Second Law Apparatus

When the latch is released, this device drops one ball vertically and projects another ball horizontally. Students are able to see and hear that both balls hit the floor at the same time.

Why is this device is being marketed as a "Second Law Apparatus"?

(Neither Pasco nor Vernier offer such a device; each instead has a sensor-based lab for the Second Law.)

IMHO it's a great device to start a 2-D kinematics unit, to demonstrate the independence of horizontal and vertical motion. But I fail to find any good reason why it demonstrates the 2nd Law:

  1. To show the 2nd Law, you should use different masses. Do students perceive these two balls as the same mass? They "look" the same by size and material. The dropped ball usually has a hole; sometimes the other has a hole, sometimes not. Even if the masses are technically different, it's not as convincing as say 2x or 10x the mass.

  2. Suppose we assume the masses are different. The experiment has two independent variables: mass and method of release. It's not a controlled experiment.

  3. Presumably, you've recently taught them that mass does not matter for the acceleration of gravity. This device confirms that mass does not matter. But to teach the 2nd Law, you want to show them that mass does matter.

  4. The results of this device can be explained without introducing any concept of force; you are just refining the kinematics of free-fall. Yet, force is essential to the 2nd Law. Why make this demonstration more complicated than it needs to be?

  5. Many students are going to think that the force was the push the ball got when it was triggered, not the force of gravity when in the air.

  6. How does this establish that force is the cause of the acceleration? Why not some other (incorrect) explanation?

  7. Okay, suppose you accept this as a demonstration of the 2nd Law. Great, you have 2 data points. It's not as convincing as the Pasco or Vernier labs, which can collect lots of data.

  8. There are no measurements in this demonstration that establish the formula of the 2nd Law, or to establish the Newton unit. (But the Pasco and Vernier labs do this.)

Can someone give me a convincing argument that this is a good way to demonstrate the 2nd Law?

  • 1
    $\begingroup$ "Can someone give me a convincing argument that this is a good way to demonstrate the 2nd Law?" I cannot do this. I am on your side here. While N2L is technically at play while the balls are falling, I don't think it does a good job at showing N2L. I would say this demonstration is better suited to show that horizontal motion and vertical motion are independent. i.e. we can look at the horizontal and vertical motion of a projectile separately. $\endgroup$ Sep 28, 2018 at 20:48
  • $\begingroup$ By the way, it don't know whether or not Newton himself used this scenario as justification of his 2nd Law. If someone posts a verifiable reference to Newton doing such, I would be willing to accept it on the historical basis, but I still don't think it's good teaching practice. $\endgroup$
    – DrSheldon
    Sep 28, 2018 at 22:03
  • $\begingroup$ "Why is this device is being marketed as a "Second Law Apparatus"?" A cynic would say: in order to sell more of the devices. Teachers know the key role of N2L, but would probably consider the independence of horizontal and vertical motion somewhat peripheral. $\endgroup$ Sep 28, 2018 at 22:27

1 Answer 1


In the Principia, Newton credits Galileo with a similar experiment:

By the first two Laws and the first two Corollaries, Galileo discovered that the descent of bodies observed the duplicate ratio of the time, and that the motion of projectiles was in the curve of a parabola; experience agreeing with both, unless so far as these motions are a little retarded by the resistance of the air.

Axioms, or Laws of Motion (1)

Newton then explains the results of Galileo's experiment using the first two laws (2). It's plausible that Newton's explanation of this experiment led to the name of the device.

The apparatus in question also resembles one of Newton's thought experiments. This occurred in Newton's definition of centripetal force, and does not reference any of the laws of motion. We now call this thought experiment "Newton's cannonball":

Newton's cannonball

If a leaden ball, projected from the top of a mountain by the force of gunpowder with a given velocity, and in a direction parallel to the horizon, is carried in a curve line to the distance of two miles before it falls to the ground; the same, if the resistance of the air were taken away, with a double or decuple velocity, would fly twice or ten times as far. And by increasing the velocity, we may at pleasure increase the distance to which it might be projected, and diminish the curvature of the line, which it might describe, till at last it should fall at the distance of 10, 30, or 90 degrees, or even might go quite round the whole earth before it falls; or lastly, so that it might never fall to the earth, but go forward into the celestial spaces, and proceed in its motion in infinitum.

Definition V: Centripetal Force

So should we call this device a "Second Law Apparatus"? Newton did indeed use such an experiment to defend the 2nd Law. But it was Galileo's experiment, so maybe we ought to name it after him. I still stand by the claims I posed in the question; notably, that it's a great device to introduce 2-D kinematics, but it's not a best teaching practice to convince students of the 2nd Law.

(1) The Principia was originally written in Latin. The citations I provide here are from an 1846 translation into English.

(2) Interestingly, this is followed by Newton's description and explanation of colliding pendulum experiments, which Newton credits to Christopher Huygens and others. We now (incorrectly) call such a device "Newton's Cradle".

Newton's cradle


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