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?