298UNIT5 MOTIONANDFORCECalculating accelerationThe equation foraccelerationTo calculate acceleration, you divide the change in speed by the time over which the speed changes. For example, if a bicycle’s speed increases from 2 m/s to 6 m/s, its change in speed is 4 m/s. Because two speeds are involved, subscripts are used to show the difference. The starting speed is v1, and the ending speed is v2. Positive andnegativeaccelerationIf an object speeds up, it has a positive acceleration. If it slows down, it has a negative acceleration. In physics, the word acceleration is used to describe to any change in speed, positive or negative. However, people sometimes use the word decelerationto describe the motion that is slowing down.ACCELERATIONAcceleration(m/s2)Change in speed (m/s)a= v2- v1tTime (s)Calculating accelerationA sailboat moves at 1 m/s. A strong wind increases its speed to 4 m/s in 3 seconds (Figure 14.12). Calculate the acceleration.1. Looking for:You are asked for the acceleration in meters per second. 2. Given:You are given the starting speed in m/s (v1), final speed in m/s (v2), and the time in seconds.3. Relationships:Use the formula for acceleration: 4. Solution:Your turn...a.Calculate the acceleration of an airplane that starts at rest and reaches a speed of 45 m/s in 9 seconds. Answer: 5 m/s2b.Calculate the acceleration of a car that slows from 50 m/s to 30 m/s in 10 seconds. Answer:-2 m/s221vvat−=24 m/s1 m/s3 m/s1 m/s3 s3 sa−===Figure 14.12: An acceleration example with a sailboat.