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All questions of System of Particles and Rotational Motion for NEET Exam

A mixer grinder rotates clockwise, its angular velocity will be :
  • a)
    zero
  • b)
    negative
  • c)
    uniform but not zero
  • d)
    positive
Correct answer is option 'B'. Can you explain this answer?

Ashish Roy answered
**Explanation:**

A mixer grinder is a device that is used for grinding and mixing various ingredients. It consists of a motor and a set of blades that rotate at high speeds to perform the grinding and mixing tasks. When the mixer grinder is turned on, the motor starts rotating the blades in a clockwise direction.

**Angular Velocity:**
Angular velocity is a measure of how quickly an object rotates or moves around a central point. It is defined as the rate of change of angular displacement with respect to time. The direction of the angular velocity is determined by the direction of rotation. In the case of a mixer grinder rotating clockwise, the angular velocity will be negative.

**Direction of Angular Velocity:**
The direction of angular velocity is determined by the right-hand rule. According to the right-hand rule, if the fingers of the right hand curl in the direction of rotation, the thumb will point in the direction of the angular velocity vector. In the case of a mixer grinder rotating clockwise, the fingers of the right hand curl in the clockwise direction, and the thumb points in the opposite direction, which is counterclockwise or negative.

**Significance of Negative Angular Velocity:**
A negative angular velocity indicates that the object is rotating in the opposite direction compared to the conventional positive direction. In the case of a mixer grinder, a negative angular velocity means that the blades are rotating counterclockwise when viewed from above. This counterclockwise rotation is necessary for the blades to effectively grind and mix the ingredients.

**Conclusion:**
In conclusion, a mixer grinder rotates clockwise, which means its angular velocity will be negative. The negative angular velocity indicates that the blades are rotating counterclockwise when viewed from above, allowing them to efficiently perform the grinding and mixing tasks.

A boy is playing with a tire of radius 0.5m. He accelerates it from 5rpm to 25 rpm in 15 seconds. The linear acceleration of tire is
  • a)
    70 m/s2
  • b)
    7 m/s2
  • c)
    10 m/s2
  • d)
    0.07 m/s2
Correct answer is option 'D'. Can you explain this answer?

Naina Sharma answered
r=0.5m, t=15s
n1=5rpm=5/60 rps
n2=25rpm=25/60rps
… ω1=2π(5) rad/s
… ω2=2π(25) rad/s
As,
a=r∝          [∝=ω2- ω1/t]
so, a=0.5[2π(n2-n1)/t]
a=0.5x6.28x20/60x15
a=6.28x2/60x3
a=6.28/90
a=0.697 m/s2
a≈0.7 m/s2

Can you explain the answer of this question below:

Let 19825_493f965b17f1e90d9c8381c8500ac576H and 19825_493f965b17f1e90d9c8381c8500ac576E respectively be the angular speeds of the hour hand of a watch and that of the earth around its own axis. Compare the angular speeds of the earth and hour hand of a watch.

  • A:

    19825_493f965b17f1e90d9c8381c8500ac576H =  19825_493f965b17f1e90d9c8381c8500ac576E

  • B:

    ωH = 219825_493f965b17f1e90d9c8381c8500ac576E

  • C:

    ωH = (1/2)19825_493f965b17f1e90d9c8381c8500ac576E

  • D:

    ωH = 419825_493f965b17f1e90d9c8381c8500ac576E

The answer is b.

Gaurav Kumar answered
Given,
Angular velocity of the hour hand-H
Earth’s speed around its own axis-E
So, time taken by hour hand to complete a rotation is=TH=12hr
Time taken by earth to complete a rotation =TE=24 hours
So,
ωEH =(2π/TE )/2π/TH=TH/TE=1/2
⇒ ωEH=1/2
⇒ 2 ωE= ωH

 Consider the following two statements :
A. Linear momentum of a system of particles is zero.
B. kinetic energy of a system of particles is zero.Then 
[AIEEE 2003]
  • a)
    A does not imply B and B does not imply A
  • b)
    A implies B but B does not imply A
  • c)
    A does not imply B but B implies A
  • d)
    A implies B and B implies A
Correct answer is option 'C'. Can you explain this answer?

We know momentum of a system = mass* change in velocity.
but kinetic energy = 1/2 * mass * (velocity)²
If change in velocity is zero that doesn't mean that the velocity of the system is zero.....
If velocity of a system is zero then there is no change in velocity.....means change in velocity=0
so the answer is option (c).....

The angular velocity of a second hand of a clock is :
  • a)
    π/ 30 rad/s
  • b)
    π/5 rad/s
  • c)
    2π rad/s
  • d)
    π /3 rad/s
Correct answer is option 'A'. Can you explain this answer?

Krishna Iyer answered
Time taken to complete one rotation is 60 seconds.
One rotation involves 360 degree = 2π
Formula of angular velocity = theta/time taken
Hence, omega = 2π/60
= π/30 rad/s

All the points of a rigid body rotating about the given axis have same :
  • a)
    Linear acceleration
  • b)
    Angular velocity
  • c)
    Linear velocity
  • d)
    Angular momentum
Correct answer is option 'B'. Can you explain this answer?

Rajeev Saxena answered
Thus, it is the velocity of a reference point fixed to the body. During purely translational motion (motion with no rotation), all points on a rigid body move with the same velocity. However, when motion involves rotation, the instantaneous velocity of any two points on the body will generally not be the same.

Direction of linear velocity of a particle rotating in a circular motion is :
  • a)
    Along the tangent
  • b)
    Along the radius away from the centre
  • c)
    Along the axis of rotation
  • d)
    Along the radius of the centre
Correct answer is option 'A'. Can you explain this answer?

Mira Sharma answered
Unlike linear motion, where velocity and acceleration are directed along the line of motion, in circular motion the direction of velocity is always tangent to the circle. This means that as the object moves in a circle, the direction of the velocity is always changing.

A linear velocity of a particle performing circular motion, which is directed along the tangent to the circular path at given point on the circular path at that instant is called instantaneous velocity. It is also called as tangential velocity.

Which of the following is not a unit of angular displacement?
  • a)
    Degrees
  • b)
    revolutions
  • c)
    metre
  • d)
    radian
Correct answer is option 'C'. Can you explain this answer?

Nitin Nair answered
Angular displacement is measured in units of radians. Two pi radians equals 360 degrees. The angular displacement is not a length (not measured in meters or feet), so an angular displacement is different than a linear displacement.

A demo CD, 8 cm in diameter, spins 200 rev/min. Determine its linear velocity in cm/s for a point 3 cm from the center.
  • a)
    62.8 cm/s
  • b)
    20/6 cm/s
  • c)
    20.9 cm/s
  • d)
    40 π/6 cm/s
Correct answer is option 'A'. Can you explain this answer?

Given:
Diameter of CD = 8 cm
Radius of CD = 4 cm
Angular velocity = 200 rev/min
Point from center = 3 cm

To find:
Linear velocity in cm/s

Solution:
We can use the formula:

v = rω

where,
v = linear velocity
r = distance from center
ω = angular velocity

Substituting the given values, we get:

v = 3 cm × (200 rev/min × 2π rad/rev) / 60 s/min
v = 62.8 cm/s

Therefore, the linear velocity of the point 3 cm from the center of the demo CD is 62.8 cm/s.

Hence, the correct option is (a) 62.8 cm/s.

A mass m moves with a velocity v and collides inelastically with another identical mass. After collision the 1st mass moves with velocity in a direction perpendicular to the initial direction of motion. Find the speed of the second mass after collision.   
[AIEEE 2005]
  • a)
     v
  • b)
  • c)
     
  • d)
     
Correct answer is option 'C'. Can you explain this answer?

Let mass A moves with velocity v and collides inelastically with mass B, which is at rest.  
According to problem mass A moves in a perpendicular direction and let the mass B moves at angle θ with the horizontal with velocity v.
Initial horizontal momentum of system 
(before collision) = mv ....(i) 
Final horizontal momentum of system 
(after collision ) = mVcosθ.................(ii)
From the conservation of horizontal linear momentum mv
= mvcosθ ⇒ v = Vcosθ .....(iii) 
Initial vertical momentum of system (before collision) is zero.
Final vertical momentum of system mv/√3 − mV sinθ
From the conservation of vertical linear momentum 

Rotational analogue of force is :
  • a)
    Moment of Inertia
  • b)
    Radius of gyration
  • c)
    Gyration
  • d)
    Torque
Correct answer is option 'D'. Can you explain this answer?

Nitin Nair answered
The farther the force is applied from the pivot, the greater the angular acceleration. The equation τ = m(r^2)α is the rotational analog of Newton's second law (F=ma), where torque is analogous to force, angular acceleration is analogous to translational acceleration, and mr2 is analogous to mass (or inertia ).

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