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Physics > Moment of Force and Equilibrium > Flashcards

Moment of Force and Equilibrium Flashcards

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1
Q

What happens when force is applied on a rigid vs a non-rigid body?

A

When force is applied on a rigid body, it can only cause motion in it, while when applied on a non-rigid body it can cause both a change in its size and shape as well as motion.

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2
Q

What is force defined as in a quantitative way?

A

It is defined as the rate of change in its linear momentum.

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3
Q

Give the formula for force with reference to linear momentum.

A

F = dp/dt = d(mv)/dt (where F, p, v and a are changing)
or
F = ma

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4
Q

What is the SI unit for momentum?

A

The SI unit for momentum is kg⋅m/s

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5
Q

What is the derivation for F=ma?

A 
Change in momentum is Δp=Δ(mv).
If mass is constant, then Δ(mv)=mΔv.
For constant mass, F=Δp/Δt=mΔv/Δt.
Because Δv/Δt=a, 
F=ma
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6
Q

What type of quantity is force and why?

A

Force is a vector quantity as both magnitude and direction is needed to express it completely.

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7
Q

What is the SI unit of force?

A

The SI unit of force is newton (N) or kilogram-force (kgf).

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8
Q

What is the relationship between kgf and N?

A

1 kgf=g N, where g is the acceleration due to gravity at that place. Thus, 1 kgf=9.8N

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9
Q

Name the kinds of motion a rigid body can have when acted upon by a force

A
  1. Linear or translational motion

2. Rotational motion

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10
Q

Define linear motion

A

When a force acts on a stationary rigid body which is free to move, the body starts moving in a straight line path in the direction of the applied force. This motion is called linear motion.

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11
Q

Define rotational motion.

A

If a body is pivoted at a point and a force is applied on the body at a suitable point, it rotates the body about the axis passing through the pivoted point. This is the turning effect of the force and the motion of the body is called the rotational motion.

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12
Q

Name the factors which affect the turning of a body.

A
  1. The magnitude of the force applied

2. The perpendicular distance of the line of action of the force from the axis of rotation (or pivoted point)

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13
Q

Define moment of force.

A

The moment of force is the product of magnitude of the force applied and the perpendicular distance of the line of action of force from the pivoted point. It produces a turning effect on a body about its axis.

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14
Q

How to produce maximum torque on a body by a given force?

A

To do this, the force is applied on the body at a point for which the perpendicular distance of the line of action of the force form the axis of rotation is maximum. In this situation, the given force produces maximum torque because torque is a product of…

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15
Q

What is the SI unit of moment of force?

A

Since the SI unit of force is newton and that of distance is metre, the SI unit of torque is N m.

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16
Q

Why is the unit N m of torque not written as joule?

A

Because torque is a vector, while work or energy is a scalar quantity.

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17
Q

What is the CGS unit of moment of force?

A

dyne*cm

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18
Q

If force is measured in gravitational unit, what is the unit of torque in both SI and CGS systems?

A

In SI system, kgf * m

In CGS system, gf * cm

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19
Q

How are the SI+CGS units of force normally related to the SI+CGS units of force gravitationally?

A

1 N m = 10^5 dyne * 10^2 cm
= 10^7 dyne cm

I kgf m = 9.8 N m
1 gf cm = 980 dyne cm

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20
Q

When is the moment of force called anti-clockwise moment and what is it taken as?

A

Conventionally, if the effect on the body is to turn it anti-clockwise. It is taken positive.

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21
Q

When is the moment of force called clockwise moment and what is it taken as?

A

When the effect on the body is tot run is clockwise. It is taken negative.

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22
Q

The direction of anti-clockwise moment is along the axis of rotation ___ .

A

outwards

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23
Q

The direction of clockwise moment is along the axis of rotation ___ .

A

inwards

24
Q

Name the two ways by which the direction of rotation can be changed.

A
  1. By changing the point of application of force.

2. By changing the direction of force.

25
Q

Why do we apply a force at the handle of the door to open it?

A

This is because the handle is provided at the maximum distance from the pivoted point of the door i.e. the hinges. By using the handle to open the door, a smaller force at a larger perpendicular distance from the hinges produces the maximum torque to open or shut the door.

26
Q

Why is the upper circular stone of a hand flour grinder provided with a handle near its rim?

A

A smaller force at the handle is enough to rotate the grinder about the iron pivot at its centre as that the handle is provided near the rim at the maximum distance from the iron pivot. Since torque is the product of magnitude of force and the perpendicular distance of line of action of force from pivoted point, the max distance of handle from pivot makes up for the small force at the handle.

27
Q

Why is a force applied tangentially to turn a steering wheel?

A

The sena of rotation of the wheel is changed by changing the point of application of force but without changing the direction of force.

28
Q

How, in a bicycle, is the rear wheel turned anti-clockwise?

A

A small force is applied on the foot pedal of the toothed wheel of size bigger than the rear wheel so that the perpendicular distance of point of application of force from the axle of the wheel is large. The toothed wheel is joined tot when rear wheel by a chain passing over their teeth.

29
Q

Why does a spanner have a long handle?

A

A spanner, used to tighten or loosen a nut, has a long handle to produce a large moment of force by applying a small force normally at the end of its handle.

30
Q

How is a spanner turned to loosen or tighten a nut?

A

A spanner is turned anti-clockwise to loosen a nut and clockwise to tighten a nut.

31
Q

What provision does a jack screw have in order to lift heavy loads?

A

A jack screw has a long arm so that less effort applied normally at its arm can produce a large moment of force, which is enough to rotate it so as to raise or lower the load table.

32
Q

Rotation is always produced by a ___ of forces.

A

Pair

33
Q

In pivoted bodies, why does rotation occur?

A

It occurs due to the force externally applied and the force of reaction produced at the pivoted point.

34
Q

How are the applied force and the force of reaction produced at the pivoted point related?

A

The force of reaction produced at the pivot is equal in magnitude, but opposite in direction to the applied force.

35
Q

What is the moment of force of reaction about the pivot and why?

A

The moment of force of reaction about the pivot is zero, because its distance from the axis of rotation is zero.

36
Q

What is a couple?

A

Two equal and opposite parallel forces, not acting along the same line, form a couple. A couple is always needed to produce a rotation.

37
Q

What is always needed to produce a rotation?

A

A couple is always needed to produce a rotation, as it constitutes the external force and the force of reaction at the pivoted point.

38
Q

What do we do when we require a larger turning effect?

A

We apply two forces, equal in magnitude but opposite in direction, on the body explicitly such that both the forces turn the body in the same direction.

39
Q

What do we do to open the nut of a car wheel?

A

We apply equal forces, each at F, at the two ends of a wrench’s arm in opposite direction.

40
Q

Name some instances where a couple is applied for rotation.

A
  1. Turning a water tap
  2. Tightening the cap of an ink pot
  3. Turning the key in the hole of the lock
  4. Winding a clock with a key
  5. Turning the steering wheel of a car
  6. Pushing the pedals of a bicycle
41
Q

What is a couple arm?

A

The perpendicular distance between the two forces that are equal in magnitude but opposite in direction is called the couple arm.

42
Q

Why can two forces, which are equal in magnitude but opposite in direction, not produce translational motion?

A

This is because their resultant sun in any direction is zero, but each force is capable of producing a turning effect on a pivoted body in the same direction.

43
Q

What is the formula for moment of couple?

A

Moment of couple = Either force * couple arm

44
Q

What are the 2 effects of the no. of forces which produce equilibrium in the body?

A
  1. The resultant of all the forces is zero, so they do not change the state of rest or of linear motion of the body.
  2. The algebraic sum of moments of all the forces about the fixed point is zero, so they do not change the rotational state of the body.
45
Q

When is a body said to be in equilibrium?

A

When a no. of forces acting on a body produce no change in its state of rest or of linear or rotational motion, the body is said to be in a state of equilibrium.

46
Q

What are the two kinds of equilibrium?

A
  1. Dynamic equilibrium

2. Static equilibrium

47
Q

Define static equilibrium

A

When a body remains in the state of rest under the influence of several forces, the body is in static equilibrium.

48
Q

Why is a book lying on a tabletop said to be in static equilibrium?

A

The weight of the book exerted on the table vertically downwards is balanced by an equal and opposite force of reaction exerted by the table on the book vertically upwards.

49
Q

When is a beam balance balanced in a horizontal position?

A

When the clockwise moment of force due to the object on its right pan balances the anti-clockwise moment of force due to the objects in its left pan and the beam has no rotational motion, i.e., it is in static equilibrium.

50
Q

Define dynamic equilibrium.

A

When a body remains in the same state of motion (translational or rotational), under the influence of several forces, the body is said to be in dynamic equilibrium.

51
Q

How does a rain drop reach the earth’s surface with a constant velocity?

A

This is because the weight of the falling raindrop is balanced by the sum of the buoyant force and the force due to friction (or viscosity) of air. Thus, the net force on the drop is 0, so it falls with constant velocity.

52
Q

How does an aeroplane move at a constant height?

A

The upward lift on it balances its weight downwards

53
Q

What conditions need to be satisfied for a body to be in equilibrium?

A
  1. The resultant of all the forces acting on the body should be zero
  2. The algebraic sum of moments of all the forces acting on the body about the point of rotation should be zero i.e., the sum of the anti-clockwise moments about the axis of rotation must be equal to the sum of the clockwise moments about the same axis.
54
Q

What is the principle of moments?

A

According to the principle of moments, for a body to be in equilibrium, the algebraic sum of moments of all the forces acting on the body should be zero i.e.,
sum of anti-clockwise moments = sum of clockwise moments

55
Q

Which instrument works on the principle of moments?

A

A physical balance or a beam balance

Physics (19 decks)

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