Gen mech

Force

An interaction that can change an object's motion; force has magnitude (amount) and direction.

Magnitude

The amount or strength of a force.

Direction

The way a force acts or points.

Forces in the same direction

Combine together to produce a greater total force.

Opposing forces

Subtract from each other; the stronger force determines the direction.

Newton's Third Law

Every action has an equal and opposite reaction.

Action-Reaction Forces

Forces that are equal and opposite but act on different objects.

Example of Newton's Third Law

If you push against a wall, the wall pushes back on you with an equal and opposite force.

Equilibrium

A condition in which balanced forces produce no change in velocity.

Balanced Forces

Forces that are equal in magnitude and opposite in direction.

Object sitting on a table

Gravity pulls downward while the table pushes upward, creating equilibrium.

Gravity

The attractive force between objects with mass.

Effect of increasing mass on gravity

More mass produces a stronger gravitational attraction.

Effect of increasing distance on gravity

More distance produces a weaker gravitational attraction.

Friction

A force that opposes motion between surfaces.

Effect of friction on motion

More friction makes an object harder to move.

Lubrication

Reduces friction between moving surfaces.

Work

The amount of energy transferred when a force moves an object through a distance.

Work Formula

Work = Force × Distance.

Increasing force while distance stays constant

Increases the amount of work.

Increasing distance while force stays constant

Increases the amount of work.

Power

The rate at which work is done.

Power Formula

Power = Work ÷ Time.

More power

Doing the same amount of work in less time.

Pressure

Force applied over a certain area.

Pressure Formula

Pressure = Force ÷ Area.

Effect of decreasing area

Same force on a smaller area produces greater pressure.

Effect of increasing area

Same force on a larger area produces lower pressure.

Lever

A rigid bar that rotates around a fixed point to move a load.

Fulcrum

The fixed point around which a lever rotates.

Effort

The force applied to a lever.

Resistance/Load

The object or force that the lever is moving.

Mechanical Advantage (Lever)

Mechanical advantage = effort arm ÷ resistance arm.

Effort Arm

The distance from the fulcrum to where the effort is applied.

Resistance Arm

The distance from the fulcrum to the load/resistance.

Longer effort arm

Requires less effort force to move the same load.

Shorter effort arm

Requires more effort force to move the same load.

Mechanical Advantage

How much a machine multiplies the force applied to it.

Inclined Plane

A sloped surface, such as a ramp, that reduces the force needed to move a load.

How an inclined plane makes work easier

It reduces the required force by increasing the distance over which the force is applied.

Longer inclined plane

Requires less force but increases the distance the load must travel.

Shorter inclined plane

Requires more force but reduces the distance the load must travel.

Mechanical trade-off of an inclined plane

Less force required means more distance traveled.

Pulley

A wheel with a groove that uses a rope or cable to move loads.

Fixed Pulley

A pulley attached to a fixed point that primarily changes the direction of the applied force.

Mechanical Advantage of a Fixed Pulley

Usually 1; it changes direction but does not significantly multiply force.

Supporting Rope Sections

The sections of rope directly supporting the load.

Pulley Mechanical Advantage

Approximately equal to the number of rope sections supporting the load.

More supporting rope sections

Increase mechanical advantage and reduce the force required.

Trade-off of multiple pulleys

Less force is required, but more rope must be pulled.

Gears

Wheels with teeth that transfer rotational motion and force.

Two touching gears

Rotate in opposite directions.

Three gears in a row

The first and third gears rotate in the same direction.

Gear train

A series of connected gears used to transfer motion and change speed, force, or direction.

Gear rotation rule

Adjacent gears rotate in opposite directions.

Hydraulics

Systems that use a confined liquid to transmit force.

Pascal's Principle

Pressure applied to a confined fluid is transmitted throughout the fluid.

Hydraulic Pressure

Pressure in a confined fluid is transmitted through the fluid in all directions.

Hydraulic Jack

A device that uses hydraulic pressure to multiply force and lift heavy loads.

Larger hydraulic piston

Produces greater output force when the pressure is the same.

Smaller hydraulic piston

Moves a greater distance when used with a larger output piston.

Hydraulic Trade-off

Greater output force comes with less output distance.

Force and Area in Hydraulics

Larger piston area can produce greater force from the same pressure.

Basic Mechanical Advantage Rule

A machine can reduce the force required, but usually increases the distance over which the force must be applied.

Force × Distance

Basic relationship for mechanical work.

Work ÷ Time

Basic relationship for power.

Force ÷ Area

Basic relationship for pressure.

Effort Arm ÷ Resistance Arm

Formula for the mechanical advantage of a lever.

Mechanical Comprehension ASVAB Focus

Know force, motion, friction, work, power, pressure, levers, inclined planes, pulleys, gears, and hydraulics.

Machine Trade-Off Principle

Machines make tasks easier by reducing force, but often require more distance or time.

If friction increases

More force is generally required to move an object.

If friction decreases

Less force is generally required to move an object.

If force doubles and distance stays the same

Work doubles.

If distance doubles and force stays the same

Work doubles.

If work stays the same and time decreases

Power increases.

If force stays the same and area decreases

Pressure increases.

If force stays the same and area increases

Pressure decreases.

If a lever's effort arm gets longer

Mechanical advantage increases and less effort is needed.

If a ramp gets longer for the same height

Less force is required, but the load travels farther.

If pulley support sections increase

Mechanical advantage increases and required input force decreases.

Gear size and speed

Changing gear sizes can trade speed for force/torque.

Large driving gear to small driven gear

The driven gear generally rotates faster but with less torque.

Small driving gear to large driven gear

The driven gear generally rotates slower but with greater torque.

Torque

A twisting force that causes rotation.

Rotational Motion

Motion in which an object turns around an axis.

Linear Motion

Motion in a straight line.

Axis

The fixed line around which an object rotates.

Mass

The amount of matter in an object.

Weight

The force of gravity acting on an object's mass.

Acceleration

A change in velocity over time.

Velocity

Speed in a particular direction.

Inertia

The tendency of an object to resist changes in its motion.

Newton's First Law

An object remains at rest or in uniform motion unless acted upon by an unbalanced external force.

Newton's Second Law

Force = Mass × Acceleration (F = ma).

Increasing force while mass stays constant

Increases acceleration.

Increasing mass while force stays constant

Decreases acceleration.

Balanced forces and acceleration

Balanced forces produce zero net force and therefore zero acceleration.

Net Force

The overall force remaining after all forces acting on an object are combined.

Zero net force

An object has no acceleration; it may be stationary or moving at constant velocity.

Simple Machine

A device that makes work easier by changing the size or direction of a force.