Reviewing Main Ideas Work Work is the transfer of energy when a force makes an object move. Work is done only when force produces motion in the direction of the force. Power is the amount of work, or the amount of energy transferred, in a certain amount of time.

Reviewing Main Ideas Using Machines A machine makes work easier by changing the size of the force applied, by increasing the distance an object is moved, or by changing the direction of the applied force. The number of times a machine multiplies the force applied to it is the mechanical advantage of the machine. The actual mechanical advantage is always less than ideal mechanical advantage.

Reviewing Main Ideas Using Machines The efficiency of a machine equals the output work divided by the input work. Friction always causes the output work to be less than the input work, so no real machine can be 100 percent efficient.

Reviewing Main Ideas Simple Machines A simple machine is a machine that can do work with a single movement. A simple machine can increase an applied force, change its direction, or both.

Reviewing Main Ideas Simple Machines A lever is a bar that is free to pivot about a fixed point called a fulcrum. A pulley is a grooved wheel with a rope running along the groove. A wheel and axle consists of two different-sized wheels that rotate together. An inclined plane is a sloping surface used to raise objects. The screw and wedge are special types of inclined planes. A combination of two or more simple machines is called a compound machine.

Reviewing Main Ideas The Nature of Energy Energy is the ability to cause change. Energy can have different forms, including kinetic, potential, and thermal energy. Moving objects have kinetic energy that depends on the object’s mass and velocity, and can be calculated from this equation:

Reviewing Main Ideas The Nature of Energy Potential energy is stored energy. An object can have gravitational potential energy that depends on its mass and its height, and is given by this equation: GPE = mgh

Reviewing Main Ideas Conservation of Energy Energy can change from one form to another. Devices you use every day transform one form of energy into other forms that are more useful. Falling, swinging, and projectile motion all involve transformations between kinetic energy and gravitational potential energy.

Reviewing Main Ideas Conservation of Energy Friction converts mechanical energy into thermal energy, causing the mechanical energy of a system to decrease. Mass is converted into energy in nuclear fission and fusion reactions. Fusion and fission occur in the nuclei of certain atoms, and release tremendous amounts of energy.

Chapter Review Question 1 How does adding a lubricant affect the friction, output force, and efficiency of a machine? Answer Lubricants fill spaces between surfaces and reduce friction. Therefore, the output force and efficiency of the machine are greater.

Chapter Review Question 2 What is the difference between kinetic and potential energy? Answer Kinetic energy is the energy a moving object has because of its motion; potential energy is stored energy due to position.

Chapter Review Question 3 A lightbulb converts electrical energy into _______and _______. Answer The filament of a lightbulb converts the electrical energy supplied to the bulb into thermal energy and radiant energy.

Chapter Review Question 4 What is the difference between energy and power? Answer Power is the rate at which energy is converted from one form to another.

Chapter Review Question 5 The SI unit of energy is the __________. A. calorie B. joule C. Newton D. watt

Chapter Review Answer The answer is B. One joule is 1 kg·m2/s2.

Chapter Review Question 6 What type of energy is stored due to the bond between atoms? A. chemical kinetic energy B. chemical potential energy C. elastic potential energy D. elastic kinetic energy

Chapter Review Answer The answer is B. Chemical potential energy is energy stored due to chemical bonds.

Standardized Test Practice Question 1 You move a 130-kg dresser at an acceleration of 0.5 m/s2 over a distance of 5 m. How much work do you do on the dresser? A. 65 J B. 260 J C. 325 J D. 1300 J

Standardized Test Practice Answer The answer is C. Calculate the force applied by multiplying mass by acceleration. Calculate work done by multiplying force by distance.

Standardized Test Practice Question 2 How long will it take a jogger to convert 1,000 J of energy if her power is 125 W? A. 8 s B. 125 s C. 250 s D. 1250 s

Standardized Test Practice Answer The answer is A. Use the equation P =E/t and solve for t.

Standardized Test Practice Question 3 Calculate the mechanical advantage of a crowbar if the input force is 200 N and the output force is 1800 N A. 2000 0.1 C. 0.9 D. 9

Standardized Test Practice Answer The answer is D. Mechanical advantage is equal to the output force divided by the input force.

Standardized Test Practice Question 4 What is the kinetic energy of a 7.5-kg salmon swimming at 0.67 m/s? A. 1.7 J B. 2.5 J C. 3.4 J D. 5.0 J

Standardized Test Practice Answer The answer is A. Kinetic energy is equal to one-half the mass multiplied by the square of the velocity.

Standardized Test Practice Question 5 A runner has a mass of 60 kg and a kinetic energy of 750 J. What is the runner’s speed? A. 4 m/s B. 5 m/s C. 9 m/s D. 10 m/s

Standardized Test Practice Answer The answer is B. Use the formula KE = ½ mv2 and solve for v.

Standardized Test Practice Question 6 Find the approximate kinetic energy of a ball with a mass of 0.05 kg moving at 35 m/s. A. 1225 J B. 61 J C. 31 J D. 1 J

Standardized Test Practice Answer The answer is C. Use the formula KE = ½ mv2.

Standardized Test Practice Question 7 Use the table to determine approximate how long a person would need to run in order to use the same number of calories as a person uses walking in 1 h.

Standardized Test Practice A. 5 min 10 min C. 15 min D. 20 min

Standardized Test Practice Answer The answer is C. A runner burns calories about four times as fast as a walker.