Tag: the turning effect of a force

Questions Related to the turning effect of a force

For a rigit body, we know that if verious forces act at various points in it , the resultant motion is as if a net force acts on the CM(centre of mass)  causing translation and a net torque at the CM causing rotation around an axis through the CM. for the earth-sun system (approximating the earth as a uniform density sphere).

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. the torque is zero

  2. the torque causes the earth to spin

  3. the rigid body result is not applicable since the earth is not ever approximately a rigid body

  4. the torque causes the earth to move around the sun

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Correct Option: B

The angular momentum of body remains conserve if:

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. applied force on body is zero

  2. applied torque on body is zero.

  3. applied force on body is constant

  4. applied torque on body is constant.

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Correct Option: B
Explanation:

Just as linear momentum is conserved when there is no net external forces$,$

angular momentum is constant or conserved when the net torque is zero$.$
Hence,
option $(B)$ is correct answer.

A body is acted upon by two forces each of magnitude $F$, but in opposite directions. State the effect of the forces if the two forces act at two different points of the body at a separation $r$. Which of the following is/are true?

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. Resultant force $=0$

  2. moment of forces $=Fr$

  3. The forces tend to rotate the body about the mid-point between the two forces.

  4. All of the above

Show answer
Correct Option: D
Explanation:

Here we apply two forces of same magnitude but opposite in direction.Hence we have F and - F which makes resultant force as zero.


Moment of force for F is $r \times F $. For force F it has distance r therefore it has moment rF and same with force -F.Hence moment of force is rF.

The forces tend to rotate the body about the mid-point between the two forces.

Consider a cylindrical shaft of length $\lambda$ and radius $R$. It is twisted by torque r. If shear modulus of material is $S$, then angle of twist should be.

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. $\theta =\dfrac { \pi SR^{ 4 } }{ 2\tau \ell } $

  2. $\theta =\dfrac { 2\tau }{ \pi S\ell S^{ 4 } } $

  3. $\theta =\dfrac { 2\tau \ell }{ \pi SR^{ 2 } } $

  4. $\theta =\dfrac { 2\tau \ell }{ \pi SR^{ 4 } } $

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Correct Option: D

A body is acted upon by two unequal forces in opposite directions, but not in same line. The effect is that:

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. the body will have only the rotational motion.

  2. the body will have only the translational motion.

  3. the body will have neither the rotational motion nor the translational motion.

  4. the body will have rotational as well as translational motion.

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Correct Option: D
Explanation:

Due to unequal forces, the net force will act on the body, so the forces are unbalanced and the unbalanced forces cause a change in motion. This means body having translational motion.
There also be torque acting on the body due to unbalanced force. So there will be rotational motion too.

A uniform meter rule is pivoted at its mid-point. A weight of $50gf$ suspended at one end of it. Where should a weight of $100 gf$ be suspended to keep the rule horizontal : 

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. At distance $50  \ cm$ from the pivoted end.

  2. At distance $12.5 \ cm$ from the other end.

  3. At distance $37.5 \  cm$ from the other end.

  4. At distance $25 \ cm$ from the pivoted end.

Show answer
Correct Option: D
Explanation:

As we have one meter rule pivoted at midpoint there will be a perpendicular distance of 50 cm from the contact of force of 50gf.
 Here we have moment of force of $r \times F$.
After substituting we get $moment\ of\ force = 50\times50 gf cm$.
Here this must be balanced by 100gf's moment of force.
 Let us think that there would be x perpendicular distance.
On equating we get 
$ 50\times50  gf cm = 100\times x gf cm$
$ x = 50\times 50/100 = 25 cm$. 

Hence $100 gf$ must be applied at a distance of $25 cm $.

A body is acted upon by two forces each of magnitude $F$, but in opposite directions. State the effect of the forces if both forces act at the same point of the body. Which of the following is/are true?

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. Resultant force $=0$

  2. Moment of forces $=0$

  3. No motion

  4. All of the above

Show answer
Correct Option: D
Explanation:

Here, we are applying two force in same magnitude but in opposite that is one is F and -F. Hence resultant force is $F + (-F) = 0 $. 

Hence, resultant force is zero. Here perpendicular distance is same for both the forces.
Hence, if we calculate $r \times F$ cross product. We get the same magnitude of moments but opposite in direction as force are of opposite in direction. Hence moments are also zeroes. Without any resultant and moment there will be no motion.

Name the factor on which moment of inertia of a body depends .

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. Mass

  2. Force

  3. Distance from rotating axis.

  4. Density

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Correct Option: A,C
Explanation:

Moment of inertia of a body depends  on the body's mass distribution and the axis of rotation.
The moment of inertia of a body is directly proportional to its mass and increases as the mass is moved further from the axis of rotation.
Answer (A) Mass & (C) Distance from rotating axis.

A uniform metre rule of mass $100g$ is balanced on a fulcrum at mark $40cm$ by suspending an unknown mass $m$ at the mark $20cm$ . To which side the rule will tilt if the mass $m$ is moved to the mark $10cm$?

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. on the side of mass $m$

  2. on the other side of mass $m$

  3. can't say

  4. it won't tilt

Show answer
Correct Option: A
Explanation:

As 100 g is balanced at fulcrum it will have a perpendicular distance of 40 cm from itself and mass m will have $100 - 20 = 80 cm $.


Here moment of forces are equal that is balance each other.Hence $ m\times 80 = 100\times  40 \Rightarrow $

$ m = 100/2 = 50 g $.

If there is mark at 10 cm then perpendicular distance would be 90 cm and moment of force would be $50\times90 = 4500\times g \ dyne - cm$ 
where as first moment of force is $100\times40 = 400\times g\ dyne- cm $. 

Hence it tilts on mass m side as it has more moment of force.

Which of the following playground equipment make use of the turning effect of forces?

physics turning on a pivot the turning effect of a force moment of force or torque turning effect of force couple

  1. See-saw

  2. Slide

  3. Swing

  4. Sand-box

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Correct Option: A,C
Explanation:

The forces acting on the see-saw and swing produces torque and make them to rotate. See-saw and Swing make use of the effect of forces.