Inertial Mass Measurement

While I was checking the worksheets at the end of class I was noticing that a few groups had "interesting" values for their cart masses on the final table of Unit 5.

• The mass values you put in the table derive from the accelerations you measure, not the spring scale measurements
• If you did things right then the Force column (the forces from the fan) should be the same for all cases where the same number of batteries were used.
• The masses you calculate this way are systematically wrong because you ignore friction.  But if they are wrong by more than about 20% then they are too wrong. (Later we'll learn how to correct for the friction to get a better value.)
• If they are too wrong then check your method of calculation. The ratio of accelerations should be equal to the ratio of the total mass that is being accelerated. That means you have to do some algebra to solve for the mass of the fan-cart in terms of the bar's mass.
• The question asking for g want the value implied by your data. If you measured the weight of the cart with a spring scale that weight would be in newtons. If you used to the spring scale and read the mass scale you have to use g = 9.8 N/kg to get the weight. Then you report another value of g  in the question which will be somewhat wrong because friction was ignored.
• If you find you just can't get any reasonable values from your data then try to understand what's wrong and explain your analysis of the issues. We don't expect you to always get perfect results or to fudge your data and we don't necessarily grade on the nearness of your answer to the "right" answer.

One-question physics test

There's a one-question physics test that lets me know if we're making progress. It's the last question of Unit 3.  

"Consider the ball toss carefully. Assume that upward is the positive direction. Indicate in the table that follows whether the velocity is positive, zero or negative during each of the three parts of the motion. Also indicate if the acceleration is positive, zero or negative."

Why?

Learning physics requires revising preconceptions we come to the class with.  The meaning of acceleration illustrates one such preconception. As commonly used, the word acceleration means moving faster. The reasoning goes like this: 

If you're not moving then you can't be accelerating. Zero velocity means not moving doesn't it? — therefore if v = 0 then a = 0.   

Right? 

Wrong!

The precise definition of acceleration in the physics context is the rate of change of velocity: a= dv/dt

The velocity can be instantaneously at zero, but if the velocity is changing then acceleration is not zero.

If you understand this then you're changing your preconceptions based on your physics lessons. If not then either you're not listening, or you hear but don't want to accept something that violates your sense of what's right.

The reason we need to get this is that forces lead to acceleration and physics is all about forces and what they do to things.

If you think you got it then try this question:

Mike jumps out of a tree and lands on a trampoline. The trampoline sags 0.5 m before launching Mike back into the air. At the very bottom, where the sag is the greatest, Mike’s acceleration is:

A. Upward B. Downward C. Zero

Have the courage of your convictions and enter your answer as an (anonymous) comment. Say why.

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Update  For answer click here.