125 Physics Projects for the Evil Genius (21 page)

Figure 26-1
Equal and opposite reactions
.

Putting a propeller on a cart with wheels, as shown in
Figure 26-2
, propels the cart forward (or backward if turning the other way).

What would you expect to happen if a sail is put in front of the propeller to catch the air, as shown in
Figure 26-3
? Some people would say the cart will move faster because the force from the fan will “push” the cart. However, what we find is this: with the sail in place, the cart does not move as it did without the sail. This is a surprising result for many people seeing this for the first time. The reason for this is, without the sail, the equal and opposite reaction of the propeller causes the cart to move forward. However, with the sail in place, the force of the propeller balances the reaction force. As a result, there is no net force and the cart does not move.

Figure 26-2
Without a “sail” the fan pushes the cart
.

Figure 26-3
With a “sail” the cart does not move
.

Linear momentum is conserved in the absence of external forces. For every action, there is an equal and opposite reaction.

In this experiment, you launch a 2-liter soda bottle into the air. Your fuel is water, which is propelled downward by air pressure forcing the rocket upward. This experiment is a good illustration of Newton’s third law and the law of conservation of momentum, and it lends itself to a nice, friendly, competitive “space race.”

• 2-liter soda bottle (water bottles are not necessarily capable of sustaining internal pressure, as soda bottles are)
  
• nose cone fabricated from a cardboard party hat or a cone formed from poster board and tape
  
• cardboard for fins
  
• glue gun or tape
  
• water
  
• hard rubber stopper that just fits the top of the bottle (the stopper should be snug enough to seal the bottle while it is being pressurized, but not oversized to the extent that it prevents the bottle from launching)
  
• bicycle pump with a one-way valve (or an electric pump or compressor)
  
• optional: support to serve as a “launch pad” for the bottle (for instance, made from a tripod built from PVC pipe sections). See
  Figure 27-1
  .
  

1. Slide the open end of the bottle over the vertical rod of a ring stand for easier assembly.

2. Use the glue gun to attach fins to the rocket (remembering that the flat side of the bottle is the top of the rocket). Be careful not to apply excessive heat, which could melt a hole in the bottle.

3. Attach a nose cone to make the bottle more aerodynamic. Use poster board or a coneshaped party hat.

Figure 27-1
Bottle rocket ready for launch
.

4. Fill the bottle from about one-quarter to one-third full.

You can do this in several ways. If you are planning many launches, you may want to go for something more elaborate. The basic parts are:

1. An air pump or compressor.

2. A one-way valve: The simplest way to do this is to insert a needle (available at any sporting-goods supply store) used to inflate footballs and basketballs through the stopper. With this method, no release mechanism is needed because the rocket will take off as soon as enough pressure builds up to overcome the force holding the stopper in the bottle.

3. A release mechanism: A metal “claw,” which holds the bottle in place until the pressure builds to a certain level, allows a greater pressure to build up in the bottle. This can be mounted on a wooden or PVC tripod structure. You can also hold this in your hand, but be prepared to get wet as the “fuel” surges downward from the bottom of the rocket.

1. Insert the stopper into the bottle.
   
2. Secure the bottle onto the launcher. Move the holding mechanism into place (or hold it if that is what you are doing).
   
3. Pressurize the bottle. The maximum air pressure should not go above 80 to 100 psi (pounds per square inch) to avoid bursting the bottles.
   
4. Use a string to remotely release the release mechanism.
   

The rocket will ascend vertically. The upward leg path can take as long as about 4 seconds corresponding to a maximum height of more than 75 meters (over 250 feet).

The air pressure forces the water downward with a high velocity. The mass of the water times the velocity of the water represents the downward momentum of the water. Conservation of momentum requires an equal momentum upward that is applied to the mass of the bottle, which acquires a velocity to take it upward. Another way to say this is the action of the downward force of the water is counterbalanced by an equal and opposite reaction that drives the bottle upward.

Bottle rockets can be made more elaborate by adding fins. A parachute, made of the clear plastic used by dry cleaners, can be added to keep the rocket in the air for a longer time or to release a payload consisting of a tennis ball or other object.

You may want to see the
Mythbusters
episode, where they explore the use of bottle rockets to propel a person. Note, for safety reasons, they confined their efforts to dummies.

This is another example of conservation of linear momentum and Newton’s third law.

Figure 27-2
Sir Isaac Newton’s laws of motion describe the motion of bottle rockets, satellites and planets
.

Newton’s third law states that for every action (force), there is an equal but opposite reaction (force). This project illustrates how this concept can be applied to a particular physical situation. The outcome may be different than what many people expect.

• 1 ping-pong ball attached to a string
  
• 2 beakers (or jars) filled with enough water to immerse the ping-pong ball
  
• balance scale
  
• counterweights
  

1. Set each of the beakers on the opposing pans of the scale and establish a balance, as shown in
Figure 28-1
.

2. Predict what will happen when the ping-pong ball is lowered into the beaker of water. Will the side with the ping pong ball

a. Rise?

b. Fall?

c. Remain balanced?

3. Lower the ping-pong ball into the beaker and observe what happens.

4. Remove the ping-pong ball. What is the effect on the balance?

Lowering the ping-pong ball into the beaker forces that side of the balance down, as shown in
Figure 28-2
.

When the ping-pong ball is withdrawn, the balance is restored.

There is a buoyant force on any object immersed in water (or partially immersed in water such as a floating ping-pong ball). For every action there is an equal and opposite reaction. In this case if the action (the buoyant force) is up, the reaction must be down, causing the observed effect.

Figure 28-1
What will be the effect of a floating object? Will it tip the balance
?