Use this friction calculator to calculate the friction force between an object and the ground. It is based on a simple principle: that friction is proportional to the normal force acting between the object and the ground. Read on to learn how to apply the force of friction equation and the difference between static and kinetic friction. You can use this tool as a coefficient of friction calculator, too.

Force of friction equation

The formula that lets you calculate the friction force is very simple:

F = μN

where:

  • F is the force of friction, measured in Newtons;
  • μ is the dimensionless coefficient of friction; and
  • N is the normal force (perpendicular to the ground surface), expressed in Newtons.

How to find force of friction

  1. Choose the normal force acting between the object and the ground. Let's assume a normal force of 250 N.
  2. Determine the friction coefficient. We can choose a coefficient of friction equal to 0.13.
  3. Multiply these values by each other: (250 N) * 0.13 = 32.5 N.
  4. You just found the force of friction! Perhaps you want to check what work it performs?

Static friction vs kinetic friction

Static friction acts when the object remains stationary. Imagine you try to pull a heavy box. If we don't take friction into account, even the smallest force should cause some acceleration of the box, according to Newton's second law. In reality, you need to pull quite hard for the box to start moving because of the static friction force.

Kinetic friction acts on a moving object or, in other words, on an object with nonzero kinetic energy. If there were no kinetic friction, any object that you nudge (for example, a toy car) would never stop moving, as, according to Newton's first law, no force would act on it, so it would keep on going with a constant velocity.

Even though the formula is the same for static and kinetic friction, you need to remember that the coefficients of friction are different. The coefficient of kinetic friction is usually lower than the one of static friction.

FAQ

How do you measure the coefficient of friction?

There are two easy methods of estimating the coefficient of friction: by measuring the angle of movement and using a force gauge. The coefficient of friction is equal to tan(θ), where θ is the angle from the horizontal where an object placed on top of another starts to move. For a flat surface, you can pull an object across the surface with a force meter attached. Divide the Newtons required to move the object by the object’s mass to get the coefficient of friction.

What would happen in a world without friction?

If there were no friction, there would be no anything. The friction that holds atoms together would disappear, so nothing would be able to form. Life wouldn't exist, as atoms wouldn’t be near each other long enough to form simple molecules. The world would become a dangerous place to live, as cars in motion would lose the ability to stop. You’d probably die as well, as your blood would steadily move around quicker and quicker. Good thing this scenario is physically impossible!

How do you calculate the energy lost to friction?

The energy lost due to friction is equal to the work done by friction, or Ffricd, where d is the distance traveled. Ffric can be broken down to μkFN, where μk is the coefficient of friction, and FN is the normal force. FN can further be broken down to be mgcos(θ). So, in summary E = μk(mgcos(θ))d.

What are the 4 types of friction?

The 4 types of friction are: static, sliding, rolling, and fluid. Static is between two surfaces when neither are moving (with respect to each other). Sliding is between two objects sliding over each other (oddly enough) - like when you slide over a wooden floor in your socks. Rolling is between a surface and a rollable object (wheel, ball, etc.). Fluid friction is the friction between an object in movement and the medium it's traveling through, e.g., a plane through the air or a fish through water.

What is the SI unit of friction?

Like all forces, the unit for friction is the Newton, which is equal to 1 kg·m·s-2. In Imperial, the force unit is the pound of force, lbf, 1 of which is roughly 4.45 N. The coefficient of friction is dimensionless, and therefore has no units.

What is the difference between static friction and dynamic friction?

Static and dynamic friction differ with respect to movement. Static friction is the friction between two surfaces that are not moving relative to each other. If these two surfaces were moving relative to each other, it would be dynamic friction. For example, a block on a table experiences static friction. This continues as you incline the table until the block moves, at which point the friction becomes dynamic.

How does friction affect motion?

Friction is what makes motion possible. When one object moves against another, two opposite but equal forces are formed due to friction, without which you would be running on the spot. Friction also allows us to stop - when an object is slowed, friction turns the kinetic energy into thermal or vibrational energy. The more slippery a surface is (the higher the coefficient of friction), the slower the rate of this energy transfer is.

Bogna Haponiuk