1. The Nature of Force
Force causes an object to stop moving, start moving, or change direction.
In science, the word force has a simple and specific meaning. Force is a push or pull.
When one object pushes or pulls another object, we say that the first object is exerting a force on the second object. Examples of force is you exert force on a nail when you hammer it, on a wagon when you pull it, and on a pebble when you skim it across a pond.
Like velocity and acceleration, forces are described not only by how strong they are, but also by the direction in which they act. If you push a door open, you exert force in a different direction than if you pull on the door.

2. Unbalanced Forces
Turn to page 45 and we will read about unbalanced forces. Answer the following questions.
When you and a friend push a box across the room, you (add or subtract) the two forces?
If you are pushing one way and your friend is pushing the other way, you (add or subtract) the forces?
According to the book, the (width or length) of each arrow tells you the strength of the force?
The (wider or narrower) the arrow, the more strength the force has.

3. Try it yourself + = Unbalanced forces in opposite directions
+ =
Unbalanced forces in opposite directions
Balanced forces in opposite directions
+ =

4. The overall force on an object after all the forces are added together is called the net force. When there is a net force acting on an object, the forces are said to be unbalanced.
Unbalanced forces can cause an object to start moving, stop moving, or change directions.
Unbalanced forces acting on an object will change the object’s motion. In other words, an unbalanced force will cause an object to accelerate.
For example, if two unequal forces acting in the opposite directions are applied to a box, the box will accelerate in the direction of the greater force.

5. Balanced Forces
Equal forces acting on one object in opposite directions are called balanced forces.
One force is exactly balanced by the other force.
Balanced forces acting on an object will not change an object’s motion.
When you add equal forces exerted in the opposite directions, the net force is zero. Therefore, the box will not move.

6. Take a Look
Open your books to page 47 to see examples of unbalanced forces in the same direction, unbalanced forces in opposite directions, and balanced forces in the same direction.

7. Newton’s First Law of Motion
The ancient Greeks observed that objects have natural resting places.
Objects move toward those places. For example, a rock falls to the ground. A ball rolls to the bottom of the hill.
Once an object is in its resting place, it cannot move by itself. For an object ot move, a force has to act on it.

8. Inertia
In the early 1600’s an Italian astronomer, Galileo Galilei, questioned the idea that a force is needed to keep an object moving. He suggested that once an object is in motion, no push or pull is needed to keep it in motion.
Whether an object is moving or at rest, every object resists any change to its motion.
This resistance is called inertia.
Inertia is the tendency of an object to resist change in its motion.

9. Sir Isaac Newton
Galileo’s ideas paved the way for the English mathematician, Sir Isaac Newton.
Newton discovered the three basic laws of motion in the late 1600’s.
Newton’s first law of motion states that an object at rest will remain at rest, and an object that is moving at constant velocity will continue moving at constant velocity unless acted upon by an unbalanced force.
Newton’s first law of motion is also called the law of inertia.
Inertia explains why when a car suddenly stops, inertia causes you to continue moving forward. Passengers in a moving car have inertia. Therefore, a force is required to change their motion. That force is exerted by the safety belt. If the safety belt is not worn, that force may be exerted by the windshield instead. What do we have now in most cars that will exert the force to change the passenger’s motion?

10. Mass
Think about this… Which is more difficult to move, a jar of pennies or a jar of foam peanuts? What is the difference between the jar of pennies and the jar of foam peanuts? They both have equal volume which means they take up the same amount of space. The difference is the amount of mass each one has.
Mass is the amount of matter in an object.

12. How do we Measure Mass? Mass is the amount of matter in an object.
The standard unit of measurement of mass is kilograms. For example, a bicycle might have a mass of 10 kilograms and a rider might have the mass of 45 kilograms. Which one has more mass? The rider of course.
A kilogram is equal to 1000 grams. The mass of a nickel equal about 5 grams.
The amount of inertia an object has depends on its mass. The greater the mass of an object, the greater its inertia. Mass, then, can also be defined as a measure of the inertial of an object.