Contact force

In physics, a contact force is a force that acts at the point of contact between two objects,^[1] in contrast to body forces. Contact forces are described by Newton's laws of motion, as with all other forces in dynamics.

Contact forces are ubiquitous and are responsible for most visible interactions between macroscopic collections of matter. Pushing a car up a hill or kicking a ball are some of the everyday examples where contact forces are at work. In the first case the force is continuously applied by the person on the car, while in the second case the force is delivered in a short impulse. Certain contact forces describe specific phenomena and are important enough to have been given unique names. The most common instances of this include friction, normal force, and tension.

In the Standard Model of modern physics, the four fundamental forces of nature are known to be non-contact forces. The strong and weak interaction primarily deal with forces within atoms, while gravitational effects are only obvious on a macroscopic scale. Molecular and quantum physics show that the electromagnetic force is the fundamental interaction responsible for contact forces. The interaction between macroscopic objects can be roughly described as resulting from the electromagnetic interactions between protons and electrons of the atomic constituents of these objects. Everyday objects do not actually touch each other; rather contact forces are the result of the interactions of the electrons at or near the surfaces of the objects (exchange force).

Example

An example of contact force commonly encountered in college-level physics is the force between two masses A and B which are lying next to each other and a force F is being applied on one of the masses, for example A. In such a case, the contact force will be proportional to the mass of B.

Hence, we can see the many examples of contact forces in everyday life. Contact forces can act through a rigid connector or a non rigid connector.

For example when a boy pulls a cart through a rope he is connecting the force applied through a non rigid connector (the rope). He could also pull the cart through the handle of the cart hence transferring the force through the rigid connector (the handle).

But from case A the boy cannot push the cart (disadvantage of non rigid connector).

See also

• Body force
• Surface force

References

1. ^ Knight, Randall (2008), Physics for Scientists and Engineers: A Strategic Approach (2 ed.), California: Pearson Addison-Wesley, ISBN 0-321-51671-0 p. 127