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Re: Pushing a car uphill



My opinion is that your lab activity was more about biology and not about
physics. You did not have any of your trials do the "roll back" first. It
is likely that after you warm up your muscles, any next task is easier, at
least in perception. I would repeat the trials, asking each team to do some
number of push-ups, then the trial, then the opposite, then the trial
again. For some the trial would start uphill, for others the trial would be
downhill first. I like the other comments this has drawn, but they likely
do not apply here, since the question is about perception. Chuck's notion
about actually measuring time and force would, of course, make a "physics"
task of it.

Tom Ford

At 09:02 PM 10/24/01 -0400, you wrote:
Hi,

In my high school Conceptual Physics class, we currently are focusing on
the concepts impulse and change in momentum.

One of my students asked, "Why is it easier to push a car (by hand) up a
slight incline if you first let it roll back a bit?"

I replied that I doubted the idea. I added that if it's in fact true, I
would be at a loss to explain it.

The following day (yesterday) I took the class outside to experiment a
bit with the idea. I positioned my car on a very slight incline (I
estimate the acceleration when rolling backward to be about 0.10 m/s/s.)
We ran the same test four times:

Test 1) One set of three guys first pushed the car forward from rest,
then again after it had rolled back about 0.5 m. [Two of these guys said
they though it was a little easier the second time, one said it seemed
about the same.]

Test 2) Another group of three guys pushed it forward first from rest and
again from the backward roll. [One guy said it seemed easier with the
backward roll, two were not sure.]

Test 3) One student pushed by himself. [He said it seemed a little
easier with the backward roll.]

Test 4) I pushed the car by myself. [to me, it seemed equally difficult
both ways.]

So, I have two questions:

#1) Why was the roll-back condition not more difficult for any of us?
(The change in momentum is greater, so one might expect the task to seem
harder to do.)

#2) If the roll-back condition actually does make the task easier, why is
it easier?

Any thoughts?

Thanks.

Ed Seppa
Shelton (CT) High School