Physics: Work and Power: Using Force to Move Objects
Calculating work, power, force, distance, and time
Physics: Work and Power: Using Force to Move Objects
Calculating work, power, force, distance, and time
Physics - Grade 6-8
- 1
A student pushes a box with a force of 50 newtons for a distance of 4 meters. How much work does the student do on the box?
Use work = force x distance.
The student does 200 joules of work. Work = force x distance, so 50 N x 4 m = 200 J. - 2
A student pushes hard on a heavy cabinet with a force of 100 newtons, but the cabinet does not move. How much work is done on the cabinet by the student? Explain.
The student does 0 joules of work on the cabinet because the cabinet does not move. In physics, work requires both force and distance in the direction of the force. - 3
A mover does 120 joules of work while pushing a crate 6 meters across the floor. What force did the mover use?
Rearrange the formula to solve for force.
The mover used a force of 20 newtons. Since work = force x distance, force = work ÷ distance, so 120 J ÷ 6 m = 20 N. - 4
A student lifts a stack of books and does 90 joules of work in 3 seconds. What is the student's power output?
Power tells how fast work is done.
The student's power output is 30 watts. Power = work ÷ time, so 90 J ÷ 3 s = 30 W. - 5
Two students each do 600 joules of work moving boxes. Student A takes 10 seconds, and Student B takes 5 seconds. Which student has the greater power output, and what is each power output?
Student B has the greater power output. Student A's power is 600 J ÷ 10 s = 60 W, and Student B's power is 600 J ÷ 5 s = 120 W. - 6
A person pulls a sled 5 meters using a horizontal force of 40 newtons. How much work is done on the sled in the direction of motion?
Only the force in the direction of motion is used in this calculation.
The work done on the sled is 200 joules. Work = force x distance, so 40 N x 5 m = 200 J. - 7
A force-distance graph shows a constant force of 10 newtons acting over a distance of 8 meters. What is the work done?
For a rectangle on a force-distance graph, area = height x width.
The work done is 80 joules. On a force-distance graph, the work is the area under the graph, so 10 N x 8 m = 80 J. - 8
Complete the calculation: A machine does 240 joules of work in 12 seconds. What is the machine's power output?
The machine's power output is 20 watts. Power = work ÷ time, so 240 J ÷ 12 s = 20 W. - 9
Write the correct unit for each quantity: force, distance, work, time, and power.
Match each formula part to its standard metric unit.
Force is measured in newtons, distance is measured in meters, work is measured in joules, time is measured in seconds, and power is measured in watts. - 10
A student with a weight of 450 newtons climbs stairs that are 3 meters high. How much work does the student do against gravity? If the climb takes 9 seconds, what is the student's power output?
Use the vertical height of the stairs, not the distance along the steps.
The student does 1,350 joules of work against gravity because 450 N x 3 m = 1,350 J. The student's power output is 150 watts because 1,350 J ÷ 9 s = 150 W. - 11
A person pushes a lawn mower with a force of 80 newtons for 12 meters. The push takes 6 seconds. How much work is done, and what is the person's power output?
The work done is 960 joules because 80 N x 12 m = 960 J. The power output is 160 watts because 960 J ÷ 6 s = 160 W. - 12
A student carries a backpack while walking 20 meters down a level hallway. The student's upward force on the backpack is 60 newtons. How much work does the upward force do on the backpack in the direction of walking? Explain.
Compare the direction of the force with the direction of the motion.
The upward force does 0 joules of work in the direction of walking because the upward force is perpendicular to the horizontal motion. Work is done only when force acts in the same direction as the movement. - 13
A ramp lets a worker move a box to a platform 2 meters high. Lifting the box straight up requires 300 newtons of force for 2 meters. Pushing it up the ramp requires 100 newtons of force for 6 meters. In an ideal situation with no friction, how much work is done in each case?
A ramp can reduce the force needed, but it increases the distance.
In both cases, 600 joules of work are done. Lifting straight up gives 300 N x 2 m = 600 J, and pushing up the ramp gives 100 N x 6 m = 600 J. - 14
A motor uses 500 joules of input energy to lift an object, but only 400 joules of useful work is done on the object. What percent of the input energy becomes useful work?
The motor turns 80 percent of the input energy into useful work. The calculation is 400 J ÷ 500 J = 0.80, which equals 80 percent. - 15
A small motor has a power output of 100 watts. How long will it take the motor to do 800 joules of work?
Rearrange the power formula to solve for time.
It will take the motor 8 seconds to do 800 joules of work. Since power = work ÷ time, time = work ÷ power, so 800 J ÷ 100 W = 8 s.