Have you ever while buttering toast had the bread slip away, tumble down and land on the floor butter-side-down? Would you blame the butter landing on bad luck—or physics? Time for science to tell us! Take a slice, try it out and learn about the science behind a falling slice of toast!
Objects close to Earth fall because our planet, a very heavy object, pulls on them. The effect of this attraction is very familiar but our intuition about how fast items fall can let us down. Intuitively, one would think the heavier an object is, faster it falls. But if you take two balls of the same size—a light one and a heavy one—and release them from the same height, both will reach the ground at the same time. This shows the rate at which objects fall straight down does not depend on their masses. This observation was made by Galileo Galilei in the 16th century, and it still surprises a lot of people today.
In addition to this fact of physics, some objects tumble as they fall because one side of the object starts falling before the other. How fast an object rotates while it falls depends on how its mass is distributed with respect to the axis of rotation.
In this activity you will study how a slice of toast falls or tumbles down and how the laws of physics—and not luck—determine which side faces down when your toast reaches the ground.
- Old newspaper (or other paper)
- A few slices of toasted sandwich bread
- Butter, peanut butter or any other kind of sticky spread
- Butter knife
- A table 75 centimeters (about 30 inches) high
- Protect the floor around the edge of the table with the newspaper.
- Spread a few slices of toasted bread or toast on one side with a sticky spread such as butter or peanut butter. Spread some of the slices thin and some thick but always distribute the spread evenly over one full side of the bread.
- Perform all tests over the protected area on the floor.
- Hold a slice buttered side up with your two hands (one on each side of the slice), parallel to the ground, at the height of the table, and release. Watch as it falls. Which side faces down when the toast reaches the ground?
- Pick up the slice and place it near the edge of the table, again with the buttered side up. Slide the slice over the edge of the table with moderate speed: not too fast, not too slow. Observe as it falls. Did it fall in the same way as the first test, or was it different?
- As the slice lies on the ground, observe which side is down this time. Is it the same or different from your first test?
- Do you think the outcome of the first test (releasing it with two hands) was the result of a random process (sometimes this side faces down and other times the other one faces down) or would one side of your slice preferably end facing down every time?
- What about the second test (sliding it of the table)? Would the side facing down be arbitrarily determined or would one side be preferred? Why do you think this is the case?
- Repeat both tests with the same or a different buttered slice. Which side hit the ground this time?
- Even if you obtained the same results, this could easily be due to luck. Repeat at least 10 more times. Do you get the same side facing down repeatedly or does it look like a random toss? That is, sometimes the buttery side faces up and sometimes it faces down.
- If one side is more likely to land face down, why would that be? Would the heavier (buttered) side predominantly land up or down? What happens when the buttered side faces down when you release the slice or slide the slice off? Perform a new set of tests to find out!
- Extra: What would happen if the height over which the slice falls is larger or shorter? Perform another set of tests to find out! Use considerably different heights such as a small chair, windowsill or balcony. How does it land now?
- Extra: Can you design a test to see if the mass of the slice of toasted bread makes a difference? Would a heavy hardcover book fall and tumble in the same way? Predict the outcome and test it out!
- Extra: Can you design a test to see if the distribution of the mass of the slice of buttered bread makes a difference? Would a slice with most of its mass near the center fall and tumble in the same way as a slice that has the same mass mainly located at the edges of the slice? Predict the outcome and test it out!
Observations and results
Did the slice fall straight down, landing buttered side up after being released with two hands? On the other hand, did your slice almost always tumble down and land buttered side down after sliding off the table?
When released with two hands from a horizontal position, all parts of the slice start falling at the same instance. Thus, the slice falls straight down. Earth’s gravitational attraction determines how fast the slice falls—but at any instance, each part of the slice falls at the same speed, and the slice lands with the same face up as it had when released.
When you slide a slice off the table, one side tips over the edge of the table, causing the slice to rotate while it falls. Earth’s pull still determines how fast the center of the slice falls, but now the slice also rotates around an axis through this center. How many times the slice can rotate during its fall will depend on the duration of the fall, which is determined by the height of the table. How fast your object rotates is another factor, which is determined by the distribution of the mass of the rotating object. With a table 75 centimeters high, the side of your uniformly buttered toast that was initially facing up will generally land facing down.
The results will depend on the height of the table. When sliding off a low table, the fall takes less time and thus, the slice will rotate during a shorter time. On the contrary, it has more time to rotate when falling off a higher table.
Dispose of your protective paper and throw the used bread in the compost bin.
This activity brought to you in partnership with Science Buddies