Accelerated MotionScience 10

Accelerated Motion

• is changing either the speed or direction, or both, of motion.

• Acceleration is the rate of change of velocity, in other words, how fast velocity is changing.

Acceleration Formula

a = v = vf - vi

t tf - ti

Acceleration is a vector quantity and therefore must always be either positive or negative to indicate direction.

Example Problem #1

a = v = vf - vi

t t

The cheetah can reach a speed of 19 m/s in 2.0 s from a standing start. What was the cheetah’s acceleration? (Assume the cheetah was moving to the right.)

= +19 - 02.0

= +9.5 m/s2

Example Problem #2

a = v = vf - vi

t t

A car slows down from 15 m/s to 8 m/s in 3.5 s. What is its acceleration. (Assume the car is moving to the right.)

= +8 - +153.5

= - 2 m/s2

Positive vs. Negative

• A positive acceleration can mean– an increase in velocity, or– a decrease in velocity in the negative

direction (left or down)

• A negative acceleration can mean– a decrease in velocity, or– an increase in velocity in the negative

direction (left or down)