ASVAB PRACTICE TEST A

Answers and Explanations

Part 8: Mechanical Comprehension (MC)

  1. A

    The principle of conservation of mechanical energy says that the total mechanical energy of an object (PE + KE ) remains constant as an object moves. As the car rolls down the hill, its speed increases because its potential energy (PE ) is being converted into kinetic energy (KE).

  2. C

    During the ball’s journey upward after being thrown, the ball has its maximum kinetic energy when it leaves the thrower’s hand. This kinetic energy is converted into potential energy as it rises. Maximum potential energy (PE ) occurs when the ball stops rising, and this is where its velocity is zero.

  3. B

    Inertia is a function of an object’s mass. A large mass will require a large amount of force to cause it to accelerate. The larger the mass, the greater the inertia of the object. Momentum is a quality of an object in motion and describes how difficult it is to change the velocity of a moving object. Torque is a rotational force. Pressure is force per unit area.

  4. C

    Isaac Newton discovered that every particle in the universe is attracted to every other particle. This concept became a physical law known as Newton’s law of universal gravitation. The gravitational force that each body exerts on the other grows stronger as the bodies get closer. The force also increases as the mass of the bodies increase.

  5. D

    Based on Newton’s second law, F = ma. Rearrange that equation to solve for acceleration: , where units of force are in Newtons and mass is expressed in kg. Here: .

  6. C

    Work is defined as force times distance. Lifting a box straight up to the top of a ramp will require the same amount of work as pushing the box up the ramp (neglecting friction). The ramp allows a smaller force to be applied over a larger distance. However, the end result or net work done is the same in both cases.

  7. B

    It is not possible to get more work out of a machine than what is put in. Machines do not reduce work, they simply manage it and make it easier by diminishing the amount of force that is required.

  8. A

    A chisel is an example of a wedge.

  9. B

    The wedge is a variation of the inclined plane, but is made to move whereas the inclined plane is made to stay in one place.

  10. A

    The distance from post A to the load is less than the distance from post B to the load; therefore, it holds the greater part of the load.

  11. C

    Work and kinetic energy are both measured in joules. This is not a coincidence, as any work that is done to accelerate an object at rest will be converted into the kinetic energy of that object. This principle is known as the work-energy theorem.

  12. B

    A screwdriver gains mechanical advantage through the use of a large wheel (screwdriver handle) rotating a small wheel (screwdriver tip), much like a wheel and axle. Large movements converted into small movements will amplify the force applied.

  13. B

    Any time that force is increased through mechanical advantage, there will always be a proportional decrease in the distance moved.

  14. A

    A prybar can increase mechanical advantage, and can change the direction of motion, so it is a first-class lever.

  15. B

    One horsepower is equal to 550 foot-pounds of work per second or 746 watts.

  16. C

    Moving the fulcrum toward the object will increase the force applied to the object, but decrease the distance that the object can be moved.

  17. C

    If a machine were able to produce more work than was put into the machine, there would be a net increase in energy. This would violate the laws of thermodynamics.  The work put out by a simple machine can never exceed the work put into it.

  18. D

    If a small gear drives a large gear, a speed reduction takes place. The large gear will turn more slowly than the small gear and the speed of the output will be slower.

  19. B

    A block-and-tackle requires a minimum of two pulleys, with a rope, belt, or chain used to operate them. There is at least one fixed and one moveable pulley in the block and tackle.

  20. A

    The force exerted to lift an object using a single pulley is the same as the weight of the object itself.

  21. B

    A doorknob is similar to a wheel and axle in that a large wheel (the doorknob or handle) is used to operate a small wheel (the latch mechanism).

  22. A

    Mass is a measure of the amount of matter in a substance. It does not change from place to place. Weight, however, depends on the gravitational constant, and does change depending on the location.

  23. C

    Consider “Torque” and “Speed” to be two people on a teeter-totter. Whenever “Torque” goes up, “Speed” goes down, and vice versa. These two quantities can be described as inversely proportional.

  24. C

    An object’s potential energy (PE) can be calculated using the formula

    PE = mgh

    where m is mass in kilograms (kg), g is acceleration due to gravity (9.8 m/s2), h is the height of the object in meters (m), and PE is gravitational potential energy measured in joules (J).

  25. A

    No work is done because the box does not move. W = Fd, where F is force (newtons), and d is distance (m). If an object experiences no change in position, then no net work is done.