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Resistive Force

by Ron Kurtus (revised 29 September 2015)

A resistive force is one that inhibits or resists the motion of an object. It acts in a direction opposite of any motion or applied force that is trying to move the object. It is considered passive in that it is not active or applied and there is no apparent agent of force.

The most common resistive force is friction, where an object is held back from sliding across a surface. Another form of resistive force is fluid resistance, where the object is trying to plow through a fluid material.

If the object or material is moving with respect to another object, it can provide an applied—as well as a resistive—force on the object.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Friction resistive force

When a freely moving object slides across a surface, it can encounter friction resistive force between the surfaces that can slow down and even stop the motion of the object.

If the object is pushed or pulled by an applied force, its acceleration will be reduced from the friction. If the force of friction is greater than the applied force, the object will not move forward.

When an applied force and a resistive force are acting on an object, the resulting acceleration with be the difference between the forces, provided the applied force is greater.

Moving against friction

For example, in the case of an object being pushed along the floor against the force of friction:

FR = F − Fr

where

Applied force less than resistive force

If the applied force is less than the resistive force (F < Fr), the object will not move. For example, you can push on extremely heavy box but not be able to move it.

(Note that although your effort may be great, the amount of work you have done is zero, because the box did not move.)

A box that is pushed along the surface of a deck experiences the resistive force of friction. If the deck is moving at some velocity in either direction, the friction still holds because it is not a function of speed.

(See Resistive Force of Friction for more information.)

Fluid resistance

Fluid resistance can be air resistance, water resistance, or resistance from other fluid material—even mud.

Fluid resistance occurs when a solid object pushes its way through a liquid, such as water, or a gas, such as air.

For example, once a car has reached some speed and allowed to coast freely on a level road, air resistance and rolling friction will slow it down until it finally stops. Also, you can apply the brakes to slow and stop the car.

However, fluid resistance is a function of the relative velocity of the object moving through the fluid.

Applied resistive force

Friction and fluid resistance are resistive forces when the material is stationary. However, both can also contribute as an applied force when the materials or objects are moving relative to each other.

For example, a boat moving through still water experiences the resistive force of water resistance. If the water is moving in the same direction as the boat is moving but at a slower speed, the force of the water resistance is reduced. However, if the the water is moving in the same direction as the boat but at a faster speed, the water acts as an applied force and pushes the boat forward.

If the water is moving in an opposite direction than the boat is moving, it is applying a force on the boat, slowing it down.

Summary

A resistive force inhibits or resists motion and acts in a direction opposite of any motion or applied force. Friction is the most common resistive force. A freely moving object can be slowed down and even stopped by a resistance. The rate of acceleration of an pushed object can be reduced by a resistive force. An applied force may not even be able to overcome a large resistance.


Don't resist change


Resources and references

Ron Kurtus' Credentials

Websites

Resistive force - Wikipedia

Motion in the Presence of Resistive Forces - India Online Academy

Average Resistive force exerted by a wooden block on a bullet - Exercise

Resistive Force - Forces against a dragster vehicle

Drag (Physics) - Wikipedia

Physics Resources

Books

Top-rated books on Physics of Force


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