In previous chapters, we studied how objects move. In this chapter, we will study why objects move as they do. We will study Newton's Laws of Motion, which explain the relationship between acceleration and force. We will also use Newton's Laws for problem solving.
1. What is a Force? 2. Newton's First Law of Motion 3. Newton's Second Law of Motion 4. Newton's Third Law of Motion 5. Mass and Weight 6. Friction 7. Chapter 4 Quiz
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.
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.
Newton's First Law deals with an object with 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 formula
where:
From this formula, we can say that force is something that accelerates an object.
QUESTION: If you apply a net force of 1 N on 200 g-book, what is the acceleration of the book? m/s2
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.
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
If your mass is 70 kg on Earth, your weight is W=(70 kg)(9.8 m/s2) = 686 N.
Weight is measured by Newton (N).
You will have to learn another vocabulary before you proceed: the 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 is
For example, the normal force acting on a 70 kg-person would be FN = - (70 kg)(-9.8 m/s2) = 686N
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
The value of depends on surface you are dealing with. The following table shows some example of .
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
Try the Chapter 4 Quiz and see how well you can do!
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