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Posts Tagged ‘vertical motion’

The other week I was asked to explain how a cylinder (or ball) rolling down a slope differs from e.g. a ball being dropped vertically. It is an interesting question, because it illustrates some things which are not immediately obvious. We all know that, if you drop two balls, say a tennis ball and a cannon ball, they will hit the ground at the same time. This is despite their having very different masses (weights). Galileo supposedly showed this idea by dropping objects of different weights from the tower of Pisa (although he probably never did this, see our book Ten Physicists Who Transformed Our Understanding of Reality).

With a tennis ball and a cannon ball, they clearly have very different masses (weights), but will fall to the ground at the same rate. This fact, contrary to the teachings of Aristotle, was one of the key breakthroughs which Galileo made in our understanding of motion. But, what about if we roll the two balls down a slope? If we build a track to keep them going straight, will a tennis ball roll down a slope at the same rate as a cannon ball? The answer is no, and I will explain why.

Rolling rather than dropping

When a ball rolls down a slope, it starts off at the top of the slope with gravitational potential energy. When it starts rolling down the slope, this gravitational potential energy gets converted to kinetic energy. This is the same as when the ball drops vertically. But, in the case of the ball dropping vertically, the kinetic energy is all in the form of linear kinetic energy, given by

\text{ linear kinetic energy } = \frac{ 1 }{ 2 } mv^{2}

where m is the mass of the ball and v is its velocity (which is increasing all the time as it falls and speeds up). The gravitational potential energy is converted to linear kinetic energy as the ball drops; by the time the ball hits the bottom of its fall all of the PE has been converted to KE.

If, instead, we roll a ball down a slope, the kinetic energy is in two forms, linear kinetic energy but also rotational kinetic energy, which is given by

\text{ rotational kinetic energy } = \frac{ 1 }{ 2 } I \omega^{2}

where I is the ball’s moment of inertia, and \omega is the ball’s angular velocity, usually measured in radians per second. The key point is that the the moment of inertia for the two balls in this example (a tennis ball and a cannon ball) have a different value, because the distribution of the mass in the two balls is different. For the tennis ball it is all concentrated in the layer of the rubber near the ball’s surface, with a hollow interior. For the cannon ball, the mass is distributed throughout the ball.

Two cylinders rolling down a slope

Let us, instead, consider the case of two cylinders rolling down a slope. One is a solid cylinder, the other is a hollow one with all of its mass concentrated near the surface. We will make the two cylinders have the same mass; this can be done by making the material from which the hollow cylinder is made denser than the material for the solid cylinder. So, even though the material of the hollow cylinder is all concentrated near the surface of the cylinder, and there is a lot less of it, if it is denser it can have equal mass.

img_5185-2

A solid cylinder on an inclined plane. We will make the mass of this solid cylinder the same as that of the hollow cylinder, by making it of less dense material. Although it will have the same mass m and the same radius R, it will not have the same moment of inertia I.

We will start both cylinders from rest near the top of the slope, and let them roll down. We will observe what happens.

img_5185-1

A hollow rolling down an inclined plane. We will make the hollow cylinder denser than the solid one, so that they both have the same mass m and the same outer radius R. But, they will not have the same moment of inertia I.

When things are dropped, the rate at which they fall is independent of the mass, but when they roll the rate at which they roll is not indpendent of the moment of inertia. In particular, it is not independent of the distribution of mass in the rolling object. As this video shows, the solid cylinder rolls down the slope faster than the hollow one!

But, why??

Why does the solid cylinder roll down quicker?

The reason that the solid cylinder rolls down faster than the hollow cylinder has to do with the way that the potential energy (PE) is converted to kinetic energy. Because the cylinder is rolling, some of the PE is converted to rotational kinetic energy (RKE), not just to linear kinetic energy (LKE). The only way that a cylinder can roll down a slope is if there is friction between the cylinder and the slope, if the slope were perfectly smooth the cylinder would slide and not roll.

The torque (rotational force) \tau is related to the angular acceleration \alpha in a similar way that the linear force F is related to linear accelerate a. From Newton’s second law we know that F = ma where m is the mass of the object. The rotational equivalent of this law is

\tau = I \alpha

where I is the moment of inertia. The moment of inertia I is different for a hollow cylinder and a solid cylinder. For the solid cylinder it is given by

I_{sc} = \frac{ 1 }{ 2 } mR^{2} = 0.5mR^{2}

where m is the mass of the cylinder and R is the radius of the cylinder. For the hollow cylinder, the moment of inertia is given by

I_{hc} = \frac{ 1 }{ 2 }m(R_{2}^{2} + R_{1}^{2})

where R_{2} \text{ and } R_{1} are the outer and inner radii of the annulus of the cylinder. We are going to make the hollow cylinder such that the inner 80% is hollow, so that R_{1} = 0.8R_{2} = 0.8R. We will make the mass m of the two cylinders the same.

Thus, for the hollow cylinder, we can now write

I_{hc} = \frac{ 1 }{ 2 }m(R^{2} + (0.8R)^{2}) = \frac{ 1 }{ 2 }mR^{2}(1+0.64) = \frac{ 1 }{ 2 }mR^{2}(1.64) = 0.82 mR^{2}

The cylinder accelerates down the slope due to the component of its weight which acts down the slope. This component is mg sin(\theta) where g is the acceleration due to gravity and \theta is the angle of the slope from the horizontal. To make the maths easier, we are going to set \theta = 30^{\circ}, as sin(30) =0.5.

Friction always acts in the opposite direction to the direction of motion, and in this case the friction F_{f} is related to the torque \tau via the equation

\tau = F_{f}R \text{ (1)}

so we can write

F_{f}R = \tau = I \alpha \text{ (2)}

where \alpha is the rotational acceleration. Re-arranging this to give F_{f}, we have

F_{f} = \frac{I \alpha}{ R }

The force down the slope, F (=ma) is just the component of the weight down the slope minus the frictional force F_{f} acting up the slope.

ma = mg\sin(30) - F_{f} = 0.5mg - \frac{ I \alpha }{ R } \text{ (3)}

The angular acceleration \alpha is given by \alpha = a/R where a is the linear acceleration. So, we can re-write Eq. (3) as

ma = 0.5mg - \frac{ Ia }{ R^{2} } \text{ (4)}

Now we will put in the moments of inertia for the solid cylinder and the hollow cylinder. For the solid cylinder, we can write

ma = 0.5mg - \frac{ 0.5mR^{2}a }{ R^{2} } = 0.5mg - 0.5ma

The mass m can be cancelled out, and assuming g=9.8 \text{ m/s/s}, we have

a = 0.5g - 0.5a \rightarrow 1.5a = 4.9 \rightarrow \boxed{ a = 3.27 \text{ m/s/s  (5)} }

Notice that Equation (5) does not have the mass m in it, as this cancels out. It also does not have the radius R of the cylinder in it; the acceleration of the cylinder as it rolls down the slope is independent of both the mass and the radius of the cylinder.

For the hollow cylinder, again using Eq. (4), we have

ma = 0.5mg - \frac{ 0.82maR^{2} }{ R^{2} } = 0.5mg - 0.82ma

This simplifies to

a = 4.9 - 0.82a \rightarrow 1.82a = 4.9 \rightarrow \boxed{ a = 2.69 \text{ m/s/s} (6)}

As with Equation (5), Equation (6) is independent of both mass and radius.

So, as we can see, the linear acceleration a for the hollow cylinder is 2.69 m/s/s, less than the linear acceleration for the solid cylinder, which was 3.27 m/s/s. This is why the solid cylinder rolls down the slope quicker than the hollow cylinder! And, the result is independent of both the mass and the radius of either cylinder. Therefore, a less massive solid cylinder will roll down a slope faster than a more massive hollow one, which may seem contradictory.

Summary

All objects falling vertically fall at the same rate, but this is not true for objects which roll down a slope. We have shown above that a solid cylinder will roll down a slope quicker than a hollow one. This is because their moments of inertia are different, it requires a greater force to get the hollow cylinder turning than it does the solid cylinder. Remember, the meaning of the word ‘inertia’ is a reluctance to change velocity, so in this case a reluctance to start rolling from being stationary. A larger moment of inertia means a greater reluctance to start rolling.

The solid cylinder will start turning more quickly from being stationary than the hollow cylinder, and this means that it will roll down the slope quicker. This result is independent of the masses (and radii) of the two cylinders; even a less massive solid cylinder will roll down a slope quicker than a more massive hollow one, which may be counter-intuitive.

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