OAR Mechanical Comprehension: Forces Flashcards

Two mechanics are trying to loosen a stubborn bolt. Mechanic A applies a 50 lb force to the end of a 1-foot wrench. Mechanic B applies a 25 lb force to the end of a 2-foot wrench. Assuming the forces are applied perpendicularly to the wrenches, which of the following statements is true?

Answer: They both apply the same amount of torque.

Torque is calculated as Force multiplied by the perpendicular distance from the pivot point (the lever arm). For Mechanic A, the torque is 50 lbs × 1 ft = 50 ft-lbs. For Mechanic B, the torque is 25 lbs × 2 ft = 50 ft-lbs. Since both calculations result in 50 ft-lbs, they apply the same amount of torque.

A person is trying to push a heavy crate across a flat, rough floor. They find it requires a large amount of force to get the crate to start moving, but once it is moving, it takes less force to keep it sliding. Which of the following best explains this phenomenon?

Answer: The coefficient of static friction is greater than the coefficient of kinetic friction.

Static friction is the force that must be overcome to initiate motion between two surfaces. Kinetic friction is the force that opposes motion once it has started. For most materials, the coefficient of static friction is higher than the coefficient of kinetic friction, meaning more force is required to start an object moving than to keep it moving.

Three identical containers are filled to the same height with three different liquids: saltwater, fresh water, and oil. Saltwater is denser than fresh water, and fresh water is denser than oil. At the bottom of which container is the pressure the greatest?

Answer: The container with saltwater.

Hydrostatic pressure at a certain depth is calculated by the formula P = ρgh, where P is pressure, ρ (rho) is the fluid density, g is gravity, and h is the height of the fluid. Since the height (h) and gravity (g) are the same for all three containers, the pressure is directly proportional to the density (ρ). Because saltwater is the densest liquid, it will exert the greatest pressure at the bottom.

A 100 lb traffic light is suspended motionless by two cables. Cable A makes a 30-degree angle with the horizontal ceiling, and Cable B makes a 60-degree angle with the horizontal ceiling. Which cable experiences greater tension?

Answer: Cable A (the one with the smaller angle to horizontal).

For an object in equilibrium, the vertical components of the tension forces must sum to equal the object's weight. As the angle of a support cable becomes more horizontal (smaller angle with the horizontal), it must provide a much larger total tension to contribute the same amount of vertical lift. Therefore, Cable A, with the smaller angle of 30 degrees, will bear a significantly greater tension than Cable B.

Which of the following modifications to a large cargo truck would most effectively increase its stability and make it less likely to tip over on a sharp turn?

Answer: Loading the heaviest items on the bottom of the cargo area.

An object's stability is increased by lowering its center of gravity. Loading the heaviest items on the bottom of the cargo area lowers the truck's overall center of gravity, making it more stable and less prone to tipping during a turn. Increasing the load height would raise the center of gravity, decreasing stability.

A jet engine provides thrust to propel an airplane forward. According to Newton's Third Law of Motion, what is the 'reaction' force that pushes the airplane forward?

Answer: The force of the hot exhaust gases being pushed backward out of the engine.

Newton's Third Law states that for every action, there is an equal and opposite reaction. The 'action' is the engine forcefully expelling a mass of hot gases backward. The equal and opposite 'reaction' is the force of these gases pushing the engine (and the airplane) forward.