Forces in action!

Contributed by Andrea Melendez

Introduction

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

A “force” is an effect that changes the motion of an object in either speed or direction. For example, the force of gravity causes objects to fall towards the ground with increasing speed, changing its motion. The gravity from Earth exerts an attractive force on objects that pulls them toward it. Another attractive force is the elastic force experienced by two objects connected by a rubber band. When two objects connected to a rubber band are pulled apart, the elastic between them exerts a force to pull them closer together. You can physically feel this force increase the more you try to stretch the rubber band. Other forces, like the force you feel when you try to bring magnets with the same polarity close to each other, are repulsive. Try bringing the N pole of one magnet close to the N pole of another, and you’ll notice that the magnet that you are not holding will move and change direction to avoid a close interaction with the N pole of the magnet in your hand. 

An interesting feature of forces is that their strength is not affected by other forces, even though it might seem like it because the overall force on the object they are acting on will change. For example, suppose you are exerting a force on a car that changes the speed of the car by 3 mile per hour every hour going to the left, you then introduce another force changes the speed of the car by 1 mile per hour every hour to the right. The net force that the car experiences after the second force is introduced is now 2 miles per hour every hour to the right. However, the strengths of the forces that are applied to the object do not change: the car still experiences a 3 mile per hour every hour force going to the left and a 1 mile per hour every hour force to the right. This feature is what allowed us to simply subtract the forces to figure out what the total force on the car is.

This property is called the superposition of forces: forces going in the same direction add and those going in opposite directions subtract. When the forces are not going in the exact same or opposite direction, a weighted average of the forces and their directions determines the net force and direction. 

Now let’s test our knowledge of forces by thinking about the system shown in the video below. Can you predict where the water balloon will pop if I let go of the bar before pushing play? Do you think the water balloon will pop while it falls, or upon impact with the ground? Note that there is a needle sticking through the center of the wooden bar. 

To understand why the balloon popped where it did, it is helpful to make a diagram of the forces acting on both the bar and balloon. These diagrams are called free body diagrams. 

Since neither the balloon nor the bar are moving when I hold the bar, we can deduce that the forces acting on both objects cancel. This means the elastic force pulling the balloon up has the same strength as the downward force from gravity. From the bar’s perspective, the elastic force pulling it toward the water balloon and the force of gravity are cancelled by the force I use to hold the bar above the ground.

When I let go of the bar, the force that was keeping the system’s forces balanced is gone. As a result, the bar will have two downward forces pulling it toward the balloon. As the bar moves downward, the elastic force holding the balloon upward will decrease in magnitude until it is no longer stretched, but this force (with help from the gravitational force) is so strong that it happens fast, only allowing the balloon to move a tiny amount. The balloon can move a tiny amount because the elastic force diminishes as the bar falls. This means that the upward elastic force cannot balance the downward force of gravity anymore, so it moves, but only very little since the attractive forces bring the bar and the balloon together act so fast.

Materials

To build your own at home, you’ll need:

• Wooden stir stick (if you ask nicely, your local home improvement store might let you have one for free)
• A pencil 
• A balloon 
• A ruler 
• Two push pins 
• A sewing pin 
• A rubber band 
• A mask 

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. 
   

Further Questions

Questions

1. If we were to replace the wooden bar with a heavier bar, do you think it would take a longer or shorter amount of time for the balloon to pop? What about a bar made of a lighter material?
2. If we were to use a stiffer rubber band, do you think it would make a difference? If so, how?
3. 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?