Answers and Explanations

Part 8: Mechanical Comprehension (MC)

1. A

   Torque results in a twisting motion in an object. This is very different from horsepower (the rate that work is done), or force (either (C) a push or pull, or (D) a force that travels in a circle).

2. B

   Speed is different from velocity, in that velocity (which is a vector quantity) implies both speed (a scalar quantity) and direction.

3. B

   The relationship between force, mass, and acceleration is described using the formula F = ma. If mass increases, more force is required to achieve the same acceleration rate.

4. A

   Force is a vector quantity. This means that it expresses both magnitude and direction.

5. C

   The definition of mechanical advantage is the advantage gained by the use of a mechanism in transmitting force.  For example, a lever and fulcrum can be used to multiply the force applied to an object.

6. D

   A hockey puck experiences kinetic friction (however small) that causes it to lose velocity.

7. B

   This is an example of meshed gears. Meshed gears always revolve in opposite directions.

8. B

   This is not a true statement because weight is dependent on both mass and acceleration due to gravity (W = mg).

9. C

   The coefficient of static friction is always greater than the coefficient of kinetic friction. If a force is applied, but the box does not move, no work has been done (W = Fd). The nature of the surface the box rests on will define the coefficient of friction between the box and that surface.

10. A

    The mechanical advantage of a wheel and axle system is determined by the ratio of the radius of the wheel where the force is applied to the radius of the wheel where the force is transferred. In this case, the ratio of the radii is 10:2.5 or 4:1.

11. B

    Three inches of movement multiplied by 20 pounds per inch is 60 pounds of force.

12. A

    Torque (twisting force) can be increased by increasing the length of the wrench, or by increasing the force applied to the wrench.

13. B

    In accordance with Newton’s first law of motion, a vehicle traveling at a constant speed has no net forces acting on it.

14. B

    The outlet pipe on tower B is nearer to the bottom of the cistern than the outlet pipe for tower A. Therefore, it will release more water than tower A. While tower B will release water faster due to increased pressure lower in the cistern, there is no way of knowing how much faster from the information that is provided.

15. D

    Because there are three cords holding sign B, they are under the tension of the cord holding sign A.

16. A

    The illustration shows a first-class lever because the fulcrum is between the load and the effort (force).

17. C

    Accelerating at the rate of will result in a velocity of at the end of ten seconds.

    

18. A

    A wheelbarrow is an example of a second-class lever because the fulcrum is at one end, the effort is at the other, and the load is in between.

19. A

    Gears that are an odd number away from the indicated gear move in the opposite direction, while gears that are an even number away move in the same direction as the indicated gear. In this case, gear Y is three gears away from gear X, which means it will move in the opposite direction as gear X. Gear X is moving clockwise, so gear Y must move counterclockwise.

20. A

    Using the formula W = Fd, it can be seen that 1 pound of force applied through a distance of 1 foot will result in 1 foot-pound of work being done.

21. B

    The principle of conservation of energy tells us that the amount of energy in the universe is constant.

22. A

    Regardless of the two configurations, both are simple pulleys with no mechanical advantage. Since the objects both exert the same downward force (weight), the force required to counteract that weight will be the same for both pulleys.

23. B

    Choice (B) is not a true statement; the use of hydraulic force relies on the fact that liquids are effectively incompressible.

24. C

    A machine’s efficiency is expressed as the percentage of the source energy that it converts into usable energy.

25. B

    The velocity at which the ball travels will depend upon the impulse that was applied to it. Impulse is determined by multiplying the force by the amount of time that the force was applied. To increase the impulse, the player should hit the ball with greater force and make contact with the ball for a longer period of time.