Section 2: Acceleration

Chapter 2: Motion

Warm-up:A train travels 150 km in 3 hours. It is traveling directly from south towards the north. What is the speed of the train? What is the displacement of the train? What is the trains velocity?

Learning Goals:Identify how acceleration, time, and velocity are related.

Explain how positive and negative acceleration affect motion.

Describe how to calculate the acceleration of an object.

AccelerationAcceleration: the rate of change of velocity.

When the velocity of an object changes, the object is accelerating.

AccelerationA change in velocity can be either a change in how fast something is moving or a change in the direction it is moving.

So acceleration occurs when an object changes its speed, its direction, or both.

Speeding Up and Slowing Down

When you think of acceleration, you probably think of something speeding up. However, an object that is slowing down is also accelerating. Acceleration has direction, just as velocity does.

Speeding Up and Slowing Down

If the acceleration is in the same direction as the velocity, the speed increases and the acceleration is positive.

Speeding Up and Slowing Down

If the speed decreases, the acceleration is in the opposite direction from the velocity, and the acceleration is negative.

Changing DirectionAny time a moving object changes direction, its velocity changes and it is accelerating.

Changing DirectionThe speed of the

horses in this carousel is constant, but the horses are accelerating because their direction is changing constantly.

Calculating AccelerationAcceleration = change in velocity/timea = (vf-vi)/t

Vf = final velocity (m/s)

Vi = initial velocity (m/s)

t = time (s)

a = acceleration (m/s2)

Calculating Acceleration

a = (vf-vt)/t

Change in v =t =

Example 1Determine the acceleration of a car

that starts at rest and has a final velocity of 20 m/s north after 10 s.

Example 2 A ball is dropped from a cliff and has an

acceleration of 9.8 m/s2. How long will it take the ball to reach a speed of 24.5 m/s?

Example 3 A sprinter leaves the starting blocks with

an acceleration of 4.5 m/s2. What is the sprinters speed 2 s later?

Positive AccelerationSuppose a jet airliner starts at rest at

the end of a runway and reaches a speed of 80 m/s in 20 s.

The airliner is speeding up, so the final speed is greater than the initial speed and the acceleration is positive.

Negative AccelerationImagine that a skateboarder is moving

in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s.

The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.

Check-in:Describe the motion of an object that has an acceleration of 0 m/s2.