# BJU 6th Science – Chapter 9

energy The ability to do work.
potential energy Stored energy due to its position.
kinetic energy Energy caused by motion.
mechanical energy The ability to get something moving.
motion Change of an object's position.
reference point A fixed, unmoving object needed to determine whether another object has changed position.
distance The amount of space traveled from where an object starts to where it is at any given moment.
speed The rate at which an object travels; determined by dividing the distance by the amount of time the object takes to travel that distance.
instantaneous speed The speed of an object at any one particular moment.
velocity The distance an object moves over a given amount of time in a certain direction.
acceleration The change in velocity during a period of time.
force A push or pull.
friction A force that keeps objects from moving.
momentum The mass and velocity (speed and direction) of an object.
first law of motion An object tends to stay at the same velocity; an object at rest tends to stay at rest and an object at motion tends to stay in motion unless acted on by some outside force.
inertia The resistance to a change in motion.
gravity The pull of one object on another.
second law of motion The acceleration of an object is related to the objects mass and the amount of force being exerted on the object; can be written as mass x acceleration = force.
third law of motion All forces come in pairs; when an object exerts a force on another object, the second object exerts and equal force back on the first object; also called the law of action and reaction.
work A force acting on something and moving it a distance.
newton The unit used to measure force (weight).
joule The unit used to measure work; equal to one newton meter of work.
machine An object that makes work easier.
effort force The force applied to a simple machine.
resistance force The force that works against the effort force.
lever A simple machine consisting of any bar that turns on a fulcrum.
fulcrum The fixed point on which a lever turns.
first-class lever A type of lever that has the fulcrum between the effort and resistance.
second-class lever A type of lever that has the resistance between the effort and fulcrum.
third-class lever A type of lever that has the effort between the resistance and the fulcrum.
pulley Simple machine consisting of a wheel with a chain or rope wrapped in the groove of the wheel.
fixed pulley A type of pulley that is attached to something so that the pulley does not move; it changes the direction of a force.
moveable pulley A type of pulley that moves along a rope where the effort is above the pulley.
block and tackle An arrangement of fixed and movable pulleys connected by ropes.
mechanical advantage The decrease in effort needed to move an object.
wheel and axle A simple machine consisting of a wheel and a rod running through the axle.
inclined plane A simple machine consisting of a flat, slanted surface, such as a ramp.
wedge A simple machine consisting of two inclined planes back-to-back.
screw A simple machine consisting of an inclined plane that is wound around a cylinder or cone.