125 Physics Projects for the Evil Genius (19 page)

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.
   

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.
   

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

Figure 24-1
Inertia keeps the upper chips in place while the lower one is removed
.

Figure 24-2
Where the string breaks depends upon how fast you pull
.

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

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

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.

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

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.

Figure 24-3
It is just about impossible to make the center piece of paper fall by pulling the other two pieces sideways
.

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.

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.

This classic experiment explores the connection between an object’s acceleration and the force applied to it. This fundamental principle of physics was first formulated by Sir Isaac Newton in the famous second law of motion that bears his name. To measure acceleration, you use either the stopwatch or the motion sensor technique of measuring acceleration, which we used in previous experiments. The force will be provided courtesy of the Earth, in the form of the gravitation force on a mass hanging from a string.

• low-friction cart (or an air track and glider, if available)
  
• spring scale
  
• mass set (including 50 g, 100 g, 200 g)
  
• tape
  
• string
  
• pulley (low mass and low friction is preferable)
  
• clamp to attach the pulley to the table
  
• table top (at least 1 meter in length)
  
• stopwatch and meterstick or motion sensor
  

1. Determine the mass of the cart in grams. Divide by 1000 to get kilograms.
   
2. Place a 100g (0.1kg) mass in the cart. Secure it with tape, if necessary.
   
3. Set the cart at one end of the table, and attach the pulley to the other end.
   
4. Attach the string to the cart, run it over the pulley, and tie a loop that extends a few inches below the edge of the table, in the other end, as shown in
   Figure 25-1
   .
   
5. While holding the cart in position at the far end of the table, hang a mass on the loop on the other side of the string.
   
6. Next, you release the cart and let the weight of the hanging mass pull the cart across the table. As you do this, you measure the acceleration of the cart using either of the previous methods:
   

– Stopwatch: measure the time (in seconds) for the cart to be pulled a measured distance (in meters). The acceleration (in m/s
²
) is determined by a = 2d/t
²
, where
d
is the distance that the object is pulled across the table (in m) during time,
t
(in seconds).

Figure 25-1
Newton’s second law apparatus. Courtesy PASCO
.

– Motion sensor: record the position of the cart as it is drawn across the table. Display the velocity versus time graph and determine the acceleration of the cart by finding the slope of that graph. This can be done either using the Slope tool from the DataStudio menu or more simply by obtaining the acceleration as the change in velocity divided by the change in time.

7. Repeat this measurement, but make the following changes:

– Vary the mass in the cart, but keep the applied force constant, as indicated in
Figures 25-2
and
25-3
.

– Vary the applied force by adding or removing some of the
hanging weight
, but keep the mass in the cart constant, as shown in
Figure 25-4
.

Figure 25-2
Finding acceleration as a function of mass, while keeping the force constant. Courtesy PASCO
.

Figure 25-3
Adding mass to the cart while keeping the force constant. Courtesy PASCO
.