1. A Closer Look at Forces
How do forces affect us?

2. Four basic forces in nature Gravitational force Strong nuclear force
Electromagnetic force
Weak nuclear force

3. The Four Basic Forces
We do not normally encounter the strong and weak nuclear forces in everyday life.
These are the forces that make nuclear power stations and nuclear bombs work.
Gravitational force (or gravity) is the force that pulls objects toward Earth’s surface.
All things that have mass have a gravitational force that naturally attracts them to each other.
The force is very weak when objects have a small mass.
It is the electromagnetic force that causes many of the motions and activities we deal with on a daily basis. (Magnetic force, electromagnetic force, frictional force, compressive force, etc.)

4. Forces All forces have two components—magnitude and direction.
Magnitude is a measure of how strong the force is.
Direction refers to which way the force is pulling or pushing.
The unit we use to measure force is the newton (N), named after Sir Isaac Newton, an English physicist who developed many of our current ideas about force and motion.

5. Mass and Weight
Mass is a measure of the amount of matter or material in an object.
Weight is a measure of how strong the force of gravity is between two objects.
For example, if you hold a rock in your hand, Earth pulls the rock toward its centre, and the rock pulls Earth toward its centre.
However, the rock’s mass is so small in comparison with the mass of Earth that when you let go of the rock, the rock moves much farther toward Earth than Earth moves toward the rock.

6. Mass and Weight
Earth exerts a force of 9.8 N for every kilogram of an object’s mass, so we say that on Earth, the force of gravity is 9.8 N/kg (9.8 newtons per kilogram)
To pick up a container of milk, you have to overcome the force of gravity keeping the container of milk on the counter.
Since a litre of milk has a mass of about 1 kg (Figure 4), you have to apply approximately 9.8 N of force upward to lift the container.

7. Friction
Friction is the force that resists motion between two objects in contact with each other.
We depend on friction for much of what we do.
Friction allows us to walk without slipping and keeps things from sliding off surfaces.
There are two types of friction:
Static friction is the force that prevents surfaces at rest from sliding against one another.
For example, you must overcome static friction before you can make a textbook slide across a desk.
Sliding friction is the force that acts like a brake when one surface slide over another surface.
The force that acts against the movement and causes the book to slow and stop.

8. CHECK YOUR LEARNING
You place a book on your desk, and then you begin to lift one edge of the desk. You make the following observations:
At first the book does not move.
When you lift the desk edge higher, the book begins to slide slowly.
As you lower the desk slightly, the book slows down, then stops.
Use your knowledge of forces and friction to explain your observations.
State two components common to all forces. How are these indicated in a diagram?
Name three forces involved in brushing your teeth.