Chapter 3-Lesson 1

Forces and MotionWhat is Motion?

The Big Question:

How can motion be described and measured?

Lesson Objectives:

Students will understand how an object’s mass affects the amount of force needed to move it and how the Earth’s gravity affects objects. (GLE 27,38)

Vocabulary Words:1. motion- a change in a position of an object.2. force- any push or pull.3. reference point- a place or object

used to determine if an object is in motion.

4. gravity- the force that pulls all objects toward each other.

MOTIONAll kinds of things move around you in different ways. Objects can move in a straight line, a curved path, back and forth as a vibration, or as a rotation. You can describe and measure their motion in different ways.Motion is a change in the position of an object.

RELATIVE MOTIONEvery day, you compare objects that change position with objects that do not. The change in one object’s position compared with another object’s position is called relative motion. (look at picture on p. 92)

FRAME OF REFERENCEObjects that do not seem to move define your frame of reference. Your frame of reference is like your point of view. How an object seems to move depends on your frame of reference.One way to define your frame of reference is to find a reference point. (When you ride in a car, you can tell your car is moving by observing signs, trees.)

QUESTIONS1. Suppose there are passengers seated on a boat.

Are the passengers moving compared to the boat? Why or why not?

2. Is a person rowing a boat moving compared to a person standing on the shore? How do you know?

3. What is the relative motion of a person who is running past you?

ANSWERS1. No, because the distance between

the passengers and the boat stays the same.

2. Yes, because the distance between the person rowing and the person standing changes.

3. The person is passing you.

• Why should you choose an object in a fixed point as a reference point?

If you choose a moving object as a reference point, it is more difficult to compare how another object moves in relation to a moving object.

FORCES AFFECT OBJECTS

Forces make objects move or stop. Force can make an object that is standing still start to move in the direction of the force. It can also make a moving object move faster, slow down, stop, or change direction.• A contact force must touch an object to affect it.• Pushing or pulling can change both the position

and motion of an object.• All forces have size and direction.

FORCE AND MOTIONForce causes a change in motion in an object. The amount of force acting on an object affects how that object changes speed, direction, or both.

A moving object changes its motion only when a force acts on it. If balanced forces are applied to a moving object, it will keep moving at the same speed and in the same direction.

Balanced forces that act in opposite directions cancel each other.

FORCE AND MASS How an object moves also depends on how much mass it has. More force is needed to change the motion of an object with more mass.

FORCE OF GRAVITYThe strength of the force of gravity depends on the masses of the objects and how much distance is between them.

Gravity is the force that makes an object such as a ball fall to the ground.

MASS AND WEIGHT

Mass and weight are not the same thing.

Mass measures how much matter is in an object.

Weight measures how strong the force of gravity is on an object.

Mass never changes, but weight can change.