Journal Entry 1/11

1. What is the difference between a "force" and a "net force"?

"Force" is the result on an interaction between objects. When a ball is resting on the ground there are two forces interacting with it. The Earth is a force on the ball, pulling in down. The ground is also a force on the object, pushing it up, preventing it from being drawn into the core of the Earth. There is also force on an object when that object is pushed, or pulled, or lifted up. The "net force" is another term for an unbalanced force. In the previous example, the force of the Earth on the ball was balanced by the force of the ground on the ball. If that same ball was rolling along the ground and then a meter stick was pressed against the side of the ball, that meter stick would represent an unbalanced force. In this scenario, the forces of the Earth and the ground are still balanced, but the meter stick is unbalanced.

2. How do you determine the direction of the net force"

The direction of the net force of an object can be determined by using the arrows in a force diagram or by using simple addition and subtraction. Let's say that there is a large block sitting on the sidewalk, and Jen and Dan both stand on opposite sides of the block. Dan pulls the block 50 units to the right and Jen moves the block 35 units to the left. If we were representing this situation with arrows, the forces of the earth on the block and the ground on the block would be balanced, so their vertical arrows would be the same length. Their horizontal arrows would not be the same length however. Because Dan is exerting a greater force on the box (50> 35), the arrow to the right would be longer than the arrow to the left. From the diagram, we would be able to see that the direction of the net force would be to the right. If we were to use basic arithmetic to represent the same situation, we would represent 50 units to the right as (+50) and 35 units to the left as (-35). We would then add these numbers together, to get (+25). Because our answer is positive, we know that Dan is exerting a greater force and therefore the direction of the net force is to the right.

3. What is the connection between the net force exerted on on object by other objects, and the motion of the object?

Often times, the net force represents the direction in which the object moves. In the last example, the box moved to the right because Dan was exerting a greater force than Jen. The net force was going towards the right, so the box was moving to the right. The idea that an object always moves in the direction of the unbalanced force, however, is false. In the first example, when a meter stick is pushed against a ball that is rolling along the ground, the meter stick represents the net force. If the ball was rolling right and the meter stick was pressed against it, the ball would not suddenly start to move left. The ball continues to move to the right. This proves that objects don't always move in the direction of the net force.