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9Sci CH3

9th Science CH3 Exploring Motion and Forces

acceleration the rate of change in velocity (a change in direction or a change in speed); the amount of acceleration depends on velocity change and time interval
average speed a rate of motion determined by dividing the total distance traveled by the total travel time
balanced forces forces that are equal in size and opposite in direction; balanced forces acting on a body do not change the motion of the body
constant speed speed that does not change
force a push or a pull one body exerts on another; unbalanced forces always change the motion of the body
friction the force that opposes motion between two surfaces that are touching one another; the amount of friction depends on the types of surfaces and the kinds of force pressing the surfaces together
gravity force exerted by every object in the universe on every other object; the amount of gravitational force depends on the masses of the objects and the distance between them
inertia the tendency of an object to resist any change in its motion; if motionless, it tends to remain at rest; if moving, it tends to keep moving at the same speed and in the same direction
instantaneous speed the rate of motion at a given instant in time, such as that given on a car’s speedometer
net force the sum of the forces on an object when unbalanced forces are applied to it; a net force changes the object’s speed or direction or both
speed the rate of motion, or the rate at which a body changes position
velocity the speed and direction of a moving body, such as a storm or a basketball thrown across a court
weight the measure of the force of gravity on an object; weight changes with changes in fravity and is measured in newtons (N)