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UNIT 1 EXAM

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Part 1 Simple Machines (50 pts) Complete all 5 problems.

1) You are babysitting your younger cousin Scott and he wants to go to the playground and use the see-saw.

a. If you weigh 136 lbs. and your cousin weighs 75 lbs. what would the actual mechanical advantage of the see-saw be? (Assume you are the one applying a force to the system.)

b. If your cousin is sitting 5 feet from the fulcrum, how far would you need to sit from the fulcrum in order to balance the see-saw?

c. What class lever best represents a see-saw?

2) A crane with a block and tackle system which has 7 supporting strands is used to lift cargo containers off a ship.

a. If each cargo container weighs 2500 lbs., how much force is required from the motor to lift each container?

b. A second system has two cranes (each with 7 supporting strands) working together to lift a larger container weighing 3500 lbs. Assuming the motor on each crane provides an equal amount of power, what is the force required by each motor to lift the cargo?

3) A car has a steering wheel with a 15 inch diameter connected to an axle with a 2 inch diameter.

a. What is the Ideal mechanical advantage of the system?

b. If it takes 80 ft-lb of force to make the axle move, what force must be applied at the steering wheel to make the wheels turn?

4) You have taken a summer job working at a UPS warehouse, there is a ramp which you must push packages up. The platform at the top of the ramp is 3 feet above the ground and the base of the ramp is 12 feet long.

a. What is the ideal mechanical advantage of the ramp?

b. If you can push with 100 lbs. of force what is the total weight you can push up the ramp?

5) You are using a wedge to split firewood. The wedge is 3 inches wide and 8 inches long as pictured below.

3 in

a. What is the ideal mechanical advantage of the wedge?

8 in

b. If you can hit the top of the wedge with 60 lbs. of force, what is the splitting force (resistance force) generated.

Part II Gear, Pulleys and Sprockets (30 pts)

6) A gear system used to feed paper in a printer contains 5 gears arranged in series. Gears A, B, C, D and E have 36, 16, 40, 18 and 10 teeth respectively. Gear A is connected to the motor and provides a torque of 1.2 ft-lb.

a. What is the overall gear ratio of the system?

b. How much torque is used to move the paper?

c. Are gears A and C spinning in the same or opposite directions?

7) A chain drive system is used on a bicycle to transmit power from the pedals to the rear wheels. Johnny prefers using two gear ratios when riding, one for going up hills and one for flat terrain. The first ratio is 36 teeth on the pedals to 12 teeth on the rear wheel and the other is 18 teeth on the pedals to 40 teeth on the rear wheel.

a. What is the gear ratio for each of the combinations?

b. Which one would be most suitable for going up hills? Explain your answer by using gear ratios.

Part three: Short Answer (10 pts)

8) When a system is in motion why is the AMA different from the IMA?

9) Why should a compound gear train be used over a series gear train?

10) If you are using an 8-32 screw what is the pitch of the screw?

11) If there are 57 gears connected one after the other, and the first one is spinning clockwise.

a. What direction is the last gear spinning?

b. What direction is the 24th gear spinning?

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