Acceleration Reading
by Kristina Wilhelm
| 9 Questions
Describe what happens to the speed of a bicycle as it goes uphill and downhill.
Velocity and Acceleration
A car sitting at a stoplight is not moving. When the light turns green, the driver presses the gas pedal and the car starts moving. The car moves faster and faster. Speed is the rate of change of position. Acceleration is the rate of change of velocity. When the velocity of an object changes, the object is accelerating.
Remember that velocity is a measure that includes both speed and direction. Because of this, a change in velocity can be either a change in how fast something is moving or a change in the direction it is moving. Acceleration means that an object changes it speed, its direction, or both.

Acceleration is
speed and direction.
rate of change in speed.
change in direction.
rate of change in velocity.
Velocity is
speed and direction.
a really fast dinosaur.
rate of change in speed.
a change in speed only.

How are speeding up and slowing down described?
When you think of something accelerating, you probably think of it as speeding up. But an object that is slowing down is also accelerating. Remember that acceleration is a change in speed. A car that is slowing down is decreasing its speed. It is also accelerating, because its speed is changing.
Imagine a car being driven down a road. If the speed is increasing, the car has positive acceleration. When the car slows down, the speed decreases. The decreasing speed is called negative acceleration. In both cases, the car is accelerating, but one acceleration is positive and one is negative.
Acceleration has direction, just like velocity. In the figure below, both cars are accelerating because their speeds are changing. When a car’s acceleration and velocity are in the same direction, the speed increases and the acceleration is positive. Car A has positive acceleration. When a car is slowing down, the acceleration and velocity are in opposite directions. The acceleration is negative. Car B has negative acceleration.
Summarize: How are speeding up and slowing described?

Describe the acceleration of the cars in each figure.
Does changing direction affect acceleration?
A change in velocity is either a change in an object’s speed or its direction. When a moving object changes direction, its velocity changes and it is accelerating. The speed of a horse moving around on a carousel remains constant, but it is constantly changing direction. So, the horse is accelerating.
How do you calculate acceleration?
Acceleration is the rate of change in velocity. To calculate acceleration, you first find the change in velocity. To find change in velocity subtract the beginning velocity of an object from the velocity at the end of its movement. Beginning velocity is called the initial velocity, or vi . Velocity at the end is called the final velocity, or vf.
change in velocity= final velocity - initial velocity
= vf - vi
If motion is in a straight line, the change in speed can be used to calculate the change in velocity. The change in speed is the final speed minus the initial speed. To find acceleration, divide the change in velocity by the length of time during which the velocity changed.

acceleration (a)= change in velocity /time (t) or a = (vf - vi /t)

The SI unit for velocity is meters per second (m/s). To find acceleration, velocity is divided by the time in seconds (s). So, the unit for acceleration is m/s2 .
Write what vf and vi mean.

Calculate: Suppose a bird takes off from a tree and flies in a straight line. It reaches a speed of 10 m/s. What is the change in velocity?
Explain: Why is acceleration of an object moving at a constant velocity always zero?

Think Critically: A car that is slowing down is still moving forward. Why is this considered negative acceleration?

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