QUIZ Work and Power PLTW Principles of Engineering

A student was asked to push a large box down the hallway about 35 meters. The task took the student five minutes and a force of 150 Newtons.

At what rate was the student pushing the box?

• 1286 Watts

• 18 Watts

• 70 Watts

• 1050 Watts

A winch system powered by a 7.2 volt motor is used to lift a 30 Newton load a distance of 2.5 meters. The motor's current rating is 0.370 Amps.

What is the efficiency of the electro-mechanical system if the time required to raise the load is 90 seconds?

• 3%

• 45%

• 22%

• 31%

A tow truck used a winch to lift a 10,000 N car, 15 m out of a ditch.

If the power of the winch system is 750 W, how long did it take to get the car out of the ditch?

• 3.3 sec

• 13.3 sec

• 3.3 min

• 13.3 min

The illustration shows a fixed pulley lifting an object off the ground to a height of 5 meters. The object has a weight of 200 Newtons. This work is completed in 2 seconds.

What is the power of the system?

• 250 W

• 500 W

• 1000 W

• 2000 W

Use 9.81 m/s² for acceleration due to gravity. Force = mass * acceleration.

An electric winch lifts a 20 kg crate 2.5 meters.

If the winch must not exceed 100 watts during use, what is the fastest time that could be used to lift the crate?

• 5 seconds

• 6 seconds

• 4 seconds

• 3 seconds

Two groups of students compare their winch results from an efficiency lab. The results are above.

What explanation best explains why Group 1 has a higher efficiency?

• Group 1 experiences less resistance

• Group 1 lifts their weight faster

• Group 2 performs more work

• Group 1 has a higher output power

EXTRA CREDIT: A mechanic can apply a maximum force of 25 lb to a standard wrench. Wrenches are only sold in full inch lengths.

If a bolt needs 20 ft·lb of torque applied to turn, what is the shortest length of the wrench (in inches) the mechanic would need?

• 1 in

• 2 in

• 9 in

• 10 in