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Explore More About Newton's First Law of Motion or the Law of Inertia

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Author: Jerry Silver

The Idea

This experiment further explores Newton's first law in both the horizontal and vertical directions.

What You Need

  • 5 to 10 poker chips (or coins)
  • table
  • string—strong enough to support the mass, but weak enough to break when pulled
  • 1 weight with attachment points on both the top and bottom
  • 1 support to hang the weight

Method

Chips

  1. Place the chips in a vertical stack on the table.
  2. The table should be smooth enough for the chips to slide freely across its surface.
  3. Take one chip and direct it toward the stack by flicking it with your fingers or pushing it rapidly toward the stack.

Weight on a string

  1. You are going to do this twice (two different ways), so if you have enough materials, it works best if you duplicate the set-up side-by-side.
  2. Use the string to hang the weight from the support.
  3. Attach a string on the bottom of the weight.
  4. Predict what will happen when you pull the string.
  5. First time—pull the string slowly.
  6. Second time—pull the string quickly.

Expected Results

The sliding chip should knock out the bottom chip and take its place in the stack (Figure 24-1).

Newton's first law. Poker chips, weight on a string, and a frictionless puck.

Newton's first law. Poker chips, weight on a string, and a frictionless puck.

Pulling the string slowly causes only the upper string to break.

Pulling the string quickly causes only the bottom string to break.

Why It Works

These are simple demonstrations of Newton's first law. The stack of poker chips remains are rest. The momentum of the moving chip is transferred to the chip it replaces. Momentum is explored in Section 5 in this book.

When the string is pulled slowly, the force from pulling is added to the weight pulling down on the upper string. The combined tension is greater on the upper string and that is the string that breaks.

When the bottom string is pulled rapidly, the mass, which is at rest, tends to stay at rest and the tension is applied to the bottom string, which breaks.

Other Things to Try

You can explore Newton's first law in a number of other ways. These include:

Newton's first law. Poker chips, weight on a string, and a frictionless puck.

  1. Cut or tear a rectangular sheet of paper nearly in thirds, leaving just a short ⅛ inch (1 mm) piece of paper remaining to hold the sections together. Challenge someone to pull sideways at both ends (perpendicular to the tears) to cause the center section to drop. Because of Newton's first law, this is virtually impossible.
  2. Place a handful of coins on your inner arm while it's bent. In one quick motion, swing your arm forward and catch the coins in midair. In the first one-tenth of a second, the coins fall only about 2 inches (or 5 centimeters), so if you are quick, you stand a good chance at catching them. This takes practice. Make sure no one gets hit, either by the coins or your arm.
  3. Place a coin on a card placed directly over the bottle. Flick the card away and the coin drops into the bottle.
  4. Support a coin or sugar cube on the edge of an embroidery hoop balanced on the opening of a jar (or bottle). Smoothly pulling the hoop will result in the coin or cube falling into the jar below.
  5. Slide an air puck or a slider on an air track. (An air hockey table can also work.) Without friction, an object keeps moving in a straight line until a force interacts with it, just as an object in space. This demonstrates the aspect of Newton's first law that refers to a body in motion staying in motion.

The Point

This project explores Newton's first law, which is also known as the law of inertia: a body at rest tends to stay at rest unless acted upon by an external force. A body in motion tends to stay in motion in a straight line unless acted upon by an external force.

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