Physical Science 8th grade (1).pdf

Acceleration change in speed change in time s 1 x

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Acceleration = Change in speed Change in time s 1 x Plug in values Clear the compound fraction Final units How we get units of m/s 2 50 m s s 50 m s 50 m s x s 50 m s 2
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298 U NIT 5 M OTION AND F ORCE Calculating acceleration The equation for acceleration To 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 v 1 , and the ending speed is v 2 . Positive and negative acceleration If 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 deceleration to describe the motion that is slowing down. ACCELERATION Acceleration (m/s 2 ) Change in speed (m/s) a = v 2 - v 1 t Time (s) Calculating acceleration A 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 ( v 1 ), final speed in m/s ( v 2 ), 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/s 2 b. Calculate the acceleration of a car that slows from 50 m/s to 30 m/s in 10 seconds. Answer: -2 m/s 2 2 1 v v a t = 2 4 m/s 1 m/s 3 m/s 1 m/s 3 s 3 s a = = = Figure 14.12: An acceleration example with a sailboat.
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