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Rutgers, The State University of New Jersey

University of Washington

Simon Fraser University

Hope College

03:04

Kai C.

(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?

02:55

Keshav S.

(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?

0:00

Aditya P.

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 our next section, where we will be talking about restoring and dissipated forces and how they're related to work in energy. So restoring forces and dissipated forces for either one of these types of forces when we put them into our work kinetic energy equation. What we see is that as the force is exerted over some distance, we get a reduction in kinetic energy. But a restoring force will then give it back so it will oscillate between taking away kinetic energy and then giving it back and taking it away and giving it back, whereas a dissipated force will simply take it away and you never get it back. So we'll talk about the different types of forces that fall under these two categories. Uh, the restoring force we're going to talk about here is Thea Elastic or spring force. Different classes use different words here. Some use elastic. Some you spring, uh, doesn't really matter. It means the same thing, which is something called Hook's Law. Hook's law says that force is proportional to the amount that you displace the elastic or spring in the opposite direction by some constant K. In other words, if we have a spring and we stretch it out. Some displacement. Delta X It would be to the right. Then the spring will try to exert a force back to get shorter. If, on the other hand, we were to compress it and negative Delta X so to the left, then the spring would try to push back out with the force F and that this force is proportional to negative Delta X by some constant K that we will refer to as the spring constant notice that it's units have to be kilograms per second squared. Okay, so Hook's law is essentially the model, the mathematical model we're going to use for elastics, elastic objects or spring objects. Um, now it's not 100% accurate for all springs. There are springs that do not obey Hook's law ever and even the ones that do. If you stretch them too far, it's pretty easy to damage them. And then they no longer will obey Hook's law because you've damaged it. But there are certain class of oscillators. We call them that obey Hook's law, and in fact, this idea was used with early quantum mechanics to develop quite a few other ideas. Um, on the other hand, if we think about dissipated forces a dissipated force, a great example is friction. Okay, where you have a box sliding on a surface and it comes to a stop. So over some distance, Delta X, you had a change in velocity, which means there must have been some force involved here and the energy removed by the friction force okay, is turned into something else. So this is an interesting idea here that the energy is taken from being kinetic energy, and generally it's actually turned into thermal energy. If you put your hand down underneath things that have been rubbing together, then they feel warm. That's because all of that energy went from kinetic energy into being thermal energy. We could never turn it back from thermal energy at that temperature, at least into helpful mechanical or kinetic energy. Mechanical energy being the sum of useful types of energy for doing work. Now, um, so that's what a dissipated forces dissipated force remove that energy from the system and doesn't give it back. In fact, many forces, not just friction, operate as resistive forces eso in the next few examples will consider some options for looking at both spring on in slash elastic forces and also for looking at friction forces.

Potential Energy

Equilibrium and Elasticity

Energy Conservation

Moment, Impulse, and Collisions

Rotation of Rigid Bodies

04:49

03:46

04:20

04:52

02:15

02:42

04:42

02:20