Net Force – Definition and Example

Net Force

Introduction

Most objects in real life are always subjected to multiple forces at a time. However, some of them do not move even if they are under the action of multiple forces. This is because, the forces are directed in such a way that they cancel the effect of each other leaving the body at rest.  

Explanation

Net force: 

Suppose a football is hit with a force of F and as a result it starts moving. Now, if another player kicks it in the same direction and applies a second force F₁, it will speed up. If another player applies the second force F₂ on it in an opposite direction to its motion, it will slow it down. If the goalkeeper stops this football by applying sufficient force F₃ in the opposite direction of its motion, it stops. When a boy stands near a football, the football does not move at all. It starts moving only when the body kicks it. Here the football moves only when the boy applies a force on it. Thus, a force can make a stationary body move. 

 

Thus, the effect of forces increases when two forces acting on a body are in the same direction. And it decreases when the two forces acting on a body are in opposite directions to each other. 

The motion of a body subjected to multiple forces depends on how these forces combine. For example, it is easier to push a heavy object, such as a wooden dresser, when two people push it together rather than just one person trying harder to push it. Figure below depicts the situation.  

Here, the forces exerted on the dresser by the two persons are shown to be 30 N and 65 N in the same direction. The motion of the dresser would depend upon the combined effect of the two forces.  “The total force resulting from the combination of all the forces on an object is called the net force on it.” It is generally the sum of all the forces acting on the body, taking directions of all the forces into account.   

Thus, the net force on the dresser = 30 N + 65 N = 95 N.  

The net force of 95 N is directed in the same direction as the other two forces.  

Suppose a heavy object, such as a dresser, is pushed by two people from opposite sides instead of the same side. The situation is depicted in the figure below.  

The net force in this case is given by, 

Net force = 30 N – 25 N 

​Net force = 5 N​ 

Here, two forces are applied on the dresser from opposite sides, which decreases the effect of forces on it. This is because the forces acting in opposite directions subtract up, reducing the net force on the dresser. 

The combination of all the forces on an object is called the net force on that object. Forces on an object add up or subtract depending on their relative direction of application. The direction of the net force is towards the greater force. The effect of multiple forces being exerted on a body is defined by the net force on that body due to all those forces. 

Balanced and unbalanced forces: 

Sometimes, during a game of tug of war, the rope does not seem to move to any of the sides even if the children on both the sides pull it hard. This happens because in that case, the forces exerted by both the teams are equal to each other and are opposite in direction. Thus, the net force on the rope is 0.  This means that no force is exerted on the rope. Hence, it does not move. When the net force on an object is zero, the forces on the object are called balanced forces.​ An object subjected to a set of balanced forces do not move. 

When the net force on an object is not zero, the forces on the object are called unbalanced forces.​ An object subjected to a set of unbalanced forces moves. 

Question:  

Two teams, team 1 and team 2, were playing a game of tug of war. The forces of pull exerted by all the players are shown in the figure below. Which team will win the game? Why?​ 

Solution: 

The forces exerted by the players of team 1 are 63 N, 32 N, 26 N, and the force exerted by the players of team 2 are 46 N, 35 N, and 29 N.  

 Therefore, the net force  

= (63 + 32 + 26) – (46 + 35 + 29) = 11 N  

Thus, the net force on the rope is directed towards left, as the force pointing towards left is greater. Hence, team 1 will win the game. It is because, the net force is directed towards them.  

Summary

1. The effect of force increases when two forces acting on a body are in the same
   direction.
2. The effect of force decreases when the two forces acting on a body are in opposite
   directions to each other.
3. The combination of all the forces on an object is called the NET FORCE on that object.
4. The forces applied in the same direction add up to one another.
5. The forces acting in opposite directions subtract up, reducing the net force on the
   object.
6. The direction of the net force is towards the greater force.
7. When the net force on an object is zero, the forces on the object are called balanced
   forces.
8. When the net force on an object is not zero, the forces on the object are
   called unbalanced forces.