Newton’s Kids

NewtonsnKids

Newton’s cradle is an apparatus named in honor of Sir Isaac Newton.

The device comprises a series of suspended spheres and is used to demonstrate the laws of motion formulated by Newton.

I. Every object in a state of uniform rest or motion tends to remain in that state of rest or motion unless an external force is applied to it. This is the law of the conservation of momentum or because it is essentially Galileo’s concept of inertia it is often called simply the “Law of Inertia”.

II. The relationship between an object’s mass m, its acceleration a, and the applied force F is F = ma.

III. For every action there is an equal and opposite reaction.

When one sphere on the end is lifted and released, it strikes the next stationary sphere and imparting a force which is transmitted through the adjacent stationary spheres and causes the final sphere to swing.

When one sphere on the end is pulled back and released, it wants to keep moving and the stationary balls would like to remain motionless. The collision between the moving ball and the stationary balls results in a change in the forces acting upon all the balls in the system. [Click to see video of Giant Newton’s Cradle].

The moving ball has a certain amount of momentum (a tendency to remain in motion) and the stationary balls have a certain amount of inertia (a tendency to remain stationary). When the moving ball is stopped by the collision, its momentum is transferred to the first stationary ball in the line. The ball cannot move since it is sandwiched, so it transfers the momentum to the next stationary ball in line. This transfer of momentum continues until the momentum is given to the last ball in the line.

Because its movement is not blocked, when the last ball receives the momentum it continues on the path of the first ball. This process will repeat itself, going back and forth, until the energy of the system is lost to air resistance, friction, and vibrations and all the balls again come to rest.

The first law is seen to be satisfied by observing that the final ball swings to the same height (nearly) as the initial height of the first ball. The third law is demonstrated by noting that the first ball is stopped dead in its tracks. The second law can be used to calculate the forces involved since we can measure the mass of each ball and we know that the initial acceleration is from gravity.

[Click here fore some interesting variations on this theme]