Section 2 review
The work–energy theorem states that the net work done on an object equals that object’s change in kinetic energy. Work also can change an object’s gravitational potential energy. Energy changes can be represented by the symbol ΔE, and they can be positive or negative. When acting on moving objects, friction transforms some kinetic energy into thermal energy. Air friction (also called air resistance) can prevent a falling object from converting all of its potential energy into kinetic energy. Efficiency divides the energy one gets out of a system by the energy that one puts in. Efficiency can never be greater than 100%, and often is a great deal less. Read the text aloud
work–energy theorem, friction, efficiency

W net =Δ E k
ΔE=0
ΔE=Q+W
η= E out E in

Review problems and questions

  1. A 10 kg go-kart driven by a 30 kg boy starts from rest and rolls down a hill of height 10 m.
    1. What is the predicted speed of the boy and go-kart at the bottom of the hill?
    2. The boy’s actual speed at the bottom of the hill is measured to be 10 m/s. Does this agree with your prediction? Why or why not? Read the text aloud Show
  1. A man applies 500 N of horizontal force to push a 100 kg wooden crate 20 m across a floor.
    1. What is the work that the man performs upon the crate over the 20 m distance?
    2. When sliding, the crate experiences a friction force of 200 N in the direction opposite its motion. What is the work that the friction does upon the crate over the 20 m distance?
    3. What is the net work done on the crate?
    4. What is the change in the crate’s kinetic energy?
    5. Calculate the efficiency with which the man’s work is turned into the crate’s kinetic energy. Read the text aloud Show
  1. A moving hockey stick hits a stationary 160 g hockey puck (m = 0.16 kg) and applies a horizontal force F to the puck. The two objects remain in contact for the first 0.25 m (25 cm) of the puck’s motion. The puck then slides away at a speed v of 20 m/s. What force F is delivered to the puck? Read the text aloud Show
  1. A weightlifter grabs a 50 kg barbell resting on the floor, raises it 2 m, then holds it in place.
    1. What is the barbell’s kinetic energy before being lifted?
    2. What is the barbell’s kinetic energy when the lift is complete?
    3. What is the barbell’s change in kinetic energy?
    4. What is the net work done on the barbell?
    5. How much work does the weightlifter perform on the barbell?
    6. Do your answers to questions d and e match?
    7. Is the weightlifter’s pull the only force acting upon the barbell while it is being lifted? Read the text aloud Show
Safety hazards
  1. What does each hazard represent in the illustration at right? Show
  1. When reading through an investigation, you realize that it requires you to use a chemical with which you are unfamiliar.
    1. Where can you find the information to identify whether or not the chemical is hazardous?
    2. Where can you find the information on what particular classifications of hazards the chemical poses?
    3. Where can you find information on what protective gear you might have to wear?
    4. Where can you find instructions for safe handling of the material?
    5. Where can you find information on whether or not the material, when disposed, is considered hazardous waste?
    6. Where can you find instructions or a procedure for safe disposal of the substance if it is hazardous?
    Show
  1. At the front of Nkrumah’s classroom there is a large bottle with hydrochloric acid, but he only needs to use a small amount at his bench. Nkrumah follows good handling procedures to transfer some HCl into a smaller bottle to take to his bench. What else should he do? Show

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