AccelerationThe rate at which velocity changes

Acceleration• Acceleration can be described as changes in

speed, changes in direction, or changes in both

• Acceleration is a vector• Shows how velocity changes

Acceleration • Acceleration can be caused by an positive

change in speed (increase)• Acceleration can also be caused by a negative

change in speed (decreasing)• Negative acceleration is known as

deceleration

Acceleration example

• You get on a bus and feel the bus “accelerate” as you go down the street. When the bus is keeping a constant speed, its acceleration is zero.

• After a while the bus stops, since the speed is changing the bus is accelerating. (called deceleration)

Free Fall

• The movement of an object toward Earth solely because of gravity

• It is acceleration due to a change in speed

Unit for Acceleration

• Remember the unit for velocity is meters/second

• The unit for acceleration is meters/second2 or m/s2

• Objects falling near Earth’s surface accelerate downward at 9.8 m/s2

• Each second an object is in a free fall, its velocity increases downward by 9.8 meters per second.

Free Fall

• Look at p. 343 and the rock being dropped down the well

• After 1 second the stone will be falling at 9.8 m/s

• After 2 seconds the stone will be falling at 19.6 m/s (2 x 9.8)

• The change in the stones speed (acceleration) is 9.8 m/s2

Changes in Direction• Acceleration is not always due to a change in

speed• You can accelerate even if you are going at a

constant speed• For example, if you are riding a bicycle and go

around a curve, you are accelerating even though you are keeping a constant speed but you are changing direction

• A carousel is traveling at a constant speed but is accelerating since its direction is changing constantly

Constant Acceleration

• The velocity of an object moving in a straight line shows constant acceleration

• This is when there is a steady change in velocity

• The velocity of an object changes by the same amount each second

• A plane’s acceleration may be constant at take-off time

Calculating Acceleration

• You can calculate acceleration for a straight-line motion by dividing the change in velocity over the total time

• a = acceleration, vi = initial velocity and vf = final velocity

• So acceleration = change in velocity/total time

a= (vf – vi)/t

Calculating Acceleration• If velocity increases, then the acceleration is

positive• For example, coasting down a hill on a bicycle

• If velocity decreases, then the acceleration is negative

• For example, coasting on a bike at the bottom of the hill, your velocity will decrease