Force and acceleration.

Basic competency:
To identify various kinds of forces and their influences on an object.

Force and acceleration.
You may have played with the skate-board in any place. What makes it fun is that it makes use of some basic physical laws. What is the relation between the skate-board and motion and force?
Push a door-leaf so that opens. Pull a rubber hand. Press clay lifts your books. In each case above, you give a force. Force is a push or pull given to an object by another object.
When you push a table, you must touch the table to give force to the table. It also happens when you throw the stone. The muscle force with which you push the table or the spring force with which you throw a stone is a touch force. It is called so because an object giving force must touch another object.
When you release a chalk from a certain height, then will drop, pulled by a force of gravitation to the earth. This force is non-touch force, because it emerges without a touch between the chalk and the earth.
Give examples of other touch and non-touch forces.

Drawing force.
Force is drawn as an arrow. The length of the arrow indicates the quantity of a force and the direction of arrow indicates the direction of the force.
Example:

(a)

(b)

(c)


If figure (a) describes a force of 10 N, then figure (b) is twice as long as figure (a) and describes a force of 20 N in the same direction. Figure (c) is three times as long as figure (a) and describes a force of 30 N with a direction opposite to that of figure (a)
Adding force.
If there are two forces working in same direction, the resultant force is obtained by totaling the two forces. If the two forces have opposite directions, then resultant force is obtained by subtracting the smaller one from the bigger one with the direction following the bigger force.

Activity.
1. Draw 2 forces in the same direction as F1 = 20 N and F2 = 50 N at the scale of
10 N = 10 cm.
2. Draw the resultant force from F1 and F2.
3. Draw a resultant force if force F2 is going in a direction opposite to that of force F1.

Friction.
If you slide a book across the top of a table, the book moves but its movement becomes slower and slower and finally it stops. Why does the book? An invisible force is working on the book and the table. The force is friction. Friction is a force to oppose motion between two touching surfaces. This friction causes the book’s motion becomes slower and slower, and the then book stops. The quantity of friction force depends on two factors: roughness of the two surfaces and the force pressing two touching.



Advantages and disadvantages of friction.
Friction is not always harmful. If there is no friction, your life will be very different. You do not walk or you do not grip something. The friction between your shoes and the floor causes you be able to walk. You can grip an object because there is a friction.
However, friction can be harmful. An example of it is what happens to the chain and gear in your bicycle. The friction between the chain and the gear causes them to become worn out. The worst is when both of chain and the gear never given any oil.

Gravitation.
Have you ever seen a coconut fall, as shown in figure 4-3? Why does the coconut fall down? If you see an object fall, you observe the effect of the force of gravitation. Every object in the world gives pulling force on another object. This force is called gravitation.
It is difficult to believe that drop a bowling ball and a marble from a bridge at the same time, both object fall into the water almost at the same time. It means that the acceleration of both objects is almost the same. Do you think that the bowling ball falls into the water first before the marble does? It is true that acceleration due the gravitation of bowling is bigger because by its mass is bigger. But the bigger mass makes the inertia of bowling ball become bigger too because it needs more force to change its velocity. The marble has a smaller mass than the bowling ball, but its inertia is also smaller.