# Forces and Motion

## Vocabulary

1. relative motion -- change in one object’s position compared to positionof another fixed object
2. frame of reference -- the objects you use to detect motion
3. speed -- the rate at which an object changes position
4. velocity -- the speed and the direction of an object

## Types of Motion

Objects can move in different ways. One way objects can move is in a straight line.

A baseball player running to home plate usually moves in a straight line.

Objects can also move in a curved path. The wheels on your bicycle follow a curved path around the center of the wheel.

Objects can also move back and forth. When a guitar player plucks a string, the string moves back and forth.

When you ride your bicycle, you pass many things that do not move. They are fixed in place.

When you ride your bicycle past a mailbox you know you are moving. You compare things that change position with things that do not change position. The change in one object’s position compared to another object’s fixed position is relative motion.

## How You Know You Are Moving

How do you know if a person on a slide moves? You look at the changing position of the person. You compare the changing position with the fixed position of the slide. You use the relative motion of the objects to decide what is moving and what is not

moving.

The objects you use to decide if something is moving are your frame of reference. Your frame of reference is like your point of view.

Imagine you are on a moving school bus. From your frame of reference on the bus, the buildings outside seem to be moving. But the buildings have not moved. As the bus moves down the street, people on the sidewalk see you on the bus. From their frame of reference, you and the bus are both moving.

## Measuring Motion

Speed is the rate at which an object changes position. It measures how fast an object moves. A car moving at a high speed changes position faster than a car moving at a slower speed. The unit for speed is a unit of distance divided by a unit of time.

Kilometers per hour is a unit of speed.

Velocity is the speed and direction an object is moving. Direction can be described

using different words. North, south, east, and west can describe direction. Up, down, right, and left can also describe direction.

Any change in the speed or direction of an object’s motion is acceleration. Speeding

up and slowing down are accelerations. A roller coaster accelerates as it moves up a hill because it is changing speed. A roller coaster accelerates on a curved path because it changes direction.