Bullet Block Explained!
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it as the reference problem when asking or answering questions about off-center impacts, where energy goes during collisions, and why spinning versus non-spinning cases differ.
Recommended by 1 people across 2 different months, 2016 to 2024.
What it is
Veritasium's follow-up to his bullet block experiment, in which a bullet fired into a wooden block rises to the same height whether it hits the center or off-center. The video explains the result using conservation of momentum, energy and angular momentum, with high-speed footage, simulations and demos from RIT and Dickinson College.
What people said
Recommends Farcher · 2016-08 · Physics Stack Exchange · on “Applying a force on an object in space (weightless) away from its centre of mass vs at its centre of mass”
I think that you query is about equating the work done by the accelerating force and the gain in kinetic energy of the bar. If the force $F$ is applied at the centre of mass of the bar and the centre of mass moves a distance $x$ then the work done by the force is $Fx$ and this represents the gain in the translational kinetic energy of the bar. If the same force is applied along a line which is not through the centre of mass of the rod and the centre of mass moves by a distance $x$ the work done by the force is larger than $Fx$ because, if the force is to stay in contact with the rod which is rotating about its centre of mass as well as the centre of mass undergoing transnational motion, the
Recommends Farcher · 2024-12 · Physics Stack Exchange · on “Why does a rod move faster when struck at the center rather than the edge, despite Newton's second law indicating the same acceleration?"”
One word explains why your ideal world analysis disagrees with your real world observation - friction. Ideal world analysis Linear acceleration of centre of mass of the rod is the same irrespective of where the rod is hit. Angular acceleration about centre of mass is greater when the applied force applies a larger torque about the centre of mass, ie force is applied at right angles to the rod and further from the centre of mass. Real world observation Contradicts theory, as explained above, because of the presence of friction and so much so that many think the theoretical analysis is flawed. In some ways this is like the old idea that a force needed to be applied to a cart (or any other obje
Cites Blue · 2018-11 · Physics Stack Exchange · on “Why is there discrepancy in conservation of energy and momentum in my modified version of Veritasium's bullet board experiment?”
Bullet Block Explained! - YouTube [this video] This is the Veritasium's bullet board experiment: A bullet is shot from beneath a horizontal board. The first time it is shot right underneath its center of mass, and the second time the shot is shifted a bit to the right (giving the board an angular momentum). Both of the planks rise to the same height, implying that in the second case the board had larger energy (since it also needed to rotate). That is explained by considering that the amount of waste energy lost (in heat, deformation etc.) is not fixed. So conserving the momentum, you can always imagine that the amount of waste energy lost in the spinning board (2nd case) is less by assuming that
Cites Virender Bhardwaj · 2024-08 · Physics Stack Exchange · on “Why do we only see the distance traveled inside the block by the bullet in this problem?”
In one of a very old veritasium video [this video] he discusses a problem of firing a bullet on a block. Here is the simpler version of it: Suppose we have a cuboid shaped block on a frictionless table, and we shoot a bullet with speed $V$ directly at the center of it. The bullet gets embedded in the block, and suppose the system (bullet+ deformed block) ends up with some velocity $v$ . It must be that $V>v$ , otherwise it would mean the system gained energy from the collision which is not possible. Now consider a second case where we shoot the same bullet close to one o edges of the cube. Here, the translational kinetic energy of the system would be same as in the first case (because by conserva
Cites Bob D · 2024-08 · Physics Stack Exchange · on “Why do we only see the distance traveled inside the block by the bullet in this problem?”
As a result of the many comments associated with your post, you summarized in your comment to me that your issue with the analysis is as follows: If we are saying the bullet when fired off center travels less distance inside the block due to the work done by the stopping frictional force being less on the bullet, I don’t think that’s true. That’s because to calculate work done w.r.t ground you have to look at the displacement of bullet w.r.t ground because the block is also moving. We can't just equate it to distance traveled inside the block. It is not necessary to calculate the work based on the displacement w.r.t. ground rather than w.r.t the center of mass of the block-bullet system. Alt