A force that acts on an object from outside is known as the external force. Any force that acts on a structure from within is known as internal force. Two or more objects that interact with each other collectively form a system. An external force occurs as a result of interaction between a system and the surroundings. On the other hand, an internal force is an interaction that exists within a system. External forces cause motion in an object whereas, an internal force resists the motion. Work is said to be done when an external force acts on an object, while work done due to an internal force is equal to zero. A force is neither external nor internal by nature, it depends on the system of consideration. If the pair of action-reaction forces are present inside the system, the force applied is known as internal otherwise, it is external.
1. Hitting a Ball
When a footballer kicks a ball, he exerts a significant amount of force on the ball. The pair of action-reaction forces are not present entirely in the system of consideration. Therefore, the force acting on the ball is said to be an external force. This external force acting on the ball causes it to move. The distance covered by the object is proportional to the intensity with which force is applied to the ball.
2. A Tree Trembling due to Wind
A tree is set to swing when a force is applied by the wind on it. Since the force exerted by the wind on the tree is from the outside, it is known as an external force. On the contrary, the force that helps the tree to stay in the position and prevents it from falling down is known as an internal force.
3. Bending a Scale
When a muscular force is applied on the edges of a scale, it bends. A significant amount of tension and compression gets built into the scale. The muscular force acting on the scale is the external force. This force has a magnitude that causes the scale to bend but does not break it. This is because an internal force provides support and prevents it from breaking.
4. Pushing a Bus
When a bus is pushed while sitting inside it, it does not move. However, when the people sitting on the bus come out and apply push force on it from outside, it tends to move. Motion is introduced in the bus with the help of an external force exerted by the passengers. The push force applied by the passengers while being seated inside the bus does not help the bus to move because the passengers become a part of the system. The force existing within the system that prohibits it to move and is said to be an internal force.
5. Spring Action
When a force is exerted on a spring in order to stretch it, the spring comes into motion. The force acting on the spring is the external force. The force that tends to compress the spring and regain its original shape is known as the internal force. The internal force is completely the opposite of the external force and resists motion or any change in shape.
6. Pushing a Chair
When a person standing on the ground exerts a force on a chair, it tends to move in the direction of the applied force. When the same person applies the same amount of push force to the chair while sitting on it, it does not move. The magnitude and the direction of the applied force remain the same in both cases. The only difference lies in the system of consideration. The chair moves in the first case because the person is not a part of the system. The force applied in such a case is called an external force. Whereas, the chair does not move in the second case because the person sitting on the chair becomes a part of the system itself. Hence, the force exerted here is said to be an internal force.
7. Compressing a Sponge
When a person presses his hand against the surface of a sponge, it changes its shape. The force applied by the person acts within the system therefore, the compression force is said to be an internal force. This compression force being internal in nature resists motion and therefore, produces an opposite force that helps the sponge to regain its original shape.
8. Stretching a Rubber Band
When a rubber band is pulled or stretched, it tends to develop tension force. The elastic object comes back to its original shape once the pull force is removed. Since the interaction exists within the object or the system, the force is said to be internal. However, the force exerted to stretch the band and cause motion to change its shape is called the external force.