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Objectives

Section 1 Work, Power, and Machines. Chapter 12. Objectives. Define work and power. Calculate the work done on an object and the rate at which work is done. Use the concept of mechanical advantage to explain how machines make doing work easier.

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Objectives

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  1. Section 1 Work, Power, and Machines Chapter 12 Objectives • Definework and power. • Calculatethe work done on an object and the rate at which work is done. • Usethe concept of mechanical advantage to explain how machines make doing work easier. • Calculatethe mechanical advantage of various machines.

  2. Section 1 Work, Power, and Machines Chapter 12 Bellringer In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.) 1. In one drawing, no motion is likely to occur. Which drawing is it? 2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease?

  3. Section 1 Work, Power, and Machines Chapter 12 Bellringer, continued

  4. Section 1 Work, Power, and Machines Chapter 12 What is Work? • Workis the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force. • Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work. • Work Equation work = forcedistance W = Fd

  5. Section 1 Work, Power, and Machines Chapter 12 Work

  6. Section 1 Work, Power, and Machines Chapter 12 What is Work?, continued • Work is measured in joules. • Because work is calculated as force times distance, it is measured in units of newtons times meters, N•m. • These units are also calledjoules (J). In terms of SI base units, a joule is equivalent to 1 kg•m2/s2. • Definition of joules 1 J = 1 N•m = 1 kg•m2/s2

  7. Section 1 Work, Power, and Machines Chapter 12 Math Skills Work Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? 1. List the given and unknown values. • Given: force, F = 190 N • distance, d = 2.0 m • Unknown: work, W = ? J

  8. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for work. • work = force  distance • W = f  d 3. Insert the known values into the equation, and solve. • W = 190 N 2.0 m = 380 N•m • W = 380 J

  9. Section 1 Work, Power, and Machines Chapter 12 Power • Poweris a quantity that measures the rate at which work is done or energy is transformed. • Power Equation • Power is measured in watts. • A watt (W) is equal to a joule per second (1 J/s).

  10. Section 1 Work, Power, and Machines Chapter 12 Power

  11. Section 1 Work, Power, and Machines Chapter 12 Math Skills PowerIt takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 1. List the given and unknown values. • Given: work, W = 100 kJ = 1  105 J • time, t = 20 s • The distance of 18 m will not be needed to • calculate power. • Unknown: power, P = ? W

  12. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for power. 3. Insert the known values into the equation, and solve.

  13. Section 1 Work, Power, and Machines Chapter 12 Machines and Mechanical Advantage • Machines multiply and redirect forces. • Machines help people by redistributing the work put into them. • They can change either the size or the direction of the input force. • Different forces can do the same amount of work. • A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance.

  14. Section 1 Work, Power, and Machines Chapter 12 Force and Work

  15. Section 1 Work, Power, and Machines Chapter 12 Machines and Mechanical Advantage, continued • Mechanical advantagetells how much a machine multiplies force or increases distance. • Mechanical Advantage Equation

  16. Section 1 Work, Power, and Machines Chapter 12 Math Skills Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. 1. List the given and unknown values. • Given:input distance = 5.0 m • output distance = 1.5 m • Unknown: mechanical advantage = ?

  17. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for mechanical advantage. Because the information we are given involves only distance, we only need part of the full equation: 3. Insert the known values into the equation, and solve.

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