Acceleration Notes • Acceleration describes how quickly the velocity changes. • Deceleration or a negative acceleration means slowing down.

UNITS FOR ACCELERATION

•  If a car can increase its velocity from 0 m/s to 20 m/s in 1 second then it's acceleration is 20m/s/s. If a car can increase its velocity from 0 m/s to 20 m/s in 1 minute then its acceleration is 20 m/s/min

•  An acceleration in 20 m/s/s is often written as 20 m/s2. This means that each second that passes, the car’s velocity increases by 20 meters per second.

If your speed increases by 1 m/s each second, then your acceleration is 1 m/s².

ACCELERATIONFORMULA

Practice with the formula…. As a rollercoaster starts down a hill, its velocity is 4

m/s, but 3 seconds later, at the bottom of the hill, its velocity is 22 m/s. What is its acceleration? a= 22m/s – 4 m/s = 6 m/s/s or 6 m/s2 3 s

Every second that passes, the rollercoaster speeds up by 6 meters per second!

A lizard slows down from 20 mi/hr to 5 mi/hr in 4 seconds. What is the lizard’s acceleration?

a = 5 mi/hr – 20 mi/hr = - 3.75 mi/hr/s

4 s Every second that passes the lizard slows

down by 3.75 miles per hour!

Asailboatincreasesitsvelocityfrom1m/sto4m/sin3seconds.Whatwillthevelocityofthesailboatbeat6secondsiftheacceleraGonstaysthesame?

Acceleration = (4-1) = 1 m/s2 3

3 sec = 4 m/s 4 sec = 5 m/s 5 sec = 6 m/s

6 sec = 7m/s final velocity!

Acceleration and motion graphs • The slope of a velocity (or speed) vs. time graph is the object’s acceleration. If the line on the graph is horizontal, then the car is moving at a constant speed and therefore it doesn’t have any acceleration! Thereisnochangeinspeedsothere

isnoacceleraGon.

TRY: •  Draw a velocity vs. time graph for the following

situations: 1. Constant Acceleration 2. Constant Deceleration 3. No acceleration

Time

Velocity

Constant Acceleration

Time

Velocity

Constant Deceleration

Time

Velocity

No acceleration

Time

If the speed is not changing, there is no acceleration!!

Velocity

Free fall • An object is in free fall if it is accelerating due to the force of gravity ONLY. Objects in free fall on Earth accelerate downward at afree fall = 9.8 m/s2.

Aballisinfreefall(onEarth)aMerbeingdroppedfromrest.Willthespeedoftheballbegreaterthan30m/saMerfallingforonly2seconds?Proveyouranswerusingmath.

A ball in free fall will NOT be going greater than 30 m/s after 2 seconds. It will only have a speed of 19.6 m/s (9.8+9.8). Starting from rest means at 0 seconds = 0 m/s after 1 second = 9.8 m/s after 2 seconds = 19.6 m/s (9.8+9.8)

Acceleration is a vector! (vector = direction matters!)

• This means that acceleration occurs whenever there is a change in speed, direction, or both.

PLICKER QUESTIONS

Section Review 3.3 (p.73) 1. The seconds are squared in m/s2 because one unit of time is part of the speed unit and the other is the time over which the speed changed. It is easier than saying m/s/s or meters per second per second.

Section Review 3.3 (p.73) 2. A negative acceleration means you are slowing down over time.

3. Acceleration = 6m/s-0m/s = 2 m/s2

3 sec

Section Review 3.3 (p.73)

Section Review 3.3 (p.73)

4.a.5m/s-3m/s=2m/sisthechangeinspeed.b.2m/sdividedby4s=0.5m/s2

Section Review 3.3 (p.73) 5. Yes, a car accelerates when it turns a corner at a constant speed because acceleration is a vector so direction matters.

Section Review 3.3 (p.73) 7. C. The speed is greater than zero but the horizontal line tells us the speed stays the same over time.

Section Review 3.3 (p.73)

8. A object that is dropped in free fall increases its speed by 9.8 m/s every second.