Forces behind forces

Chemical structures, which are essentially combinations of electromagnetic attractions and repulsions, give solid objects their intrinsic “springiness” and “stickiness.”

They can push, adhere and drag.

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Common Forces

With the exception of gravity, most forces that we commonly encounter appear* to result from contact between two bodies:

Pushing perpendicular to a surface (called a normal force)

Pushing parallel to a surface (called a friction force)

Pulling via contact (called a tension force)

Pulling via gravity (called the gravitational force) is one common force that doesn’t require such “contact.” It is most clearly “force at a distance.”

*Actually, all forces act at a distance; “contact” has no meaning at the atomic/subatomic level.

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Normal Force

A normal force is a push on an object by a surface—a push directed perpendicular to that surface. (“Normal” means “perpendicular.”)

Notice: The surface exerts the force. Normal forces often arise when objects are stacked or pushed together—“reaction” forces.

Caution: A normal force is not always up-or-down—and it is not always equal in magnitude to an object’s weight.

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An unknown force is pushing horizontally on Block A (10.0 kg). Block B (15.0 kg), in turn, is being pushed by block A. Both blocks are sliding together across a frictionless horizontal surface in the positive x-direction with an acceleration of 3.00 m/s2.

Find the normal force FBA.

1. 45.0 N

2. –45.0 N

3. 30.0 N

4. –30.0 N

5. None of the above.

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Static Friction Force

A friction force is a “drag” force on an object by a surface—directed parallel to that surface. There are three kinds of friction forces: static, kinetic and rolling.

Static friction is a force that prevents a stationary object from beginning to move (slide) along a surface in response to another force. Thus, the magnitude of the static friction force, FS, will vary (depending on the force it is reacting to)—up to a maximum, FSmax. This maximum is different for each object and surface and is proportional to the normal force exerted on the object by the surface: FSmax = SFN. The coefficient of static friction, S, is a unitless number (often less than 1) that describes how the maximum static friction force compares to the normal force.

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Example: A 40-kg block sits at rest on a horizontal surface. The coefficient of static friction between the surface and the block is 0.25. What is the magnitude of the frictional force acting on the block?

1. 0 N

2. 10 N

3. 98 N

4. 392 N

5. None of the above.

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Kinetic Friction

When any object is already sliding along any surface, there is a certain amount of “drag” force—resistance to that motion. This is kinetic friction force (and it’s usually less than the maximum static friction force; thus it’s harder to get an object started in a sliding motion than it is to maintain that motion.) Notice: The direction of the friction force always opposes the surfaces’ sliding motions.

For most reasonable sliding speeds, the magnitude of the kinetic friction force does not depend on that speed. It is proportional only to the normal force exerted by the surface on the object: FK = KFN The coefficient of kinetic friction, K, is a unitless number (often less than 1) that describes how the kinetic friction force compares to the normal force.

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