REVIEW

A boy pulls a wagon with a force of 6 N east as another boy pushes it with a force

of 4N east. What is the net force.

REVIEW

What is the difference between balanced and

unbalanced forces?

What is WORK? Select the activities below that

require the least amount of work.a) Carrying heavy books homeb) Reading a 300 page novelc) Lifting a 45kg massd) Holding a steel beam in place for 3 hre) Jacking up a car

What is WORK? Work: using a force to move an object

a distance Both the force and the motion of the

object are in the same direction

Work or Not?1. A scientist delivers a speech to an audience

of his peers. 2. A body builder lifts 350 pounds above his

head. 3. A mother carries her baby from room to

room. 4. A father pushes a baby in a carriage.5. A woman carries a 20 kg grocery bag to her

car?

Which is work?

What is WORK?Work is measured in Joules.

ExamplesExample 1: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

ExamplesExample 2: A student lifts a box of books that weighs 200 N. The box is lifted 2.5 m. How much work does the student do on the box?

ExamplesExample 3: If a man pushes a concrete block with 20 N of force and it does not move, how much work has he done?

What is Power? Power: the rate at which work is done

Measured in watts

Power = work ÷ time

ExamplesExample 4: A box that weighs 375 N is lifted a distance of 35.0 m straight up by a cable attached to a motor. The job is done in 5.0 seconds. What power is developed by the motor in watts?

ExamplesExample 5: How much power will it take to move a 100 N mass a distance of 10 meters in 5 seconds?

ExamplesExample 6: How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order.

Force = Mass x Acceleration Work = Force x Distance

Power = Work/Time

ExamplesExample 7: Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Who did more work?

Work = Force x DistancePower = Work/Time

ExamplesBen and Bonnie do the

same amount of work; they apply the same force to lift the same barbell the same distance above their heads.

Yet, Ben is the most powerful since he does the

same work in less time. Power and time are

inversely proportional.

Exit slip1.How much work is done by a person who uses a force of 27.5N to move a grocery buggy 12.3m?

2. What is the difference between power and work?