Practice Problems



Work Practice Problems:

1. A force of 800 Newtons is needed to push a car across a lot. Two students push the car 40 meters. How much work is done?

2. How much work is done in lifting a 60Kg crate a vertical distance of 10 meters? *remember F=ma

3. Explain the statement, "Atlas did no work".

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4. A 1000 N mountain climber scales a 100 m cliff. How much work is done?

5. You are using 50 N of force to push an empty cart down the school hall. Your friend, who weighs 800 N, jumps on the cart and wants to be taken for a ride. You push your friend down the hall for 20 meters before being caught by the Principal. How much work did you do pushing your friend on the cart?

Power Practice Problems:

• A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce?

• A box that weighs 1000 Newtons is lifted a distance of 20 meters in 10 seconds. How many watts of power are produced?

• At a tractor pull, one machine has a run that produces 28,000 Joules in 2 seconds. How many Watts of power did this tractor produce?

• What was the horsepower produced by the tractor in question #3? Use dimensional analysis.

▪ one horsepower = 746 Watts

Given the following information:

|Equations |Conversions |

|Weight = (mass) (acceleration of gravity) = (kilograms) (10 m/s2= Newtons |one pound = 0.5 kilogram |

|Work = (force) (distance)= (Newtons) (meters)= joules |one foot = 0.3 meters |

|Power = work / time = joules / seconds = Watts |one horsepower = 746 Watts |

• A person weighing 150 pounds is lifted 200 feet by an elevator. Calculate the work done by the elevator in "foot-pounds".

• Calculate the work done by the elevator in joules.

• If the elevator lifts the person in 20 seconds, how many Watts of power are produced?

• Convert the Watts to horsepower.

Machine Efficiency Practice:

• What does friction do to the efficiency of a machine?

• You use a pair of pliers to crack a pecan. It takes 1200 N of resistance force to crack the pecan, but you only exert 400 N of effort force with your hand on the pliers. What is the mechanical advantage of the pliers?

• Think of pliers as a pair of first-class levers. The fulcrum about which the levers turn is located at point B. The lever arm represented by the line AB is 6 cm long. The lever arm represented by line BC is 3 cm long. What is the mechanical advantage of this set of pliers?

• If the effort arm length of the pliers in question #3 was nine inches, what was the length of the resistance arm length?

|HOME ENERGY AUDIT: POWER WORKSHEET |

|Directions: Answer the questions below in the space provided. |

|1. Explain how work and energy are affected by power. |

|2. Using the equation for power, determine how much power in watts is involved if 300 joules of work are done in 10 seconds. |

|3. About how many times longer would a 10 watt lightbulb be able to burn compared to a 100 watt bulb with the same energy? |

|4. Rank the following items from lowest power requirement to highest power requirement. |

| __________ |

| a 100 watt lightbulb burns for 2 hours |

| |

| __________ |

| a 40 watt lightbulb burns for 4 hours |

| |

| __________ |

| a 1500 watt hair dryer is used for 0.5 hours |

| |

| __________ |

| a 500 watt microwave operates for 0.5 hours |

| |

| __________ |

| a 15 watt compact fluorescent light burns for 10 hours |

| |

|5. Calculate how much it would cost to operate a 1000 watt spotlight for one 8 hours if 1 kilowatt-hour costs $0.15.  |

|6. How do the lighting requirements differ between the hallway, the gym, and the library of your school? How would you suggest these areas be lit? |

|  |

WORK, ENERGY, AND POWER

A 60 kg box is lifted a distance of 10 meters straight up at constant speed using a rope. How much power is required to complete this task in 5 seconds?

2. Hulky and Bulky are two workers being considered for a job at the UPS loading dock. Hulky boasts that he can lift a 100 kg box 2 meters vertically, in 3 seconds. Bulky counters with his claim of lifting a 200 kg box 5 meters vertically, in 20 seconds.

Which worker has a greater power rating?

3. A 1994 Ford Mustang is driving down a road with a constant speed of 30 m/s. The engine must exert a 5000 N force to maintain this speed.

a. What is the power rating of the engine?

b. How does the Mustang's power rating compare to the 220 hp Dodge Stealth engine (1 hp = 746 W)?

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