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0:00

Aditya P.

(I) A 7150-kg railroad car travels alone on a level frictionless track with a constant speed of 15.0 m/s. A 3350-kg load, initially at rest, is dropped onto the car. What will be the car's new speed?

00:56

Donald A.

I) How much tension must a rope withstand if it is used to accelerate a 1210-kg car horizontally along a frictionless surface at 1.20 m/s$^2$ ?

07:43

Kathleen T.

(II) A person has a reasonable chance of surviving an automobile crash if the deceleration is no more than 30 $g$'s. Calculate the force on a 65-kg person accelerating at this rate.What distance is traveled if brought to rest at this rate from 95 km/h?

03:02

Averell H.

(II) Superman must stop a 120-km/h train in 150 m to keep it from hitting a stalled car on the tracks. If the train's mass is $3.6 \times 10^5$ kg how much force must he exert? Compare to the weight of the train (give as %). How much force does the train exert on Superman?

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welcome to the first section in our unit on atomic physics. In this section, we're going to be looking at bores model of the atom. Now, this is a very important model because it was the first one to take into account the ideas of electrons and having a nucleus all combined with quantity ization of energy states. Now, this is, ah really interesting concept that he was able to come up with this. It was very early before a lot of the other work on some of these things have been done and there wasn't a lot of understanding, and this led to a lot of elaboration on what we already knew. So the basic idea of boars model is that there is a central nucleus with orbiting electrons that sit at discrete energy states and being at these discrete energy states, they can move from one to the next, but only if they receive exactly the right amount of energy or release exactly the right amount of energy. So we have these electrons that are quant ties. We say their energy is quant ties and if they were to receive a photon say of the correct energy difference between two states, they could jump up into that state. Now there's other things that could cause this as well. You could have a collision with another particle. It could just get hot. The temperature of the atom could increase, and all these things could cause the electron to jump any of the ways they do it. It has toe have the right amount of energy. Now, when an electron falls back down to a lower energy state, it will tend to emit a photon that is, it will emit a little bit of light as it falls down. And that light will have exactly the energy difference between the two states. So it's almost like it borrows the energy and then gives it back. Um, it's also possible it could release something called a phone on, which has more to do with vibration and solid materials. But photons air generally how we'll explain it as uh, in the introductory physics course here. Now, Bore was able to use this model to look at the hydrogen atom to great effect, he said. Given some quantum numbers, here is what we'll call these and equals 123 I can predict the radius and energy of electrons in these different energy states. Now he actually did have this R N as an orbital radius. We know now that because of the Heisenberg uncertainty principle, it's a little unreasonable, but it's not. Uh, it wasn't a crazy thing to say at the time, and certainly it turns out to be more accurate than you might think. The Bohr radius shows up in a lot of places, so he calculates that the smallest radius would be 0.529 nanometers, and then the radio I of the subsequent energy levels increases is M squared. Meanwhile, this is what's known as the binding energy equation, which also comes from boards model and says that if we have 13.6 e v for our baseline and equals one energy level, the next one will be that divided by and squared. You notice the negative here. What it means is that it would take 13.6 TV to remove the n equals one electron from the hydrogen atoms. So essentially, this is also could also be called the ionization energy for hydrogen or for that electron at that particular energy level. Now um he had a lot of success with hydrogen, but when he tried to do other elements, it didn't seem to work out so well. But it did raise the question in people's minds of what can we do to describe the quantities nature of electron energy levels for different atoms besides the hydrogen atom? Now, uh, other atoms tend to be extremely complicated, and even the hydrogen atom is quite complicated mathematically so. We will stay away from most of those explanations, but we can learn a lot about atomic physics just by looking about what's looking at what's happening inside the hydrogen atom.

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