Gravity Does Work

Gravity Does Work

Contributed by Andrea Melendez

Introduction

We know objects fall when we release them from some altitude (video of falling objects), but how do you suppose a system of objects connected to each other fall when dropped? 

Many years ago, a man by the name of Isaac Newton discovered that forces going in the same direction are additive, and forces going in opposite directions are subtractive. For example, a person exerting a 5 N force on a boulder to the left and another person exerting a 3 N force to the right will move to the right as if 2 N of force were applied to the left. There are many ways to exert forces on objects. In this demo we will demonstrate two: gravity and the force exerted from a stretched rubber band. 

Gravity is a force of attraction between any objects that have mass. It is the force that keeps us bound to the earth, and the planets orbiting around the sun; however, gravity is also present between less massive objects. There is actually a force of attraction between any two objects with mass, like between you and your friend as you walk down the sidewalk. The force in those instances is just so, so small that it is nearly undetectable because we don’t have enough mass. The reason why earth manages to keep us from floating off into space is because it is so massive—objects with more mass have more gravity. One of the properties of gravity, is that it attracts objects at the same rate, or with the same acceleration, regardless of shape or mass. This means that a bowling ball and a tennis ball will fall to the ground at the same time if released from the same height above the earth (in the absence of air resistance.) 

Stretched rubber bands can also exert a force on objects that are attached to them. If you were to attach a rock to one side of a rubber band and pull the rock away while holding the other end fixed, then let go, the rock will move (a telltale clue that a force was applied to the rock). This force does affect how fast the object attached to it moves. A heavier rock will take longer to traverse the same distance back toward the other end than a lighter one.

In this demo, we have two forces at play: gravity pulling both objects downward, and the force holding the water balloon up (and the bar down) from the rubber bands. While there also exists a gravitational force between the bar and water balloon, it is negligible when compared to the gravitational force exerted by the earth.  When we let go of the bar, the bar will be pulled down by both gravity and the rubber bands. The balloon will be pulled upwards by the rubber bands and downward by gravity. Because there are two downward forces on the bar, and only one on the balloon, it will move down faster than the balloon and pop the water balloon near the top. 

Materials

  • Ruler 
  • Thumb tacks 
  • Old face mask 
  • Rubber band or duct tape 
  • Sewing pin 
  • Water balloon 
  • Wooden paint stirrer 

Procedure

  1. Using a ruler, find the center of the wooden stirrer 
  2. Use a thumbtack to make a hole through the center.
  3. Measure about half an inch from the edges of the wooden stirrer. Ensure that the distance from the central hole to both edges is the same distance.
  4. Push a thumbtack through the wooden stirrer on both edges where you just measured.
  5. Use the push the sewing pin through the central hole and secure it with a rubber band, tape or glue. 
  6. Loop the mask ear loops of the facemask around the thumbtacks on the edges of the wooden stirrer.
  7. Fill a water balloon and place it in the mask. 

Physics Concepts and Questions

Concepts

  1. To understand that gravity pulls objects toward the earth at the same rate, regardless of weight.
  2. To understand that forces going in the same direction are additive. 

Questions

  1. If a boulder and a pebble were dropped off the side of a cliff (in the absence of air) which would hit the ground first?
  2. If we were to replace the wooden bar with a heavy lead bar, would it take longer or shorter for the balloon to pop? What about a bar made of a lighter material?
  3. If we were to use a stiffer rubber band, do you think it would make a difference? If so, how?
  4. If we were to take our demo to outer space and pulled the balloon the same distance away from the bar as gravity did on earth, do you think it would take more, less, or the same amount of time for the balloon to pop when we let go of the ends? 

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