no, it not useless. we still learn Bohr's model in HS n dats almost 200 yr old! while there may be new models, previous one is good for explaining the basics. it is also useful to learn previous model n see how our understanding improves over time.
Answer:
Option D.
Value cannot be calculated without knowing the speed of the train
Explanation:
The speed of the beam can only be calculated accurately when the speed of the train is put into consideration. Based of the theory of relativity, the observer is on the ground, and the train is moving with the beam of light inside it. This causes a variation in the reference frames when making judgements of the speed of the beam. The speed of the beam will be more accurate if the observer is moving at the same sped of the train, or the train is stationary.
To get the correct answer, we have to subtract the speed of the train from the speed calculated.
Answer:

Explanation:
given,
charge = -5.0 μC
Electric force, F = 11 i^ N
force would a proton experience = ?
we know



we know charge of proton is equal to 1.6 x 10⁻¹⁹ C
using formula



Force experienced by the photon in the same field is equal to 
Explanation:
Precision represents that how close the different measurements of the sample one take are to one another.
- One can increase the precision in lab by paying attention to each and every detail.
- Usage of the equipment properly and also increasing the sample size.
-
Ensuring that the equipment is calibrated properly. They should be clean and functioning. Using equipment which is not functioning correctly can cause results to swing wildly and also bits of the debris stuck to the equipment can influence the measurements of the mass and the volume.
- Each measurement must be taken multiple times, especially if experiments in which combining of the substances in specific amounts is involved.
Answer:
b is the answer tennis ball