IIT JEE Main 2017 Physics Question 3 SetA

IIT JEE Main 2017 Physics Question 3 SetA: 

A body of mass m = 10–2 kg is moving in a medium and experiences a frictional force F = –kv2. Its initial speed is v0 = 10 ms–1 . If, after 10 s, its energy is 1/8mv20, the value of k will be:

(1) 10–3 kg m–1
(2) 10–3 kg s–1
(3) 10–4 kg m–1
(4) 10–1 kg m–1 s–1

Solution

Given in problem that the body of mass m is experiencing a frictional force of F = –kv2. Its initial speed is v0 = 10 ms–1. If, after 10 s, its energy is 1/8mv20, the value of k can be obtained by following method:
kf/ki = 1/8mv20/1/2mv20,
=> kf/ki = 1/4
=> vf/vi = 1/2
=> vf = v0/2
=> -kv2 = m dv/dt
=> ∫v0v0/2 dv/v2 = ∫0t0-k dt/m
=> 1/v0 – 2//v0 = -kt0/m
=> 1/v0 = -kt0/m
=> k = m/(v0t0)
=> k = 10-2/(10 x 10)
=> k = 10-4 kg m-1

Answer to IIT JEE Main 2017 Physics Question 3 SetA

Correct answer to given problem No 3 of IIT JEE Main Physics paper of 2017 SetA is option number (3) that is the value of k is 10–4 kg m–1.

IIT JEE Main 2016 Physics Question 14

IIT JEE Main 2016 Physics Question No 14

A point particle of mass m, moves along the uniformly rough track PQR as shown in the figure. The coefficient of friction, between the particle and the rough track equals µ . The particle is released, from rest, from the point P and it comes to rest at a point R. The energies, lost by the ball, over the parts, PQ and QR, of the track, are equal to each other, and no energy is lost when particle changes direction from PQ to QR. The values of the coefficient of friction µ and the distance x(= QR), are respectively close to

IIT JEE Main 2016 Physics Question No 14

(1) 0.29 and 3.5 m
(2) 0.29 and 6.5 m
(3) 0.2 and 6.5 m
(4) 0.2 and 3.5 m

Solution

PQ = (h/sin 30º) = 2h = 4m
Given that work done against friction along track PQ and QR are equal.
therefore, µ mg cos 30º  x 4 =  µ mg x
(4√3)/2=x
=> x = 2√3 = 2 x 1.732 = 3.46 ≅ 3.5 m
Loss in PE = work done against friction along PQ and QR

hence, mg 2 = µ mg( (√3)/2)x4 + µ mg (3.5)

So we get µ = 0.29

Answer

Correct answer to given problem No 14 of IIT JEE Main Physics paper of 2016 is µ = 0.29 and QR = 3.5 i.e. option number 1.