Motion, Speed& Acceleration

Review

Describe Motion

Motion

Reference Point

Frame of Reference

Frame of Reference Example

Describe Speed

Speed

Describe Speed

• A way to describe motion

– Average speed - Rate of motion calculated by dividing the distance traveled by the amount of time it takes to travel that distance

– Constant speed - Speed that does not change

– Instantaneous speed - Speed of an object at any

given time

What is the formula used to calculate speed?

What is the formula for calculating speed?

Speed is calculated by dividing distance by time –

Calculate This Speed

A football field is about 100 m long. If it takes a person 20 seconds to run its length, how fast was the football player running?

Calculate this Speed:

A football field is about 100 m long. If it takes a person 20 seconds to run its length, how fast was the football player running?

Speed = Distance ÷ Time

Speed = 100 m ÷ 20 sSpeed = 5m/s

Remember to include

the UNITS!!

Describe Velocity

Distinguish Between Speed and Velocity

• Speed describes distance and time

• Velocity describes distance, time, and direction

Describe Acceleration

• A change in velocity – which may be:– A change in speed

• Starting• Stopping• Speeding up• Slowing down

– A change in direction

• Acceleration is caused by unbalanced forces

More

Describe Acceleration

Describe Acceleration

Deceleration is called negative acceleration - it means an object is slowing down

When acceleration is calculated, it may be a negative number

- Acceleration

Calculating Acceleration•To determine the acceleration of an object moving in a straight line, you must calculate the change in speed per unit of time.

Calculating Acceleration•As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?

•Read and Understand

•What information have you been given?• Initial speed = 4 m/s• Final Speed = 22 m/s• Time = 3 s

- Acceleration

Calculating Acceleration•As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?•Plan and Solve•What quantity are you trying to calculate?• The average acceleration of the roller-coaster car = __•What formula contains the given quantities and the unknown quantity?• Acceleration = (Final speed – Initial speed)/Time•Perform the calculation.• Acceleration = (22 m/s – 4 m/s)/3 s = 18 m/s/3 s•Acceleration = 6 m/s2

•The roller-coaster car’s average acceleration is 6 m/s2.

- Acceleration

Calculating Acceleration•As a roller-coaster car starts down a slope, its speed is 4 m/s. But 3 seconds later, at the bottom, its speed is 22 m/s. What is its average acceleration?

•Look Back and Check

•Does your answer make sense?•The answer is reasonable. If the car’s speed increases by 6 m/s each second, its speed will be 10 m/s after 1 second, 16 m/s after 2 seconds, and 22 m/s after 3 seconds.

- Acceleration

Calculating Acceleration•Practice Problem

•A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s average acceleration?

•(30 m/s – 10 m/s) ÷ 2 seconds = 10 m/s2

- Acceleration

Calculating Acceleration•Practice Problem

•A certain car can accelerate from rest to 2.7 m/s in 9 seconds. Find the car’s average acceleration.

•(27 m/s – 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s2

- Acceleration

Distinguish Between Position/Distance vs. Time

Graphs and Speed/ Velocity vs.Time Graphs

Graphing Acceleration

•You can use both a speed-versus-time graph and a distance-versus-time graph to analyze the motion of an accelerating object.

- Acceleration

Position/Distance vs. Time Graphs (slope = speed)

Interpret The Graph Below:

Interpret The Graph Below:

The graph shows an object which is not moving (at rest).

The distance stays the same as time goes by because it is not moving.

Interpret The Graph Below:

Interpret The Graph Below:

The graph showsthat the objects distance increases as time passes.The object is moving and so it has velocity.

The straight line shows it is a constant (not changing).

Interpret The Graph Below:

Interpret The Graph Below:

Just like the previous graph, this graph shows an object moving with constant velocity

Interpret The Graph Below:

Interpret The Graph Below:

The curve in the graph shows that the objects velocity is changing as time passes.

This is acceleration.

Interpret The Graph Below:

Interpret The Graph Below:

In the first part of the graph the object is moving with constant velocity.

In the second part of the graph the object is at rest (not moving).

In the third part the object is again moving with constant velocity.

Velocity vs.Time Graphs(slope = acceleration)

Interpret The Graph Below:

Interpret The Graph Below:

The graph showsthat the objects velocity does not change as time passes.

It shows constant velocity.

Interpret The Graph Below:

Interpret The Graph Below:

The graph showsthat the objects velocity is increasing as time passes – it is accelerating.

The straight line shows that it is constant acceleration.