Tag: energy

Dropper Popper

Question 1:
Where does the energy of the jump come from?

Question 2:
How does this demonstration resemble hydrogen fusion?

Answer 1:
The energy of the jump comes from the elastic potential energy stored in the deformation of the popper. The gravitational potential energy merely serves as the activation energy required to unleash the stored energy. In fact, the height of the jump does not change with the height from which the popper was dropped.

Answer 2:
During hydrogen fusion, the rest-mass energy stored in the hydrogen nuclei is released (analogous to the elastic potential energy in the dropper popper). However, in order for two hydrogen nuclei (two protons basically) to fuse, they come first overcome the very strong electrostatic repulsion they are going exert on each other. This requires them to start their approach with very high kinetic energy. In the stars (and our Sun), this very high kinetic energy is provided by the gravitational pull of the stars themselves. As the hydrogen nuclei fall towards the centre of the star (analogous to the falling dropper popper), they gain the necessary KE to allow them to come close enough to other hydrogen nuclei for fusion to occur.

5.3.1 Magnetic Cannon

Question:

Where did all the momentum come from? Is the PCOM violated?

Where did all the kinetic energy come from? Is the PCOE violated?

Answer:

PCOM
Notice that the other two balls recoiled to the right after the collision. (The recoil was at quite a high speed, but friction brought them to rest quickly) So even though the outgoing ball had a large leftward momentum, after subtracting the rightward momentum of the other two balls, the total momentum is still equal to the initial leftward momentum.

PCOE
Note that the magnetic field must have an associated magnetic potential. Since the field is attractive, the balls must be losing magnetic potential energy as they come closer. So even though the outgoing ball had a large kinetic energy, after accounting for the loss in magnetic potential energy, the total energy is still equal to the initial total energy.

Delving Deeper
Notice that after the demonstration, two balls were magnetically stuck to each other. Considerable effort must be expended to pull these two balls apart. Ths is the act that stores magnetic potential energy in the system.

So the demonstration actually started with the system full of magnetic potential energy. When the balls come closer, and become accelerated by the magnetic forces (resulting in a high-speed collision), the magnetic potential energy is being converted into KE.