Practice Problem #1

• A weightlifter bench presses 80kg (approx. 175lbs.) 0.75m straight up.

• a. How much work does she do, assuming constant velocity, in one lift (just the 0.75m straight up)?

• b. How many repetitions would she have to do to burn off 1 hamburger (400 Calories). Note 1 calorie = 4.186 J.

Solution to Practice Problem #1

Solution to Practice Problem #1

Practice Problem #2

• A person pushes a stalled 2000 kg car from rest to a final speed of 2.0 m/s. During this time the car moves 20 m. Neglecting friction between the car and the road, find:

• a. the final acceleration of the car • b. the horizontal force exerted on the car

(Hint: Use Newton’s 2nd Law to find the force using the acceleration.)

• C. the work done on the car

fv

Solution to Practice Problem #2

Practice Problem #3

A person pushes a 10 kg box at a constant velocity over a distance of 4 m. The frictional force between the box and the floor is 30% the weight of the box. How much work does the person do in pushing the box?

Solution to Practice Problem #3

• The weight of the box is mg = 980 N

• The frictional force Ff = 0.3 980 N = 294 N

• The work done, W = Ff d = 294 N 4 m = 1,176 J

Practice Problem #4

• An elevator m=800 kg has a maximum load of 8 people or 600 kg. The elevator goes up 10 stories = 30 m at a constant speed of 4 m/s. What is the average power output of the elevator motor if the elevator is fully loaded with its maximum weight? (neglect friction)

(Hint: First determine the time the elevator takes to go up 10 stories, then determine the work the elevator motor exerted. Finally find the power of the motor.)

Solution to Practice Problem #4

Practice Problem #5

• A 500 kilogram car is driving at 15 meters/second. What's its kinetic energy?

• Solution

KE = ½ mv2 = ½ 500 kg (15 m/s)2 = 56,250 J

Practice Problem #6

• A cart is loaded with a brick and pulled at constant speed along an inclined plane to the height of a seat-top. If the mass of the loaded cart is 3.0 kg and the height of the seat top is 0.45 meters, then what is the potential energy of the loaded cart at the height of the seat-top?

• Solution: PE = mgh = 3 kg 9.8 m/s2 0.45m = 13.23 J

Practice Problem #7

A forklift lifting a crate of mass 100 kg at a constant velocity to a height of 8 m over a time of 4 s. The forklift then holds the crate in place for 20 s.

a. How much power does the forklift exert in lifting the crate?

b. How much power does the forklift exert in holding the crate in place?

Solution to Practice Problem #7