Two energy questions!1.A box with 4 bricks (4 kg each) is lifted at a constant velocity from the floor to a height of 90 cm. How much work is done of the box full of bricks? 12. A 1255 kg Honda is...

Two energy questions!

1.A box with 4 bricks (4 kg each) is lifted at a constant velocity from the floor to a height of 90 cm. How much work is done of the box full of bricks?


12. A 1255 kg Honda is going through a school zone at 50 km/h and then the driver quickly steps on the brakes to make the car come to a complete stop by skidding 30 m. What is the work done on the car by the frictional forces? (Hint: the answer is negative because the force and the displacement are in the opposite directions.)

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bandmanjoe | Middle School Teacher | (Level 2) Senior Educator

Posted on

In physics, the formula for calculating work is "work = force x distance".  The unit of work is in joules, while the force is expressed in Newtons.  The distance is given in meters; if not given in meters, the distance should be converted to meters. 

For number 1, we must convert the mass given to a force, which would be the weight of the bricks.  There are 4 bricks, with a mass of 4 kilograms each.  That would be 16 kg, multiplied times 9.8 m/s/s, the acceleration due to gravity.  That would give us 156.8 Newtons of force.  Multiply that times the distance (in meters), which would be .9 meters, and you get 141.12 Joules of work being done.

In problem 12, we must convert the 1255 kg of the car to a force, which would be the weight of the car.  1255 times 9.8 m/s/s, the acceleration due to gravity, gives us 12,299 Newtons of weight.  That times the distance it took to stop the car, which was 30 meters, would be 368,970 Joules of work done on the car by friction.  This number would be negative, since the car is experiencing negative acceleration, so -368,970 Joules.

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