Relationship between applied force and friction

Frictional Forces These forces act between the molecules of two different surfaces that are in close the force of static friction fs has the same magnitude as the applied force, but. The normal force is one component of the contact force between two One important difference between the two is that the kinetic friction. According to the laws of physics, the force of friction, Ffriction, always acts to oppose the force you apply when you try to move an object. Friction is proportional.

The nature of the force of gravity will be discussed in more detail in a later unit of The Physics Classroom. Always be cautious of the distinction between mass and weight. It is the source of much confusion for many students of physics.

Flickr Physics Photo A 1. The scale reads just short of Mass refers to how much stuff is present in the object.

Relationship between applied and normal force? - Physics Stack Exchange

Weight refers to the force with which gravity pulls upon the object. Even on the surface of the Earth, there are local variations in the value of g that have very small effects upon an object's weight.

These variations are due to latitude, altitude and the local geological structure of the region. Use the Gravitational Fields widget below to investigate how location affects the value of g. Sliding versus Static Friction As mentioned abovethe friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. For the purpose of our study of physics at The Physics Classroom, there are two types of friction force - static friction and sliding friction. Sliding friction results when an object slides across a surface. As an example, consider pushing a box across a floor. The floor surface offers resistance to the movement of the box.

We often say that the floor exerts a friction force upon the box. This is an example of a sliding friction force since it results from the sliding motion of the box. If a car slams on its brakes and skids to a stop without antilock brakesthere is a sliding friction force exerted upon the car tires by the roadway surface.

This friction force is also a sliding friction force because the car is sliding across the road surface. Sliding friction forces can be calculated from knowledge of the coefficient of friction and the normal force exerted upon the object by the surface it is sliding across.

The coefficient value is dependent primarily upon the nature of the surfaces that are in contact with each other.

Relationship between applied force and frictional force?

The static frictional force is given by the equation: The coefficient of friction static or kinetic is a measure of how difficult it is to slide a material of one kind over another; the coefficient of friction applies to a pair of materials, and not simply to one object by itself. Note that there is a less-than-or-equal-to sign in the equation for the static frictional force. The static force of friction has a maximum value, but when two surfaces are not moving relative to each other the static force of friction is always just enough to exactly balance any forces trying to produce relative motion.

What happens when one object is sliding over another, when there is relative motion between two surfaces? There will still be a frictional force, but because we're dealing with things in motion we call it the kinetic frictional force.

Types of Forces

There is a different coefficient of friction associated with kinetic friction, the kinetic coefficient of friction, which is always less than or equal to the static coefficient. As with the static frictional force, the kinetic frictional force acts to oppose the relative motion of the surfaces in contact.

One important difference between the two is that the kinetic friction equation has an equals sign: Applying Newton's laws Let's try a couple of examples involving friction.

Example 1 - A box of mass 3. A string tied to the box exerts a vertical force of 7. What is the kinetic coefficient of friction?

Again, start by drawing a picture. In this case, because the box is traveling down the ramp, we know the frictional force is kinetic. It opposes the motion, so the frictional force must be directed up the slope. One thing to keep in mind in this problem is that the velocity is constant - this means the acceleration is zero. The free-body diagram is also shown, with the forces split into components parallel and perpendicular to the inclined plane.

Because there is no acceleration, any coordinate system is fine - a system parallel and perpendicular to the ramp is pretty convenient, though, because two of the forces are along those directions. Applying Newton's second law to the forces in the y-direction: Again, we can use this equation to solve for the normal force: Applying Newton's second law in the x-direction gives: This tangential component opposes the applied force. The net force on the cabinet is zero. The harder you push the greater is the microscopic displacement of the surface molecules and the greater is the tangential component of the intermolecular forces. When you push hard enough, some of the projections on the surfaces will break off, i. The horizontal component of the intermolecular forces diminishes and no longer completely opposes the applied force. But while the horizontal component has diminished, it has not vanished. It is now called the force of kinetic friction.

For the cabinet to keep accelerating, you have to push with a force greater in magnitude than the force of kinetic friction. To keep it going with constant velocity you have to push with a force equal in magnitude to the force of kinetic friction.

If you stop pushing, the force of kinetic friction will produce an acceleration in the opposite direction of the velocity, and the cabinet will slow down and stop.