Rigid Body Statics

CHAPTER NINE

RIGID BODY STATICS

Moment of a Force or Torque

The torque or moment of a force about a reference point is the vector given by

 =

where r is the position vector of the point of application of the force relative to the reference point.

Fig 9.1

Note that the angle between the two vectors is the angle between their directions. Thus, we could redraw Fig. 9.1 as

Fig. 9.2

The magnitude of the moment is | | =, where is the angle between the two vectors.

Notice that the moment of a force tends to make a body the force is acting upon to rotate about the reference point O. It is important, therefore, to refer to the reference point. You would recall that you were taught at the secondary school level, about clockwise and counter-clockwise (or anticlockwise) moment. This is because, at least at this level, the moment makes the body rotate either clockwise (as in the case above) or counterclockwise about the reference point.

Let us take a look at opening a door. Do you push the door close to the hinges? Do you push the narrow edge of the door parallel to the door through the line through the hinges? No. It is easier to push the door at the farthest end (from the hinges) and perpendicular to the length of the door. Thus, if the door is of length r and the force you apply is F, in case, the moment of the force is . Of course, if , the moment of the force is zero.

The moment of a force is a vector, the direction of which is given by that of the right-handed cockscrew. It follows that if your hand should move from r to F contained on the plane of the paper, sweeping through the angle (clockwise in this case), the vector moment of the force points away from you, into the paper. The vector for anticlockwise moment would point towards you.

Equilibrium of a Rigid Body

Earlier, we saw what it takes for a point mass to be in equilibrium – the net force on it must be zero. For a rigid body (an extended body), we require, additionally, that the net moment on the body be zero. This is summed up in two laws:

(i)The net force acting on the body in any direction is zero. You have been taught the equivalent of this force: sum of upward forces is equal to the sum of downward forces. This is just only one part of it. Indeed, there cannot be a net force in any direction, otherwise, the body would move in that particular direction.

(ii)The net moment about any point on the body must be zero. This you saw as: sum of clockwise moment = sum of anticlockwise moment. This applies to any point at all, as if there is any point at which the net moment is not zero, that point would rotate about the reference point.

Example 1:

As an example, consider a uniform beam of negligible weight resting on a fulcrum as shown in Fig. 9.3. It is of length 1 m, and is resting on a knife edge placed at the 40 cm mark. There is a 50 g mass hanging from the 80 cm mark. Clearly, the beam cannot be balanced. There must be a force on the other side of the 50 g mass to ensure balance. Let us decide to hand a 200 g mass on that side. Where exactly should we put it?

Let us place the 40 g mass at a point x m from the left end of the beam.

Fig. 9.4

Condition 1: The sum of upward forces equals the sum of downward forces. The reaction at the support equals the weight of the beam. In this case, indeed, we have not bothered with the weight of the beam as it is negligible (otherwise, we would have had its weight pointing down from the center of gravity, same as center of mass in this case).

Condition 2: Net Moment about the fulcrum is zero, that is, sum of clockwise moments minus sum of anticlockwise moments is zero:

= 09.1

Hence,

0.4 – 0.25 = 0.15 m

Example 2:

A metre rule of mass 30 g is supported at the 40 cm and the 85 cm marks. Two masses 45 g and 52 g are hung from the 42 cm and the 90 cm marks respectively. Find the reaction at the supports.

Fig. 9.5

(iii)The sum of upward forces = sum of downward forces:

9.2

(iv)Taking moments about :

9.3

From equation 9.3, we can obtain . Putting this in equation 9.2 gives .

Example 3:

A man of mass m is climbing a uniform ladder of weight W and length L m resting on a rough floor and a smooth wall, as shown in Fig. 9.6. Find the reaction at the wall. How far up the ladder can he go without the ladder slipping if the coefficient of static friction is ?

Fig. 9.6

Solution

The forces acting on the ladder are already shown. is the force the ladder exerts on the floor, and is a resultant of what the reaction would have been without friction , and the frictional force . There is no friction at the wall. Hence, only the normal reaction needs be reckoned with.

Condition 1: Sum of upward forces = sum of downward forces

9.4

9.5

Let the man be x m up along the ladder. Then, if the angle the ladder makes with the floor is , then taking moments about the foot of the ladder,

9.6

If the center of gravity of the beam had been one-third of the way up the ladder, we would have replaced by . Indeed, we replace by , if the center of gravity is one-pth of the distance up the ladder.

From equation 9.4, we can calculate , since W and mg are known. From equation 9.6, we can get find and hence, (they are equal) in terms of x. But we recall that this force can be written as . We can then get since each term on the right is now known. As an example, consider the following:

A man of mass 55 kg is climbing a uniform ladder of weight 70 kg and length 10 m resting on a rough floor and a smooth wall. If the ladder touches the wall at a height of 7 m, how far up the ladder can the man go without the ladder slipping, given that the coefficient of static friction between the ladder and the rough floor is 0.3?

= 9.7

9.8

Let the man be x m up along the ladder. Then, if the angle the ladder makes with the floor is , then taking moments about the foot of the ladder,

= = , where is the coefficient of static friction between the ladder and the rough floor.

Hence, m.

CHAPTER TEN

RIGID BODY DYNAMICS

A rigid body is indeed such that the relative position of the constituent masses remain constant. Thus, if we consider Fig. 10.1., then

Fig. 10.1

Let the body rotate about O with angular frequency . Then, the total kinetic energy of the rigid body is

(since the body is rigid, each constituent rotates at the angular frequency )

Hence,

10.1

where is called the moment of inertia of the rigid body.

You would observe that the expression for the kinetic energy looks like that for kinetic energy if we make the identification and . Thus, moment of inertia plays the role of the equivalent of mass for the rotational motion of a rigid body.

Moment of Inertia of a Continuous Body

We have seen that the moment of inertia of a rigid body with discrete components is . We shall now see the situation where the body is of continuous distribution. For one, the summation becomes an integral, , and the integral evaluated as the problem demands.

Examples

1.A thin rod of length rotating about an axis perpendicular to one of its ends

Fig. 10.2

Clearly, the elemental mass contributes an elemental moment of inertia .

The total moment of inertia is then,

since , the product of the density and the volume of the element. Note that the volume is , where A is the cross-sectional area and dx is the (elemental) length of the elemental mass. and A are constant. Hence, we can write,

10.2

But , the mass of the cylinder, or . Putting this in the expression for ,

10.3

Let us now see what happens when the bar rotates around its center about an axis perpendicular to the bar.

Fig. 10.3

But , the mass of the cylinder, or . Putting this in the expression for ,

10.4

Parallel Axis Theorem

You would notice that we could have got equation 10.3 from equation 10.2 as follows:

This is indeed true in the general case of an axis through a point at a distance h from the axis through the center of mass,

Assignment

Find the moment of inertia of uniform rod through an axis parallel to its length, passing through a point one-third to one of its ends.

A body could have both rotational and translational motion. Such a body would then have both translational kinetic energy and rotational kinetic energy, so we could write the total kinetic energy as,

10.5

Thus if a uniform solid cylinder of radius R and mass M should roll down an inclined plane from height h from rest, the gravitational potential energy is fully converted to the translational kinetic energy and rotational kinetic energy at the bottom of the incline, where v and have their values at the bottom of the incline. The moment of inertia of a solid cylinder about an axis through its center, parallel to its length is .

The total kinetic energy is then

,

or

In the absence of rotation, all the potential energy is converted to translational kinetic energy,

or

This is because some of the potential energy has been converted to rotational motion.

Assignment

A solid cylinder of mass 3 kg and radius 0.2 m is propelled up an inclined plane with an initial velocity of 25m/s. How far up the incline can the cylinder travel?

ANGULAR MOMENTUM AND TORQUE

The angular momentum of a body is the moment of its momentum. Thus, if the momentum of the body is , then, its angular momentum about a reference point is,

10.6

It is therefore, a vector the direction of which is given by the right-handed cockscrew rule.

Just as the moment of a force gives the rotational effect of the force, so does the angular momentum give the rotational effect of a momentum.

We can write the magnitude of the angular momentum as

10.7

where is the angle between the two vectors.

If the vectors are perpendicular, L = r p = mvr, and zero if the vectors are parallel. Makes sense, right? Just as it was in the case of the moment of a force, the rotating effect is zero if the two vectors are in the same direction.

Let us now take the time-derivative of the angular momentum.

10.8

= = = 

Remember we said the moment of a force about a reference point is the torque.

Thus, we conclude that the torque a body in angular motion is the time rate of change of its angular momentum, that is,

=10.9

Note that differentiating a cross product means that we keep the order of appearance of the vectors involved, since cross product is not commutative, that is, .

Since  =, what happens if the torque is zero? Then, , and the angular momentum is constant. Does this equation look like the one you saw earlier? That is, . Newton’s first law! In just the same way, in angular motion, a body remains in angular motion with constant angular momentum until a net external torque acts on it. Can you also see that equation 10.9 is like , Newton’s second law? You can then see again that torque plays in angular motion the equivalent of force in translational motion.

In uniform circular motion, , hence, there is no torque on a body in uniform circular motion. This is why the angular momentum remains , because is constant in uniform circular motion.

In circular motion, r and p are perpendicular (remember?), because the velocity is along the circumference. Hence, we can write . Then, the torque is,

(r is constant, but is not)

Example

A flywheel of radius 1.2 m and moment of inertia 12.5 kgm2 starts rotating from rest and uniformly attains an angular speed of 10 rad/s2 in 5 seconds. Find the force applied along the rim of the flywheel.

Solution

The torque on the flywheel is , where r is the radius of the wheel.

The angular acceleration is given by,

= 5 rad/s2

Then,

= 20.8333 N

Notice that the moment of inertia of a thin rod rotating about an axis through the middle is less than that of the same rod through an axis at the end of the rod for instance. This is why a rod rotating about an end would be more effective or destructive than the one rotating through the middle if they are both rotating at the same angular speed - . The more the moment of inertia, the more the torque causing the motion, or the amount of destruction the rod can cause.

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