Understanding All You Need To Know About Newton’s Cradle

You see it in many movies and on the desks of many CEOs. Newton’s cradle is a swinging ball set-up that some claim to help relieve stress and inspire creativity. However, for the majority of individuals, it is a fascinating object.

Named after Sir Isaac Newton, the man who discovered gravity, Newton’s cradle demonstrates two fundamental and critical laws of physics: the law of energy conservation and the law of momentum conservation.

History of Newton’s cradle

The funny thing about Newton’s cradle is that it was not created by the man himself, nor was he the first person to discover the laws that the swinging ball set-up displays.

The physics principles that Newton’s Cradle demonstrates were first presented by three physicists to the Royal Society in 1662. Their findings discussed the collision of suspended bodies and how motion transfers from the moving body to the ones at rest. One of the three, Christiaan Huygens, was the first to discover the mechanics behind Newton’s cradle, which is the use of momentum conservation and a quantity that is proportional to mass x velocity2, which is what we know now as the calculation for the kinetic energy of a moving object.

It was not until a French physicist called Abbe Mariotte, who was the first person to conduct and record proper experiments on the pendulum balls. So, why was it named after Isaac Newton? There are two reasons for that. First, the law of momentum conservation is derived from Newton’s second law of motion (Force = mass x acceleration). Second, Newton made a far more significant contribution to the world of physics than Mariotte or Huygens.

How does Newton’s cradle work?

While Newton’s cradle has changed a fair bit since it was first designed, the fundamentals remain the same and are pretty straightforward. It typically consists of an odd number of balls, usually seven or five, that barely touch one another and are suspended on a metal or wooden frame. The balls are usually made of stainless steel and, in some designs, titanium. Stainless steel is the common material due to its low cost and superior elastic properties.

The balls are all identical in density, mass, weight, and size and are suspended using two equal-length wires that are angled from both sides of the frame, restricting the balls’ movement to a parallel plane.

When one ball from either end is lifted and released, it strikes the immediate stationary ball, transferring all of its energy to it. Through a series of energy transfers, the ball at the opposite end swings upwards, reaching the height of the first ball and repeating the same process.

The physics behind Newton’s cradle

As mentioned above, Newton’s cradle displays two fundamental laws of physics: momentum and energy conservation.

The law of energy conservation states that energy can neither be destroyed nor created. However, it can be converted from one form to another. The law of momentum conservation, on the other hand, states that momentum in an isolated system is constant and conserved.

Returning back to Newton’s cradle, the balls have no potential and kinetic energy when at rest. As such, they do not have any momentum. However, when the first ball is lifted up, gravitational potential energy acts on it. The higher it is raised, the more gravitational potential energy it generates. When released, the gravitational potential energy is converted into kinetic energy, converting completely when the ball hits the bottom. Additionally, the ball generates momentum.

Once it hits the second ball, it loses its kinetic energy and momentum completely, coming to a complete stop. However, the laws of physics state that momentum and energy cannot be destroyed. As such, they are transferred to the second ball and to the rest of the balls.

Once it reaches the last ball, there are no subsequent balls to which the energy can be transferred. As such, the energy and momentum it receives propels it upwards in a similar way to the first ball.

Conclusion

The physics and functioning principles of Newton’s cradle are pretty straightforward, and many often wonder: What if we dropped more than one ball? Will the speed at which the pendulum swings be cut? And also, once it starts swinging, will it go on forever?

First, each ball transfers just enough amount of energy to move the other ball. Hence, the number of balls that are propelled upwards will be the same as the number of balls that are dropped.

Second, as the formula for momentum (momentum = mass x velocity) states, momentum will be conserved if mass and velocity stay constant, which, in the case of Newton’s cradle, is true.

Lastly, the cradle will eventually come to a stop due to energy loss due to generated heat from the balls hitting one another, sound energy, and friction from air resistance.

All in all, the world of physics is indeed an interesting landscape. If you are interested in discovering more about interesting physics fundamentals, such as if gravity pulls or pushes, then look no further than Physics Tuition.

Our classes push our students beyond what they learn in their respective classrooms, challenging them to discover more about the subject. To find out more about our classes, visit us at our website or contact us to inquire more.