DESCRIBING MOTION AND SPEED - Bardstown City Schools · 2015. 12. 7. · DESCRIBING MOTION AND SPEED Unit 3.1 Powerpoint . What is Motion? ... sec The division sign ... Chapter 1:

DESCRIBING MOTION

AND SPEED

Unit 3.1 Powerpoint

What is Motion?

Motion – a description

of an object when its

distance from another

object is changing

Movement depends on

your point of view

Reference Points

Reference point - a place or object

used for comparison to determine if

something is in motion

An object is in motion if it changes

position relative to a reference

point

Reference Points

A stationary, or non-moving object makes an excellent reference point.

You do not want to have a moving reference point or motion will not have a good description.

For example, you are sitting on a bus while looking at a bus next to your bus. You all of a sudden think that your bus is moving backward, but then you notice that the other bus is moving forward. You made the moving bus

a reference point.

SI (International System of Units)

We use what is called the standard, Imperial, customary or, English units of measure in the U.S.

The SI system, or metric system is used throughout the world

There is a base word and in front of the base word is a prefix

The only countries that do not use the metric system (SI) are in red: U.S., Myanmar (in Asia), and Liberia (in Africa).

SI (International System of Units)

All metric units have a base unit.

Base units are:

1. Meter (m) – Unit of Measure for Distance or length

[English unit equivalent is a yard, inches, etc.]

2. Liter (L) – Unit of Measure for volume

[English unit equivalent is a quart, gallon, etc.]

3. Gram (g) – Unit of Measure for mass

[English unit equivalent is a pound, ton, etc]

The base unit may have a prefix attached.Ex. centi-meter or centimeter

Prefix Name Relative Size

Kilo (k)- Largest Prefix

Hecto (h)-

Deka (D)-

Base Unit

Deci (d)-

Centi (c)-

Milli (m)- Smallest Prefix

For example, since kilo- is a larger prefix than centi-, a kilometer is larger than a centimeter.

Measuring Distance (or Length)

To measure distance and an object’s motion, you have to have units of measurement!

In science class, we will use the metric system, or also known as SI(Systeme International, in French) (International System of Units, in English)

Measuring Distance (or Length)

A meter – the base unit (SI) for length A yard is 36 inches and a

meter is 39.5 inches

A centimeter – one hundredth of a meter, so there are 100 centimeters in a meter Used for short distances

A millimeter – There are

1,000 millimeters in a meter Used for short distances

Measuring Distance (or Length)

The kilometer – There are 1,000 meters in a kilometer Used for long

distances (in comparison to miles)

Calculating Speed

Speed – the distance the object

travels in one unit of time

Rate – tells you the amount of

something that occurs or changes

in one unit of time

Speed = distance

time

Calculating Speed

Speed = distance

time

= miles

hour

= kilometer

hour

= inches

sec

The division sign -------- is read as per

Example: miles per hour, kilometer per hour

Different Types of Speed

There are 4 different ways to describe the

speed of an object:

1. Constant Speed

2. Average Speed

3. Instantaneous Speed

4. Velocity

Different Types of Speed

Constant Speed- If the speed of an object does not change, the object is traveling at a constant speed

Ex. You drive on I-65 from Shepherdsville to Louisville on cruise control at 70 mile/hour

Different Types of Speed

Most objects do not

move at constant

speeds for very

long

Average speed -

divide the total

distance by the total

time •Car trip

Different Types of Speed

Instantaneous Speed – the rate in which an object is moving at a given instant in time.

An example of instantaneous speed is when an officer can clock the speed of a car with a speed-capturing device

Different Types of Speed

Velocity - Speed in a given direction

When you know the speed and direction of an object’s motion, you know the velocity of the object

Example 15 km/hour westward is velocity while 15 km/hour is speed

Graphing Motion

You can show the motion of an object on a line graph in which you plot distance against time

Time is along the x-axis and distance on the y-axis for a graph showing speed

There are many motion graphs (acceleration, distance, etc.), so be careful to check the axes.

Interpreting Motion Graphs

A straight line indicates a

constant speed

The steepness depends on

how quickly or slowly the

object is moving

The faster the motion the

steeper the slope

Interpreting Motion Graphs

Slope is defined as rise over run or the steepness of a line

Slope = rise

run

On the graph, the steeper (dotted) line indicates a larger distance moved in a given time. In other words, higher speed.

Both lines are straight, so both speeds are constant.

Interpreting Motion Graphs

The line on this graph is curving upwards. This

shows an increase in speed, since the line

is getting steeper

In other words, in a given time, the distance

the object moves is change (getting larger). It

is accelerating (or speeding up).

Deacceleration is slowing down (will discuss in

3.2)

Using the Speed (or Velocity) Formula

When you need to solve for the speed (or velocity when given a

direction)

Use the speed (or velocity) formula Speed = distance

time

When you need to solve for the distance an object traveled:

Rearrange the speed formula

Distance = Speed x Time

When you need to solve for time it takes an object to travel a

distance:

Rearrange the speed formula

Time = distance

speed (velocity)

The Magic Speed Triangle

Use the magic speed triangle

Remember it as:

S.DOT = Speed is distance over time

Or

V.DOT= Velocity is distance over time

V or S

D

T

The Magic Speed Triangle

If you want to find Distance

1. Cover the D on the Triangle

2. You are left with a V on the bottom and a T

3. This is means D = V multiplied by T or

D =V · T

If you want to find Velocity or Speed

1. Cover the V(or S) on the Triangle

2. You are left with a D on top and a T on the bottom

3. This is means V(or S) = D divided by T or

V = D

T

If you want to find Time

1. Cover the T on the Triangle

2. You are left with a V on the bottom and a D on top

3. This is means T = D divided by V or

T = D

V

V or S

D

T

The Magic Speed Triangle