Frictional force is the resistance to motion or opposition to motion. Friction is a type of force called Opposing Force. Friction can be classified into two
- Static Frictional force
- Dynamic frictional force
Lets take them one after the other
Static Frictional force
This is the frictional force that exists between two stationary bodies with one on the verge of sliding. Examples are a car parked on a hill, a shirt hanging on a hanger etc
Dynamic frictional force
This is the frictional force that exist between two bodies in relative motion.
Examples are a rolling football, a car moving etc.
What are the characteristics of friction. They are
- Frictional force opposes motion
- It does not depend on the area of contact
- It is directly proportional to the normal reaction.
- it depends on the nature of surface (smoothness, roughness etc)
- the coefficient of dynamic friction is less than the coefficient of static friction
Advantages of friction
- Without friction, it will be impossible to walk.
- It is applicable in safety belt
- It is use to produce charge
- It is used in bolt and nut
- It is applicable in tyre manufacturing
Disadvantages of friction
- It causes wear and tear
- It reduces the efficiency of a machine
- It leads to noise pollution
Ways to reduce friction
- By lubrication
- The use of rollers
- The use of ball and socket bearing
- Streamlining
- Banking is use to reduce sideways friction
Before we continue, let's us understand normal reaction. When a person is standing on the ground, he is applying a force on the ground, the equal force the ground is applying on you is called Normal Reaction. Normal Reaction for a horizontal surface is different from that of an inclined plane
Laws of friction
This law states that the frictional force (Fr) is directly proportional to the normal reaction (R).
i.e
Fr α R
Fr = μR
NOTE: for a horizontal surface, R=W and W = mg, so we can say that R=mg
Fr = μmg
Note the following:
- When a force is applied to a body on a horizontal surface, and the body is about to slide, the frictional force is equal to the applied force i.e. Fr = F
- When the body is moving with a constant velocity, the frictional force is equal to the applied force Fr = F
- When the body is accelerating, the difference between the applied force(F) and the frictional(Fr) force gives the resultant force(ma).
I.e F – Fr = ma
F – μR =ma
F – μmg = ma
Where μ is called coefficient of Friction, a is called acceleration, g is called acceleration due to gravity, and m is called the mass.
Let's take some questions on this
- A force of 20N applied parallel to the surface of a horizontal table, is just sufficient to make a block of mass 4kg move on the table. Calculate the coefficient of friction between the block and the table(g= 10m/s)
Solution
F = 20N, m = 4kg, g = 10m/s
- A moving car of mass 800kg experiences a frictional force of 200N. If it accelerates at 2m/s². Calculate the magnitude of the force applied to the car
Solution
m= 800kg, Fr= 200N, a= 2m/s²
- Find the acceleration of the body in the diagram below
Solution
- Find the acceleration of the body in the diagram below.
Solution
- In the figure above, the coefficient of static friction is 0.5. If the tension in the string is T. The acceleration of the system in motion is ?
Solution
There are two methods for solving this, but I will solve using the shorter method.
m1=6kg, m2= 3kg, g= 9.8m/s², μ=0.5
FOR A BODY ON AN INCLINED PLANE
Formulae for calculating Normal Reaction
R=Wcos𑁜R=mgcos𑁜
Formulae for calculating force
F=Wsin𑁜
F=mgsin𑁜
Formulae for calculating Frictional force
Fr=μR
Fr=μmgcos𑁜
Let's take some examples
Find the acceleration of the body in the diagram above
Solution
m=10kg, μ= 0.5, 𑁜=30°, g= 9.8m/s²
mgsinθ – μmgcosθ = ma
(10×9.8sin30)– (0.5× 10 × 9.8cos30) = 10a
49 - 42.4 =10a
6.6 =10a
a = 6.6/10
a =0.66m/s²
Exercises
- A 200kg box is pushed along a road with a force of 400N. if the box moves with a constant velocity, the coefficient of friction between the box and the road is (g= 10m/s²)
- Calculate the magnitude of force required to just move a 20kg object along a horizontal surface if the coefficient of friction is 0.2
- Which of the following statements about static friction is correct? (a) it is independent of the nature of surfaces in contact(b)depends on the weight of the moving body(c) depends on the relative motion between the surfaces in contact (d) depends on the area of surfaces in contact
- Find the acceleration of the diagram below
5). Find the acceleration of the body below
Answer the questions and write your answers in the comment section