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Chapter 8: Work & Machines

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1 Chapter 8: Work & Machines
Pg 191 #1-3, pg 197 #1-4, pg 202 #1-3, pg 205 #1-3

2 Pg 191 #1-3 1. Work is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it flies through the air? Explain. The pitcher is no longer doing work on the ball as it flies through the air because he is no longer exerting a force on it. However, work is being done on the ball by the Earth, which exerts a force on the ball & pulls it back toward the ground.

3 Pg 191 #1-3 (cont.) 2. Explain the difference between work & power. Work occurs when a force causes an object to move in the direction of the force, and power is the rate at which work is done. The more work you do in a given amount of time, or the less time it takes you to do a given amount of work, the greater your power.

4 Pg 191 #1-3 (cont.) 3. You lift a chair that weighs 50N to a height of 0.5m and carry it 10m across the room. How much work do you do on the chair? 50N x 10m = 500J

5 Pg 197 #1-4 1. Explain how using a ramp makes work easier.
Using a ramp makes work easier because it allows you to apply a smaller input force than you would have to apply when lifting a load straight up. However, the smaller force has to be exerted over a longer distance. 2. Why can’t a machine be 100% efficient? A machine can’t be 100% efficient because some of the work input is used to overcome friction. Therefore, work input is always greater than work output.

6 Pg 197 #1-4 (cont.) 3. Suppose you exert 15N on a machine, and the machine exerts 300N on another object. What is the machine’s mechanical advantage? MA = 300N/15N = 20

7 Pg 197 #1-4 (cont.) 4. For the machine described in question 3, how does the distance through which the output force is exerted differ from the distance through which the input force is exerted? Because the output force is greater than the input force, the distance over which the output force is exerted must be shorter than the distance over which the input force is exerted. This is an example of the force-distance trade-off.

8 Pg 202 #1-3 1. Give an example of each of the following simple machines: 1st class lever, 2nd class lever, 3rd class lever, inclined plane, wedge, & screw. 1st class lever: a screwdriver used to pry the lid off a paint can; 2nd class lever: a wheelbarrow; 3rd class lever: your leg as you kick a soccer ball (your knee is the fulcrum); inclined plane: a ramp on the back of a moving truck; wedge: a knife; screw: a jar lid.

9 Pg 202 #1-3 (cont.) 2. A 3rd class lever has a mechanical advantage of less than 1. Explain why it is useful for some tasks. A 3rd class lever helps because it increases the distance through which the output force is exerted. For example, when you move the handle of a fishing pole just slightly, the other end of the pole moves a great distance.

10 Pg 202 #1-3 (cont.) 3. Look back at Figures 6, 7, & 8 in Section 2. Identify the type of simple machine shown in each case. (If a lever is shown, identify its class.) Figure 6: The screwdriver is a 1st class lever; Figure 7: The ramp is an inclined plane; Figure 8: The handcart is a 1st class lever.

11 Pg 205 #1-3 1. Give an example of a wheel & axle.
Examples include the crank on a can opener, the reel on a fishing rod, a screwdriver, a doorknob, the crank on an ice cream maker, and the film-advance mechanism on an old camera. 2. Identify the simple machines that make up tweezers & nail clippers. Each side of the tweezers is a 3rd class lever. The sharpened edges of nail clippers are wedges, and the arm that activates the clipper is a 2nd class lever.

12 Pg 205 #1-3 (cont.) 3. The radius of the wheel of a wheel & axle is 4 times greater than the radius of the axle. What is the mechanical advantage of this machine? The mechanical advantage of a wheel & axle is determined by the ratio of the wheel radius to the axle radius. So this machine would have a mechanical advantage of 4.

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