acceleration the rate at which velocity changes. acceleration acceleration can be described as...

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