Forces, Energy, and Simple Machines

Why Mechanics Belongs in This Chapter

Official ASVAB guidance describes Mechanical Comprehension (MC) as knowledge of mechanical and physical principles. Even when this chapter is framed around science and electronics, force and machine reasoning belongs here because it connects physical science to technical diagrams, tools, vehicles, and equipment.

On PiCAT, MC does not require calculus. It rewards plain mechanical reasoning: which way a force acts, what happens when a lever arm changes, how pressure moves through a fluid, and how speed and torque trade in gears. Draw arrows and label pivots before calculating.

Force, Motion, and Balance

A force is a push or pull. Balanced forces produce no change in motion. Unbalanced forces cause acceleration, which can mean speeding up, slowing down, or changing direction. Weight is the force of gravity on mass, and it acts downward near Earth's surface.

Newton's laws appear in practical forms. An object keeps its motion unless a net force acts. Acceleration depends on net force and mass. Interacting objects push on each other with equal and opposite forces. If a thrown ball moves forward while gravity pulls down, the horizontal and vertical motions can be analyzed separately.

Friction opposes relative motion or the tendency to slide. On a flat surface, more normal force usually means more friction. Smooth lubrication reduces friction; rough surfaces increase it. Do not assume friction always points backward in a diagram. It points opposite the actual or impending sliding at the contact surface.

Work, Power, and Energy

Work is force applied through distance in the direction of motion. If a force does not move the object, no mechanical work is done on that object. Power is the rate of doing work, so the same work done in less time requires more power.

Energy comes in useful forms:

Machines transfer and transform energy. In an ideal machine, energy out cannot exceed energy in. A machine can reduce effort force only by increasing the distance over which that force is applied.

Levers and Torque

Torque is turning effect. It equals force times perpendicular distance from the pivot. A longer wrench, pry bar, or handle produces more torque from the same force. The important distance is the moment arm at a right angle to the force, not always the full object length.

Levers use a fulcrum, an effort force, and a load. In a first-class lever, the fulcrum sits between effort and load. In a second-class lever, the load sits between fulcrum and effort. In a third-class lever, the effort sits between fulcrum and load.

For balance, compare torques on both sides. A 100-pound load 1 foot from the pivot produces 100 foot-pounds of torque. A force applied 5 feet from the pivot needs only 20 pounds to balance that load, ignoring friction and lever weight.

Pulleys, Inclined Planes, Screws, and Wedges

A fixed pulley changes the direction of a pull. A movable pulley reduces effort by sharing load across rope segments. The ideal mechanical advantage is often the number of rope segments supporting the moving load.

An inclined plane spreads lifting over a longer distance. A long, shallow ramp needs less force than a short, steep ramp, but the load travels farther. A wedge is a moving inclined plane that splits or lifts material. A screw is an inclined plane wrapped around a cylinder; more turns can create large force over a small forward advance.

These machines all obey the same trade: less input force usually means more input distance. If a diagram shows a longer ramp or more supporting rope segments, expect lower effort, not free work.

Gears and Rotating Systems

Gears transfer rotation. When two gears mesh, they turn in opposite directions. A small drive gear turning a larger driven gear reduces speed and increases torque at the output. A large drive gear turning a smaller driven gear increases speed and reduces torque.

Count teeth or compare diameters. If the driven gear has twice as many teeth as the driver, it turns at half the speed but with greater turning force, ignoring losses. In compound trains, multiply the stage ratios.

Pressure and Fluids

Pressure is force divided by area. A smaller contact area produces greater pressure from the same force. This explains why a sharp blade cuts better than a dull one and why wide tracks reduce ground pressure.

In an enclosed fluid, applied pressure is transmitted through the fluid. Hydraulic systems use this idea to multiply force: a small force on a small piston creates pressure that acts over a larger piston area. The larger output force comes with shorter output movement.

PiCAT Strategy for Mechanics

For every MC-style item, label the load, effort, pivot, direction of motion, and contact surfaces. Then ask what is conserved or traded. Is the machine changing direction, reducing effort, increasing speed, increasing torque, or transmitting pressure?

Avoid memorizing one diagram shape. The same lever rule can appear as a pry bar, seesaw, crowbar, brake pedal, or wrench. The same pressure rule can appear as hydraulic brakes, a lift, a syringe, or a tank. Learn the principle behind the picture, and the picture becomes easier to read.