Mechanics (Mass, Motion, Force, Acceleration, Momentum, Work, Power, Energy)

Total Questions: 40

31. The ratio of kinetic energies of two bodies of same mass is 4:9 the ratio of their velocities will be [U.P.R.O./A.R.O. (pre) 2017]

(a) 4:9
(b) 2:9
(c) 16:81
(d) √2:√3

Correct Answer: (b) 2:9

Solution:As per question mass of the both bodies is same . Suppose velocities of the bodies 1 is v1 and the velocities of the body 2 is v2.

kinetic energy of the body 1 (k1)=1/2mv₁²

kinetic energy of the body 2 (k2)=1/2mv₂²

according to question

K1/K2=1/2mv1²/1/2mv2²=v1²/v2²

According to question,

K1/K2=4/9 ⇒v1²/v2²=4/9 ⇒〈v1/v2〉²=4/9

⇒v1/v2=√4/√9=2/3=2:3

32. Cycle and car use ball bearing because - [I.A.S. (PRE) 2013]

(a) The effective area of contact between the wheel and the axle is increased.
(b) The actual area of contact between the wheel and the axle is increased.
(c) The effective area of contact between the wheel and the axle is reduced .
(d) None of the above statement is correct.

Correct Answer: (c) The effective area of contact between the wheel and the axle is reduced .

33. Ball bearing are used to convert static friction into : [68th B.P.S.C. (PRE) 2022]

(a) rolling friction
(b) drag
(c) sliding friction
(d) none of the above

Correct Answer: (a) rolling friction

Solution:The purpose of the ball is to reduce the rotational friction and support radial and axial loads. Due to ball bearing the effective area of contact between the wheel and the axle is reduced and hence the frictional force also reduced. For example if two flat surfaces were sliding against each other. They reduce the surface contact and the friction across moving planes. Thus ball bearings are used to convert static friction into rolling friction.

34. A person is sitting in a car which is at rest.The reaction from the road at each of the four wheels of the car is R.When the car runs on a straight level road how will the reaction at either of the front wheels vary? [I.A.S. (PRE) 2008]

(a) it will be equal to R
(b) it will be less then R
(c) it will be greater then R
(d) it shall depend on the material of the road

Correct Answer: (b) it will be less then R

Solution:Dynamic friction of any object is proportional to adhesion reaction and less than static friction. Thus option (b) is the correct answer.

35. An oil tanker is partially filled with oil and moves forward on a level road with uniform acceleration .The free surface of oil then [I.A.S. (PRE) 2003]

(a) remains horizontal
(b) is inclined to the horizontal with smaller depth at the rear end
(c) is inclined to the horizontal with larger depth at the rear end
(d) assume parabolic curve

Correct Answer: (c) is inclined to the horizontal with larger depth at the rear end

Solution:When an oil tanker is partially filled with oil and moves forward on a level road with uniform acceleration, the free surface of oil then is inclined to the horizontal with larger depth at the rear end.

36. Consider the following statement : [I.A.S. ( PRE) 2003]

A 4-wheel vehicle moving in a sharp circular path at high speed will:
1.overturn about its a outer wheels
2.overturn about its a inner wheels
3.skip outwards
4.skip inwards
Which of these statement are correct ?

(a) 1 and 3
(b) 2 and 4
(c) 2 and 3
(d) 1 and 4

Correct Answer: (a) 1 and 3

Solution:In case of object moving along a circular path two types of forces centripetal and centrifugal acts upon it. As they have opposite impact thus the object remains in balance. If a four wheeler on a circular path is moving faster then the relative speed then it skids outwards but then the frictional force provide the necessary centripetal force which ensure the car to have a curved path. On sharp circular path if the outer portion of the road is not raised then the vehicle will skid outward and will overturn about its outer wheels.

37. A batsman hits a cricket ball which then roll on a level ground. After covering the short distance the ball comes to rest. The ball shows to a stop because : [M.P.P.C.S. (PRE) 2017]

(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion .
(d) there is no unbalanced force on the ball so the ball would try to come at rest.

Correct Answer: (c) there is a force on the ball opposing the motion .

Solution:A batsman hits a cricket a ball which then rolls on a level ground. After covering a short distance the ball comes to rest because there is an external force ,frictional force acting on the moving ball opposing its motion. If there is no frictional force then the ball will roll continuously .

38. What is the correct equation for finding the acceleration? [U.P.P.S.C. (PRE)2003]

(a) a=(v-u)/t
(b) a=u+vt
(c) a= (v+u)/t
(d) a=(v+u)/2

Correct Answer: (a) a=(v-u)/t

Solution:As we know v = u+at or at=v-u

∴ a=(v-u)/t

where v=final velocity of the particle

u=initial velocity of the particle

a=acceleration t=time

39. The working principal of a washing machine is :

I.A.S.(PRE) 1997
U.P.P.C.S. (MAINS) 2004
Uttarakhand Lower sub. (pre)2010
67th B.P.S.C. (PRE) (RE.exam) 2022

(a) centrifugation
(b) dialysis
(c) reverse osmosis
(d) diffusion

Correct Answer: (a) centrifugation

Solution:Centrifugation is a technique which involves the application of centrifugal force to separate particles from a solution according to their size shape density viscosity of the medium and rotor speed.

40. The separation of cream from milk by churning is due to- [Jharkhand P.C.S. (PRE) 2013]

(a) gravitational force
(b) cohesive force
(c) centrifugal force
(d) none of the above

Correct Answer: (c) centrifugal force

Solution:The separation of cream from milk by churning is due to centrifugal force .

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