Question

I'm a high school student trying to go beyond what I'm learning, and I get things pretty fast, but I don't get the full concept of what Prof. Lewin says on Classical Mechanics LEC 1about "Any measurement you make without the knowledge of its uncertainty is completely meaningless." Can anybody explain? And also, when he's trying to prove his grandmother's theory true, he adds up the 0.1 behind the 'plus of minus.' Why does he do this besides leaving it as 0.1?

Answer 1

so... the first question... so if you do know the uncertainty.. you know the range of the measurement, so it's kind of accurate. but when you don't know it can 0, and it can also be infinite. you'll never know since there's no limit. so it would be meaningless.

Answer 2

and... i don't fully understand your second question.

Answer 3

That was because the least count of the scale was 1mm. The maximum error in the measurement u make will always be less than 1mm. You cannot make a measurement with the scale and say i measured 22.45cm.

Answer 4

Here is a little more perspective on uncertainty in measurements. Suppose I'm navigating a space flight to Mars. I will need to plot a course, and to do that I need to know what direction to go. To do that, I need a precise measurement of where Mars is. Let's say that Mars is 60e6 km away from earth, and that the uncertainty in the direction of flight is + or minus 1 degree.

By the time we travel 60e6 km, we could be 1e6 km away from Mars. Knowing this error exists, we would want to recalculate our trajectory a few times before reaching Mars.

Answer 5

Okay, I'm starting to get it more, but I need another example and more explanation on what he meant by the quote I provided. But thank you guys for helping me. I know I'm probably annoying you by not quite getting it, but hey, I'm trying.

Answer 6

so... let's say you're trying to measure the length of a stick with a ruler. It says '70cm'. but the "rance of inaccuracy" could differ. It could be 65cm~75cm. It could be 69.5cm~70.5cm... but when you don't know the "range" it could be 0~infinite. then you'll never know how long it is. because it could be anything. and.. it's meaningless. when we know the "range" at least we know "something".

Answer 7

OH! Now I get it. Thank you for that great example.

Answer 8

same for me but some lectures are missing from the site could anyone help me?

Answer 9

@skoflin

http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/

This website contains the complete set of 35 lectures on Newtonian mechanics taught by Prof. Walter Lewin under 8.01

Answer 10

thanks so much!!!