Addition and subtraction of Forces
Vectors acting on the same object can be replaced by a single vector with the same effect. This single vector is known as the resultant or net vector.
Unbalanced Forces
Consider a box being pulled by a dog with a force of 120 N to the right. The box experiences a resistance of 20 N due to the friction between itself and the ground. In effect, the box is moving with a resultant force or net force of 100 N. We say that there is an unbalanced force of 100 N acting on the box.
This unbalanced force causes the box to accelerate towards the right. We can replace the two forces with a single-force arrow that represents a net force of 100 N actin towards the right.
Balanced Force
On the other hand, when two men push against a stationary car each with a force of 400 N and 500 N respectively the total force exert by th two acting on the car is 900 N. If the friction between the car and the ground 900 N, then the net force acting on the car will be zero N. This net Force of 0 N will not cause the stationary car to move. In this case, we say that the force acting on the car is balanced. when the force acting on the car are balance the car experienced no net force, or no resultant force is acting on the car.
Worked Example
Suppose two tug boats pull a stationary container ship at an angle of 30 to each other, with a horizontal force of 6000 N and 7000 N respectively. What is the resultant force acting on the container ship?
graphically
Effects of net Force
If there is net force acting on a body, the body will respond in any of the following ways
1. The shape and the size of the body will change. this effect of the force is clearly observed when you play with paste.
2. the direction of motion of the body will change when a force is applied to the volley ball, it bounce off in the direction different from its income one.
3. the body will accelerate and its velocity will change over time.
this means that : a. it will move faster if the net force is greater than zero. positive net force will speed up motion, suppose Mr. Ahmad is riding his snowmobile if the force developed by the engine is 1200 N, and the total force resisting its motion is 800 N, an unbalanced force or force (F net) of 400 N produce.
F net= F in the direction of motion - F in opposes motion =
1200 - (300+500) = 400 N
hence, Mr. Ahmad snowmobile will accelerate and move faster and move faster.
b. It will slow down if the net force is less than zero. negative net force will oppose and slow down motion. suppose Mr. Ahmad snowmobile suddenly stills half way to its journey. this will reduce the forward thrust. so the net force acting on it while it is still in motion in the opposing force of 800 N.
F net= F in the direction of motion - F opposes motion =
0- 800 = -800 N
This negative net force will slow down the snowmobile, and will finally cause it to stop moving.
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