Study Guide

Forces and Motion Terms

Forces and Motion Terms


We’re all in space. No, not in the universe, floating away from everything we’ve ever known and loved. Space is that reality continuance thingy with three dimensions that we navigate everyday while walking up the street. Space is a location with spatial coordinates.


Ever heard the line Time is of the essence? Before waxing philosophical, just know that time is essential to describe any kind of physics because interactions take place over time the same way they take place over space. It’s the invisible dimension that gives duration to an event.


Spacetime = space + time = rendezvous. If we’re planning on meeting friends, then we give them a place (space) and a time at which to meet up. Without the proper space AND time coordinates, we’ll never meet one another, because all events take place within the spacetime continuum.


Walk into a movie. Do they make you put on those horribly cheesy glasses? If yes, the movie is in 3D, just like the rest of the world with its up, over and out-there-in-your-face-ness. If not, then the movie is confined to the screen, in 2D. The “forward and backward”-ness of moving through a row of seats in the theatre is bounded by one spatial dimension, in 1D.


The number of units between two points. The distance between (1, 2) and (1, 5) is 3 units.

SI Unit

The standard international unit of whatever type of variable is being measured, such as meters for distance or kilograms for mass. Use alternate units at the peril of your grade.


The amount of spatial dimension between the thing that moves and its starting point. The distance traveled from home to the theatre and back might be two blocks, but our total displacement is zero, since we end up right back where we started. Displacement refers to a specific place. Oh, and it’s a vector quantity.


A street name for the drug methamphetamine. Speed is superpowerful and addictive, and it's a stimulant. That means that it makes your heart race and gives you tons of energy. Oh yeah, and you don't want to sleep all that much when you're on it.


A speed or rate of travel in a particular direction. Let's just look up one term and—yep, sounds like a vector to us.


The change in velocity over time. If velocity changes, it means something’s either speeding up, slowing down, changing direction, or some combination thereof. Exciting changes, all. Amusement parks build their thrills off of acceleration.

Inertial Mass

The ratio of the applied force to an object’s rotational acceleration, in SI units of kilograms. It’s related to but not equal to weight. It quantifies how difficult it is to make something spin.


The quantity of matter in a body.


A number; used in scalar multiplication.


A mathematical structure that has both magnitude and direction, represented by an ordered pair of components.


We think of a force as a great influence in our lives. Forces decide what happens to what, where, when, and how and does it all from various distances including but not limited to gravity, friction, and magnetism. Force is an influential friend with telekinesis powers.

Newton’s First Law of Motion

Objects are like people. Once they get comfy, they don’t want to move either. If you’re resting, you’ll have a tendency to keep resting. Objects that are uninfluenced by external forces always either remain at rest or continue moving at a constant velocity. This is the law of inertia, or the “couch potato law” for the motionless.

Newton’s Second Law of Motion

F = ma. The force applied on an object is equal to its mass times its acceleration. If an object ‘‘accelerates, it’s is experiencing a force. If the object isn’t moving then F = 0. If it’s moving but going at a constant speed, then F is also zero. When the total net force on the object is zero, the system is at equilibrium, a kind of boring state where not much happens and old, stiff substitute teachers from 80’s movies our parents probably liked, call out “Bueller, Bueller.”

Newton’s Third Law of Motion

For every action, there’s an equal and opposite reaction, such as our force pushing on a wall countered by the wall pushing back on our hands. That’s not to say that objects can’t move: they do all the time. Objects’ motions are governed by the forces acting on them, and not their own reactions back.

Acceleration of Gravity

the acceleration due to a gravitational force. On the surface of planet Earth, this acceleration is .

Force of Gravity

Also known as “gravity” for short. A force that objects exert on each other through a mass-sensitive field. It holds us to the surface of the planet but is endlessly important in other ways. Think of gravity as superglue for solar systems and galaxies. Without this force, no masses would bind together to form natural structures and therefore nothing solid, or even liquid, would exist.


Weight is a force, not a mass. In this case, W = mg, where g is the acceleration due to gravity of the Earth. Picture walking on the moon with Neil Armstrong. Weight is a lot less over there, though mass remains the same. This is because acceleration due to gravity on the moon is a lot weaker than on Earth.

Normal Force

In vector land, the normal force is the contact force perpendicular from the surface on which an object lies at rest. For surfaces that lay flat horizontally, the normal force points straight up, the opposite direction of the force of gravity. It's what keeps objects from falling through whatever matter lies between it and the center of the earth. For a hill or other inclined surface, the normal force points diagonally up, perpendicular to the surface.


The force opposing a motion. It's air resistance, surfaces rubbing each other and slowing down a motion, or the force creating the great grip of rubber on the road. Friction affects everything. Ice on ice has low friction, and rubber on rubber high friction.

Static Friction

The force of friction preventing objects from sliding over each other. It can be surmounted by a larger force, at which point it’s not longer static friction opposing the motion, but kinetic friction.

Kinetic Friction

The force of friction present as objects slide or rub on each other. It varies by combinations of materials and masses, etc.

Coefficient of Friction

The constant reflecting the degree of friction present between two materials. There’s a difference between the coefficient of static friction μs and the coefficient of kinetic friction, μk


If you ask us, equilibrium should've been spelled EQUALibrium. In equilibrium, every force that acts on an object is canceled out. An object in equilibrium will either stand still or move at constant speed (without acceleration) unless a force changes and puts it out of equilibrium, causing acceleration.

Free-Body Diagram

Otherwise known as “force diagram”. It’s the “stick-figure” of all the forces acting on an object. All we care about, are forces as represented by arrows, the mass, and acceleration of the object in question.

Free Fall

Whatever is dropped rather than thrown experiences free fall from the force of gravity and its corresponding acceleration of gravity alone.

Projectile Motion

This is the type of motion we can expect from a baseball, cannonball, football, or any other object thrown in a gravity field, a.k.a on Earth..The trajectory of motion makes a nice arc in the sky called a parabola.

Orbital Motion

The motion of an object imprisoned by the gravity of another more massive object, such as a satellite around Earth. The “imprisoned” object is always falling toward, but never reaching, the larger mass.

Circular Motion

The motion of an object that travels in a circle, including but certainly not limited to orbital motion, due to a force pulling in towards the center of the circle.

Orbital Speed

How fast the gravitational prisoner is stuck going around in circles.

Orbital Period

The amount of time it takes the prisoner to go around the massive body once.

Escape Velocity

The escape velocity is how fast an object needs to go to escape a gravitational field. It may not be so fast when you're running away from your out-of-shape brother, but to get off the Earth you need to go pretty dang fast.