Acceleration
Read this text. Pay attention to the examples which show how to solve equations of motion. These include how to calculate displacement, given average velocity and time, and how to calculate final velocity, given initial velocity, acceleration, and time.
Example 2.7 Calculating Deceleration: The Subway Train
Example 2.7 Calculating Deceleration: The Subway Train
Finally, suppose the train in Figure 2.22 slows to a stop from a velocity of 20.0 km/h in 10.0 s. What is its average acceleration?
Strategy
Once again, let's draw a sketch:
Figure 2.23
As before, we must find the change in velocity and the change in time to calculate average acceleration.
Solution
2. Calculate . The change in velocity here is actually positive, since
4. Convert units.
Discussion
The plus sign means that acceleration is to the right. This is reasonable because the train initially has a negative velocity (to the left) in this problem and a positive acceleration opposes the motion (and so it is to the right). Again, acceleration is in the same direction as the change in velocity, which is positive here. As in Example 2.5, this acceleration can be called a deceleration since it is in the direction opposite to the velocity.