## What is a force?

Force can be defined as a push or a pull. (Technically, force is something that can accelerate objects.) For example, when you throw a baseball, you apply a force to the ball. Force is measured by N (

**Newton**). A force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton.## What is Newton's first law?

You will have to learn a new terminology here:

The answer is 0 N. Don't forget to give a unit!

When you slide your book on floor it will stop soon. When you slide it on icy surface, it will travel further and then stop. Galileo believed that when you slide a perfectly smooth object on a frictionless floor the object would travel forever.Isaac Newton developed the idea of Galileo further. He concluded that

**net force**. Net force is the sum of all forces acting on an object. For example, in a tag of war, when one team is pulling the tag with a force of 100 N and the other with 80 N, the net force would be 20 N at the direction of the first team (100 N - 80 N = 20 N).**QUESTION****:**If both teams pull the tag with equal force, what would the net force be? If you think you've got the correct answer, select the "empty" area underneath!The answer is 0 N. Don't forget to give a unit!

When you slide your book on floor it will stop soon. When you slide it on icy surface, it will travel further and then stop. Galileo believed that when you slide a perfectly smooth object on a frictionless floor the object would travel forever.Isaac Newton developed the idea of Galileo further. He concluded that

**an object will remain at rest or move with constant velocity when there is no net force acting on it.**This is called Newton's First Law of Motion, or Law of Inertia.## What is Newton's second law?

Newton's First Law deals with an object with

F = m x a where:

The answer is 5000 N.

The answer is 5 m/s2..

*no*net force. Newton's Second Law talks about an object that has*net force. It states that***when the net force acting on an object is not zero, the object will accelerate at the direction of the exerted force. The acceleration is directly proportional to the net force and inversely proportional to the mass.**It can be expressed in formulaF = m x a where:

- F is the net force in N,
- m is the mass of an object in kg and
- a is its acceleration in m/s2.

**QUESTION****:**How much net force is required to accelerate a 1000 kg car at 5.00 m/s2? If you think you've got the correct answer, select the "empty" area underneath!The answer is 5000 N.

**QUESTION****:**If you apply a net force of 1 N on 200 g-book, what is the acceleration of the book? If you think you've got the correct answer, select the "empty" area underneath!The answer is 5 m/s2..

## What is Newton's third law?

When you kick the wall in your room, you will probably end up hurting your foot. Newton's Third Law of Motion can explain why:

The answer is 2 N.

**when one object applies a force on a second object, the second object applies a force on the first that has an equal magnitude but opposite direction**. In other words, when you kick the wall, the wall kicks you back with equal force. As a result you will get hurt. These forces are called**action-reaction forces**.Remember when you kick the wall, you exerts force on the wall. When the wall kicks you back, it exerts force on you. Therefore, the net force on the wall is not zero and the net force on your foot is not zero neither.**QUESTION:**What is the net force on 200 g ball when it hits a wall with acceleration of 10 m/s2? If you think you've got the correct answer, select the "empty" area underneath!The answer is 2 N.

## What is the difference between mass and weight?

**Mass**and

**weight**are different in physics. For example, your mass doesn't change when you go to the Moon, but your weight does. Mass shows the quantity, and weight shows the size of gravity.

If you know your mass, you can easily find your weight because

W = m x g where:

- W is weight in Newton (N),
- m is mass in kg, and
- g is the acceleration of gravity in m/s2.

W=(70 kg)(9.8 m/s2) = 686 N.

Weight is measured by Newton (N).

**QUESTION**

**:**What is the mass of an object that has a weight of 115 N on the Moon? The gravity of the Moon is 1/6 of g (which is 9.8 m/s2). If you think you've got the correct answer, select the "empty" area underneath!

The answer is 70.4 kg.

## What is friction?

You will have to learn another vocabulary before you proceed: the

FN = - m x g where:

FN = - (70 kg)(-9.8 m/s2) = 686N

The answer is 114.3 N.

Now, we will talk about friction.When you slide your book on floor, it will come to stop because of the

Force of friction can be calculated by the formula

Ff = u x FN

where:

rubber on dry asphalt ~

rubber on wet asphalt 0.95

steel on steel 0.18

steel on ice 0.010

rubber on ice 0.005

For example, if you throw a 500 g book on floor where = 0.1, the force of friction would be:

Ff = = (0.1)(0.5 * 9.8) = 0.49 N

The answer is 10.1714.3 N.

**normal force**. The normal force acts on any object that touches surface (either directly or indirectly). The normal force would be applied on a ball on a table, but not on a ball in the air, for instance. It always acts perpendicularly to the surface. The formula to calculate the normal force isFN = - m x g where:

- FN is the normal force in Newton (N),
- m is the mass in kg, and
- g is the gravitational force in m/s2.

FN = - (70 kg)(-9.8 m/s2) = 686N

**QUESTION****:**What is the normal force acting on the same person on the Moon? If you think you've got the correct answer, select the "empty" area underneath!The answer is 114.3 N.

Now, we will talk about friction.When you slide your book on floor, it will come to stop because of the

**force of friction**. Friction is the force that acts between two object in contact because of action-reaction.Force of friction can be calculated by the formula

Ff = u x FN

where:

- Ff is the force of friction in N,
- is the coefficient of friction, and
- FN is the normal force in N.

**Surface****Value of u**rubber on dry asphalt ~

rubber on wet asphalt 0.95

steel on steel 0.18

steel on ice 0.010

rubber on ice 0.005

For example, if you throw a 500 g book on floor where = 0.1, the force of friction would be:

Ff = = (0.1)(0.5 * 9.8) = 0.49 N

**QUESTION****:**What is the value of u if the force of friction on a 300 g book was 0.5 N? If you think you've got the correct answer, select the "empty" area underneath!The answer is 10.1714.3 N.

- Colegio Franklin Delano Roosevelt. "Chapter 4."
*ThinkQuest*. Oracle Foundation, July 1997. Web. 08 May 2013.