Frictional force is a force that is responsible for opposing the motion of a body that is sliding on another body. This force is created only when the two surfaces move or try to move on each other; it is not created on static surfaces. There are several types of frictional forces such as dry friction, fluid friction, lubricated friction, skin friction and internal friction. Let us know how to calculate force of friction through this article.
Frictional force between two bodies is very essential when you want to move one surface on the other. Before learning to calculate frictional force let us know some important facts about frictional force which will help you to understand the concept better. Frictional force depends upon various factors such as,
- The motion of one surface upon the other surface.
- The texture of the two surfaces which are in contact and are sliding on one other.
- The quantity of force that is applied to push the surfaces or slide the surfaces on each other.
- The SI unit of frictional force, which is Newton
Now, let us know how to calculate frictional force.
The formula for calculating frictional force is given as,
Ff = μ FN
In the above formula,
- Ff is the force of friction that is calculated in Newton
- “μ” is the coefficient of friction
- FN is the normal which is also calculated in Newton
- If you want to calculate the force of friction you need to know the coefficient of friction of the two pairs of surfaces and the normal force that is exerted on the surfaces.
Coefficient of friction:
- The coefficient of friction is different for different pairs of surfaces. Some of the coefficients of friction are given below that will be helpful to you in calculating frictional force.
- These coefficients of friction need to be calculated experimentally.
- The table below shows you some of the important coefficients of friction which you can use in your formulae.
| Pair of Surfaces | Value of coefficient of friction ( μ) |
| rubber on dry asphalt | ~1 |
| rubber on wet asphalt | 0.95 |
| steel on steel | 0.18 |
| Aluminium on steel | 0.61 |
| steel on ice | 0.010 |
| rubber on ice | 0.005 |
| Concrete on wood | 0.62 |
| Copper on glass | 0.68 |
| Wood on wood | 0.25-0.5 |
Normal force:
A normal force applied to a body is the force that is responsible for causing a change in the speed, dimensions or change in the direction of the body. The normal force applied on a body is given by Newton’s third law of motion.
The formula for normal force on a body is given as,
FN = – m × a
In the above formula,
- FN is the normal force acting on a body
- “m” is the mass of the body
- “a” is the acceleration of the body
If the normal force of the body whose frictional force is to be calculated is not given, you can use the above formula and determine the frictional force between the pair of surfaces.
Example:
If you throw a 500 g book on floor then calculate the force of friction between the two surfaces. The coefficient of friction (μ) = 0.1.
We know,
FN = m × a
Here, a = g = 9.8 m/s2
FN = 0.5 × 9.8 N
Substitute this value in the formula for calculating frictional force.
Therefore,
Ff = μ FN
Ff = (0.1) (0.5 × 9.8)
Hence,
Ff = 0.49 N
Therefore, the frictional force on the book is 0.49 N whose mass is 500g.
Hope the steps and the formulae given above are helpful for you in calculating the friction force between any of the surfaces that are in motion.
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