Bellringer

January 30th 2013

If an astronaut throws a ball in space, when will the ball stop?

WRITE THE QUESTION AND ANSWER.

Speed, Velocity, & Acceleration

What is speed?

Speed is a measure of how fast something moves or the distance it moves, in a given amount of time. The greater the speed an object has, the faster it changes position.

Review

The rate at which an object moves is its speed. Speed can be calculated this way:› Speed = Distance/Time

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Example

A train travels 300 km in 5 hours. Solve to find the train’s speed.

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Speed Word Problems

A bug crawled 25.0 cm in 5.0 seconds. Its average speed was ______.

In 2.5 seconds, a turtle was able to travel 10.0 cm. Its average speed was _______.

Speed, Velocity, & Acceleration

1. What is velocity? Velocity is a speed in a specific

direction.2. What is acceleration? The rate at which velocity changes with

time.3. What is the relationship between

velocity and acceleration? Acceleration is a measure of how quickly

the velocity changes. If velocity does not change, there is no acceleration.

Definition of Acceleration

An acceleration is the change in velocity per unit of time.

A change in velocity requires the application of a push or pull (force).

A formal treatment of force and acceleration will be given later. For now, you should know that:

• The direction of accel- eration is same as direction of force.

• The acceleration is proportional to the magnitude of the force.

ACCELERATION

Acceleration includes:a) Speeding up; also called accelerating or

positive acceleration.

ACCELERATION

b) Slowing down; also called decelerating or negative acceleration.

ACCELERATION

c) Turning, which is a change of direction.

Pulling the wagon with twice the force produces twice the acceleration and acceleration is in direction of force.

Pulling the wagon with twice the force produces twice the acceleration and acceleration is in direction of force.

Acceleration and Force

F

a

2F 2a

ACCELERATION

The formula to calculate acceleration is:

(Final velocity) – (Starting velocity)The amount of time the change takes

= Vf – Vi = Δ Vt t

“Change”

ACCELERATION

* The SI unit for acceleration is:

m/s/s or m/s2

** This is because you are dividing velocity (m/s) by time (s)

Example of Acceleration

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.Wind force is constant, thus acceleration is constant.

+

vf = +8 m/s v0 = +2 m/s

t = 3 s

Force

Acceleration is positive (+) or negative (-) based on the direction of force.

Positive acceleration is speeding up.

Negative acceleration is slowing down.

Acceleration is positive (+) or negative (-) based on the direction of force.

Positive acceleration is speeding up.

Negative acceleration is slowing down.

ACCELERATION

7. Parachute example:

A man jumps from a plane. His beginning speed is 100 mi/hr, and his ending speed with his parachute is 10 mi/hr. This took 3 sec. What was his acceleration?

(10 mi/hr – 100 mi/hr) = -30 mi/hr/s3s The negative sign

means that he was decelerating (slowing down).