Imagine shooting two identical wooden blocks with a rifle. One block is hit right through its center, while the other is hit slightly off to the side. The big question is: how high will each block rise after being shot?
Most people guessed that the block hit off-center wouldn’t rise as high as the one hit through the center. But when high-speed footage was analyzed, both blocks reached the same height. This surprised many, even those experienced in physics.
The experiment caught the attention of many viewers, sparking video responses and attempts to replicate the experiment. It became a hot topic on platforms like Scientific American and Wired, and even inspired a web comic. This shows how engaging and fun scientific inquiry can be!
The main question was: how could two identical blocks, shot with the same rifle, have different energy outcomes? Some thought the rotational energy of the spinning block was too small to matter. However, calculations showed that the rotational energy was about 50% of the block’s gravitational potential energy at its peak height, proving it was significant.
Some viewers used pixel counters to measure the heights and found slight differences. This raised questions about what counts as a significant difference. Repeated experiments showed variability, but spinning blocks didn’t consistently reach lower heights than non-spinning ones.
Another idea was that air resistance might affect the spinning block differently, as it presents a smaller surface area during ascent. However, calculations showed this difference was minimal—only about 0.6% of the block’s weight—making it an unlikely factor in the height discrepancy.
The simplest explanation is the law of conservation of momentum. When the bullet is fired, it has a certain momentum directed upwards. When it hits the block, the bullet and block together must keep that same upward momentum, no matter where the bullet hits. This means both blocks should reach the same upward velocity and, therefore, the same height.
While momentum is conserved, kinetic energy is not. When the bullet hits the wood, a lot of its kinetic energy is lost as heat, sound, and deformation. The spinning block might keep more energy due to its rotation, possibly having more kinetic energy than the non-spinning block. This could happen if the bullet doesn’t penetrate as deeply into the spinning block, causing it to lose less energy on impact.
To explore this, the bullet’s penetration depth in each block was examined. Initial tests suggested less penetration in the spinning block. However, x-ray imaging showed similar penetration depths in both blocks. Since the bullet loses about 97% of its original kinetic energy on impact, the expected difference in penetration depth was tiny—around one-tenth of a millimeter—making it hard to measure.
While the results might not provide a clear answer, they highlight the complexities of energy and momentum in physics. The experiment encourages further exploration and modifications to make differences in bullet penetration more measurable. Viewers are invited to share their ideas for future experiments, fostering ongoing interest in scientific discovery.
This analysis emphasizes the importance of understanding fundamental physics principles, like conservation of momentum, and how they apply in real-world scenarios.
Recreate the experiment using a physics simulation software. Adjust variables such as the point of impact and the mass of the blocks. Observe how these changes affect the height each block reaches. Discuss with your classmates why the results might differ from the real-world experiment.
Using the formula for rotational kinetic energy, $E_{text{rot}} = frac{1}{2} I omega^2$, calculate the rotational energy of a block when hit off-center. Assume a moment of inertia $I$ and angular velocity $omega$. Compare this energy to the gravitational potential energy at the block’s peak height.
Perform calculations to verify the conservation of momentum. Given the mass of the bullet and its velocity, calculate the expected velocity of each block after impact. Use the principle of conservation of momentum: $m_{text{bullet}} v_{text{bullet}} = (m_{text{block}} + m_{text{bullet}}) v_{text{block}}$.
Research how air resistance might affect the motion of spinning versus non-spinning objects. Conduct a small experiment using paper or lightweight objects to observe how spin influences their fall. Discuss how these observations relate to the experiment with the blocks.
In groups, design a follow-up experiment to further explore the concepts of energy and momentum. Consider variables such as block material or bullet speed. Present your experimental design to the class, explaining how it could provide more insights into the physics of spinning blocks.
Physics – The branch of science concerned with the nature and properties of matter and energy, encompassing concepts such as force, motion, and the structure of atoms. – In our physics class, we learned how Newton’s laws of motion apply to everyday phenomena.
Energy – The capacity to do work or produce change, often measured in joules or calories. – The energy of a system can be calculated using the equation $E = mc^2$, where $m$ is mass and $c$ is the speed of light.
Momentum – The quantity of motion an object has, calculated as the product of its mass and velocity. – The momentum of a car traveling at $60 , text{m/s}$ with a mass of $1500 , text{kg}$ is $p = mv = 90000 , text{kg m/s}$.
Height – The measurement of an object or point in relation to a base level, often used in calculating potential energy. – The potential energy of an object at height $h$ is given by $PE = mgh$, where $m$ is mass and $g$ is the acceleration due to gravity.
Experiment – A scientific procedure undertaken to test a hypothesis by collecting data under controlled conditions. – In our experiment, we measured the acceleration of a falling object to verify the value of $g$.
Blocks – Solid pieces of material, often used in physics experiments to study concepts like friction and motion. – By sliding blocks of different materials down an inclined plane, we observed varying coefficients of friction.
Resistance – A measure of the opposition to the flow of electric current, typically measured in ohms ($Omega$). – The resistance in a circuit can be calculated using Ohm’s Law: $R = frac{V}{I}$, where $V$ is voltage and $I$ is current.
Rotation – The action of rotating around an axis or center, often described by angular velocity and angular momentum. – The Earth’s rotation on its axis is responsible for the cycle of day and night.
Kinetic – Relating to or resulting from motion, often used to describe energy associated with moving objects. – The kinetic energy of a moving car can be calculated using the formula $KE = frac{1}{2}mv^2$.
Conservation – A principle stating that a particular measurable property of an isolated physical system does not change as the system evolves. – The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another.
