Monthly Archives: October 2013

Mass on a moving wedge

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This video contains an excellent analysis to a rather hard problem: A mass resting on a wedge, which itself is free to slide on a horizontal plane. The problem is complicated because (a) there are two accelerations to keep track of, and (b) the kinematic constraint is not-so-easy to apply. Special attention to the careful use of a ghost force (which is fine!) in the accelerating frame.

Solution by Alpar Sevgen

Physics of amusement park

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This problem involves an amusement park ride, consisting of a vertical rotating cylinder. People ride on the inside of the cylinder. In the problem, we try to calculate the conditions under which people will not slide and fall down from the cylinder.

Solution by Yaşar Safkan

There is an important note here: At the end of the problem, we work out an estimation. For a rather low coefficient of friciton of 0.05, and a radius of 5m, we find that the needed frequency of rotation is about 1 Hz. This is all fine. However, there is an additional consideration omitted in the video. In this case, the centripetal acceleration is about 200 meters per second squared, or in other words, 20g. As humans, this kind of acceleration is just too much, and definitely unrealistic for an amusement park ride. 

Cannon shooting downhill

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This problem concerns projectile motion. A cannon shoots a projectile at an angle, yet the ground is not flat, but slants downwards, which essentially lets the projectile fly longer. Here we solve both for the (horizontal) range of the projectile, and the best angle to fire the cannon to achieve maximum range.

This problem is a little intensive on the mathematical side, and is one of our longer solution videos.

Solution by Yaşar Safkan

You will find that the algebra and trigonometry is a bit tricky towards the end.

The hunter and the (poor) monkey

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This problem is about projectile motion. In a nutshell, a hunter shoots his rifle directly at a monkey hanging from a tree. Just as he fires his rifle, the monkey lets go of the branch, and starts falling. The question is to find the conditions under which the monkey will be hit by the bullet.

Solution by Yaşar Safkan

The surprising result is that, the bullet will hit the monkey independent of the speed of the bullet. In fact, it turns out letting go of the branch is the worst thing the monkey can do!

You will also find a video showing this exact experiment performed at MIT towards the end of the video. (Source: Monkey and a gun )