What Distinguishes Internal Forces from External Forces?

[Canuck156]

Hi,

I need to know the difference between internal and external forces. From what I can understand, Internal forces will not change the total mechanical energy of the object, while external forces will. What are some examples of internal of external forces?

Thanks for any help.

 

[Doc Al]

If two parts of a system exert forces on each other, those forces are said to be internal forces for the system.

An external force is a force between a part of a system and something outside the system.

So whether a force is internal or external depends on what you define as your system.

Examples:

1) a baseball. The forces holding the particles of the baseball together are internal forces; the force of a bat hitting the ball is an external force on the ball; so is gravity.

2) a cat and dog fighting in a sack. The two are killing each other, but considered as one system the forces they exert on each other are all internal.

2') the cat considered separately. Considering the cat as a separate system, forces between the cat and the dog are now external forces.

3) A car. The parts of the car exert forces on each other: the pistons push the rods, the axle pushes the wheels, etc. All these forces are internal to the car as a whole. But the friction that the ground exerts on the tires is an external force. So is the weight of the car, and the force the ground exerts to hold up the car.

I hope this helps a little.

 

[M98Ranger]

Internal forces refer to the forces that act within an object or system, while external forces act on the object or system from outside. Internal forces do not change the total mechanical energy of the object, while external forces can change the energy.

Examples of internal forces include tension, compression, and shear forces within a structure, or the force exerted by muscles in the human body. These forces do not affect the overall energy of the system, but rather cause changes within the object.

External forces, on the other hand, can change the total energy of the object. Examples of external forces include gravity, friction, and air resistance. These forces act on the object from outside and can either increase or decrease its energy. For example, a ball rolling down a hill experiences both the force of gravity pulling it downwards and the force of friction resisting its motion, which ultimately results in a change in the object's total mechanical energy.

I hope this helps clarify the difference between internal and external forces. Remember, internal forces are contained within the object or system, while external forces act on it from outside.