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Causes of Kinetic Friction
In case of Kinetic friction:
- When there is slipping there will be kinetic friction
- When there is relative motion (relative velocity is non-zero) between the contact points of the object and the contact surface, there will be kinetic friction.
- Kinetic friction always opposes relative motion. Hence it acts to reduce relative motion (or magnitude of relative velocity).
- Kinetic friction force stops acting when the velocity of the two objects become equal or in other words relative velocity becomes zero.
Magnitude of Kinetic Friction
Coefficient of kinetic friction is always less then coefficient of static friction.
Direction of kinetic friction
Kinetic friction always works opposite to the relative velocity of the object with respect to the contact surface.
Q1. Draw the kinetic friction on the block and earth.
Draw Free Body diagram first (FBD)
N = 10.g
Now, F = ma
50 = 10.a => 5 m/s^2
Q2. Draw FBD diagram and show direction of kinetic friction acting on the two blocks.
Here surface between block A & block B is rough while surface between block B and Earth is smooth; hence friction will be there only between block A & B while there won’t be any friction force acting between block B and Earth. Let’s draw FBD now –
Q3. Find out kinetic friction and acceleration of object in below diagram at this instant.
Let’s draw FBD first,
In this example, at this moment, block is moving in right direction; hence kinetic friction will work in left side. (We know that kinetic friction works in opposite direction of relative velocity.)
Apply force balance in vertical direction –
N – 10.g = 0 i.e. N = 100 N
As we have normal reaction, we can find out value of kinetic friction –
Apply force balance in horizontal direction –
1050 = 10.a
a = 105 m/s^2
Q4. Find out acceleration of object in above diagram.
Let’s draw FBD,
In above example, F is acting on an angle with horizontal so we’ve taken component of the same in horizontal and vertical direction.
Note – Object is at rest at this moment so as soon as force starts working on it, object will start moving in right direction, hence friction force will act in opposite direction i.e. left direction.
Now write force balance in vertical direction first to find out value of friction force –
(sin 37 = 3/5, cos 37 = 4/5)
N = 100 – 60 = 40 N
Hence, f = 0.5 x 40 = 20 N
Now, we have value of friction force, let’s apply force balance in horizontal direction –
80 N – 20 N = 10.a
a = 6m/s^2
Q5. State if below statement is TRUE or FALSE
Statement – When straight line is drawn on a paper then kinetic friction force will act on the paper in the direction of drawing.
Ans : TRUE
Explanation – We know that kinetic friction acts in the opposite direction of relative velocity. Now in this example, paper is on rest while pencil is moving (let’s assume in right direction). Now there are two methods of doing it.
- If we observe the pencil, pencil is moving in right direction & paper is on rest, hence relative motion is in right direction w.r.t contact surface of paper. Kinetic friction will try to oppose this movement; hence will work in right direction on other moving surface, i.e. on pencil. Now we know that friction force acting in left direction on the pencil, from third law of newton, force always acts in equal and opposite direction. Hence on paper same amount of force will act in right direction. (which is the direction of movement of drawing.)
- Let’s take a difference perspective. Let’s assume that pencil is on the rest, hence from the law of relative motion, paper will appear to move with velocity of pencil but in opposite direction. Now in new situation paper is moving in left direction. Now we know that friction will act in opposite direction of relative movement on paper. As paper is moving in left direction, kinetic friction will act in right direction. (which is direction of movement of drawing.)