Lesson 1

Requirements for Work 1. a force is applied to an object 2. the object moves in the same direction as the force W = F x d Work = force x distance (Joules) (Newtons) (meters)

Calculating Work A student pushes a desk 2.0 meters across the floor using a constant force of 25.0 Newtons. How much work did the student do on the desk?

A child holds a toy truck 2 A child holds a toy truck 2.5 meters above the floor using a force of 22 Newtons. How much work is the child doing on the truck?

Lifting Objects The work done to lift an object equals the weight of the object multiplied by the distance it is lifted. Hutchings Photography/Digital Light Source

Which force is doing work? When the applied force and the motion of the object are NOT in the same direction, only the part of the applied force that is in the direction of motion is doing work. Hutchings Photography/Digital Light Source

Work and Energy Doing work on a tray transfers energy to the tray. The added energy can be either kinetic energy or potential energy.

P = W / t What is Power? Power = Work / time Power is the rate at which work is done. OR…Power is how fast energy is transferred to the object. P = W / t Power = Work / time (Watts) (Joules) (seconds)

Two people each do 1.2 Joules of work to slide a book across the floor. One person does the work in 2 seconds. The other person does the work in 3 seconds. Calculate and compare their power.

A tractor does 5000 Joules of work to plow a field A tractor does 5000 Joules of work to plow a field. It accomplishes the work in 3 minutes. How powerful is the tractor?

A pulley takes 10 seconds to accelerate a 5 kg mass at 3 m/s/s for a distance of 6 meters. How powerful is the pulley?

MiniQuiz 1. Is work being done when a weightlifter holds barbells over his head? 2. What is the SI unit for work? 3. T or F? In order for work to be done on an object, the and force and the object’s motion are always in different directions. 4. T or F? More powerful machines work at an increased rate. 5. T or F? When you lift an object, its potential energy increases.