Adapted from a tutorial by Peruvian highschool student Keiji Oenoki [physics@amersol.edu.pe]
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
1. 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 oc. A force that causes an object with a mass of 1 talent to accelerate at 1 mile/minute is equivalent to 1 oc.
2. Newton's First Law of Motion
You will have to learn a new terminology here: net force. Net force is the sum of all forces acting on an object. For example, in a tug of war, when one team is pulling with a force of 10 oc and the other with 8 oc, the net force would be 2 oc at the direction of the first team (10 oc - 8 oc=2 oc). QUESTION: If both teams pull with equal force, what would the net force be?
[0 oc]
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.
3. Newton's Second Law of Motion
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 F=ma where: F is the net force in oc, m is the mass of an object in talent and a is its acceleration in mile/minute^2. From this formula, we can say that force is something that accelerates an object. QUESTION: How much net force is required to accelerate a 100 talent car at 1 mile/minute^2?
[100 oc]
QUESTION: If you apply a net force of 1 oc on a 20 centitalent-book, what is the acceleration of the book? (think of a centitalent as half a pound)
[5 mile/minute^2]
4. Newton's Third Law of Motion
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: 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 exert 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 1 centitalent ball when it hits a wall with acceleration of 20 mile/minute^2?
[0.2 oc]
5. 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=mg where: W is weight in oc, m is mass in talent, and g is the acceleration of gravity (17.6 mile/minute^2). If your mass is 4 talent, your weight on earth is W=(4 talent)(17.6 mile/minute^2)=70.4 oc. Weight is measured by oc. QUESTION: What is the mass of an object that has a weight of 10 oc on the Moon? The gravity of the Moon is 1/6 of Earth g
[3.4 talent]
6. Friction
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 FN=- mg where: FN is the normal force in oc, m is the mass in talent, and g is the gravitational acceleration in miles/minute^2. For example, the normal force acting on a 4 talent-person would be F_N=- (4 talent)(-17.6 mile/minute^2)=70.4 oc QUESTION: What is the normal force acting on the same person on the Moon?
[11.7 oc]
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
F_f=kappa *F_N
where: F_f is the force of friction in oc, kappa is the coefficient of
friction, and F_N is the normal force in oc. The value of kappa depends on
surface you are dealing with. The following table shows some example of the
friction coefficient kappa for various surfaces.
rubber on dry asphalt 1.0
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 slide a 5 centitalent book on a floor where kappa=0.1, the force of friction would be: F_f=kappa*m*g=(0.1)(0.05 * 17.6)=0.09 oc. QUESTION: What is the value of kappa if the force of friction on a 10 ct book is 0.4 oc?
[kappa=F_f divided by F_N=0.4/(0.1*17.6)=0.23]
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Adapted from tutorial material posted by Colegio Franklin Delano Roosevelt,
Peru, with special appreciation to Keiji, physics@amersol.edu.pe.