A pitcher throws a ball, and the batter hits a home run.

What changes did the ball go through in its motion?

It was still, sped up in the pitcher’s hand, lost speed on its way to the batter, stopped when it hit the bat, changed direction, sped up again, then slowed back down and eventually stopped.

What part of that motion is acceleration?

In everyday language acceleration means the process of speeding up.

In science, acceleration is the rate at which velocity changes.

In science, acceleration refers to :› Increasing speed› Decreasing speed› Changing directions

When an object’s speed increases it is accelerating.› This is what you normally think of when

you think of accelerating.› Example:

When a car that is stopped begins to move, the car is accelerating.

When an object slows down it is accelerating.› The term for acceleration when slowing is

deceleration or negative acceleration.› Example:

When a car stops at a red light or a ball lands in the catcher’s mitt, both the car and the ball have negative acceleration or deceleration.

Objects traveling at constant speeds can still be accelerating when they are changing direction.

Many objects change direction without changing speed.

For example: A Ferris wheel

You can use both a speed vs. time graph and a distance vs. time graph to look at the motion of an accelerating object.

If you see a curved line on a distance vs. time graph, the object will be accelerating.

If you see a slanted, straight line on a speed vs. time graph, the object will be accelerating at a constant rate.

Acceleration is increasing speed, decreasing speed, or changing direction.› When you step on the gas, the car

accelerates› When you step on the brake, the car

accelerates.› When you round a corner, the car

accelerates.PH Science Explorer, Grade 8 TE, p.321.

Acceleration = final speed minus initial speed, divided by time

Final speed – Initial SpeedTime

For an object that starts from rest, the initial speed will be zero.

A car accelerates from rest to a speed of 40 meters per second in 30 seconds. What is the car’s acceleration?› (40m/s – 0 m/s) ÷ 30s = › 40m/s ÷ 30s =› 1.33 m/s2

› Note: The unit for acceleration is meters per second squared = m/s2

› Remember – speed is m/s, Do Not Confuse the two.

1. A falling raindrop accelerates from 10m/s to 30m/s in 2 seconds. What is the raindrop’s average acceleration?

2. A certain car can accelerate from rest to 27m/s in 9 seconds. Find the car’s average acceleration.

PH Science Explorer, Grade 8 TE, p.323.

1. (30m/s – 10m/s) ÷ 2s =20m/s ÷ 2s =10m/s2

2. (27m/s – 0m/s) ÷ 9s =27m/s ÷ 9s =3m/s2

Sketch (draw pictures of) 3 ways an object can accelerate.

Show Mrs. Bradley your notes before you leave today.