7/31/2019 Acceleration Aaaaaaaaaaaaaaaaaa
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Pulling the wagon with twice the force produces twice the
acceleration and acceleration is in direction of
force.
The wind changes the speed of a boat from 2 m/s to 8 m/s in 3 s.
Each second the speed changes by 2 m/s.
Wind force is constant, thus acceleration is constant.
Acceleration is positive (+) or negative (-) based on the
direction of force.
Choose + direction first. Then acceleration a will have the same
sign as that of the force F regardless of
the direction of velocity.
Example 3 (No change in direction): A constant force changes the
speed of a car from 8 m/s to 20 m/s in 4
s. What is average acceleration?
Step 1. Draw a rough sketch.
Step 2. Choose a positive direction (right).
Step 3. Label given info with + and - signs.Step 4. Indicate
direction of force F.
Example 3 (Continued): What is average acceleration of car?
Step 5. Recall definition of average acceleration
Example 4: A wagon moving east at 20 m/s encounters a very
strong head-wind, causing it to change
directions. After 5 s, it is traveling west at 5 m/s. What is
the average acceleration? (Be careful of signs.)
Step 1. Draw a rough sketch.
Step 2. Choose the eastward direction as positive.
Step 3. Label given info with + and - signs.
Example 4 (Cont.): Wagon moving east at 20 m/s encounters a
head-wind, causing it to change directions.
Five seconds later, it is traveling west at 5 m/s. What is the
average acceleration?
Choose the eastward direction as positive.
Initial velocity, vo =+20 m/s, east (+)
Final velocity, vf= -5 m/s, west (-)
The change in velocity, Dv = vf - v0
Dv = (-5 m/s) - (+20 m/s) = -25 m/s
2 1
2 1
avg
v v va
t t t
= =
( 0)
inst
va t
t
=
2 1
2 1
avg
v v va
t t t
= =
3 m/s, rightwarda = +
