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Cornell University
University of Washington
University of Winnipeg
01:40
Keshav S.
(II) According to a simplified model of a mammalian heart, at each pulse approximately 20 $g$ of blood is accelerated from 0.25 m/s to 0.35 m/s during a period of 0.10 s. What is the magnitude of the force exerted by the heart muscle?
00:48
Averell H.
(I) What force is needed to accelerate a sled (mass = 55 kg) at 1.4 m/s$^2$ on horizontal frictionless ice?
0:00
Muhammed S.
(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?
03:02
(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 fourth example video. Looking at power in this video, we're going to consider the problem of a sprinter who is traveling some distance from rest. So v not is equal to zero meters per second. And we're interested in the power output of this particular sprinter. Okay, s. So let's say that this particular sprinter is 70 kg, pretty average sized human being, and they are going to go from rest to about 10 m per second. So we finals equal to 10 m per second and they're going to do that in Delta T equals three seconds. Okay, Eso let's find out the amount of power here. Now I see a lot of students who get to problems like this and they stumble around for a long time, saying, Well, this is work. And I know that power is work divided by time. I have ah time here. How do I find my work? And then they spend 30 minutes trying to figure out what the force is supposed to be. Remember, remember, remember that work is also equal to a change in kinetic energy. Okay? And in this case, if that's if the only force working on it is the force that causes him to move forward. Then we can simply plug this in, and we can say that power is equal to Delta K divided by adult t. So we have one half M. The final squared V, not squared, is zero. So it's minus zero, divided by three seconds, and we're done. All you have to do is plug in the numbers here. So, uh, remember that when you're doing biological problems like this, sometimes instead of p equals FV or work equals F. Delta X. As I've said before, work can be a difficult measurement to get, but it's really, really easy to measure velocities and times and things like that. So remember that as long as you can, where we're talking about total work here, total work is equal to a change in kinetic energy, and you should always try to keep track of that and use it whenever possible, because it'll probably be a pretty big shortcut for you.
Potential Energy
Equilibrium and Elasticity
Energy Conservation
Moment, Impulse, and Collisions
Rotation of Rigid Bodies