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Re: [Phys-l] the first law of motion

I have to agree with JD that WHEN TALKING WITH STUDENTS it is NOT fine to have a "tacit assumption" that we know there is a resistive force present. Students generally don't know this. Many adults don't know this. There is a simple diagram I have in my homework problems that shows an object 4 times with different forces in each of 4 directions. The question asks which diagram will result in the object continuing to move at a constant velocity. Less than 10% of the students can get it correct the first time they encounter the question (HS Senior trig based physics). They insist for quite some time that a NET force is required to keep the object moving at constant velocity in spite of data they personally collected that demonstrates the opposite. It takes time, hard work and multiple real life examples to get them to accept the no net force means constant velocity. To assume there is a resistive force that is being "balanced" causes BIG problems.

On 11/1/11 1:04 PM, Josh Gates wrote:
JD:
b) If you talk about some "force to keep the car moving"
that is not OK. That's not physics. We cannot even begin
to quantify that.
I feel like there was a tacit assumption there that we need a force to keep the car moving _because we know that there's a resistive force present_. That's fine.

jg

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