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Re: Pushing a car uphill



It is always amusing to see the collision between the world of physics
models and the world of brute reality.

The seeming paradox of an easier push against a larger force, is writ
particularly large for people who tow large boats with small cars
into and out of sandy uneven ramps.
(Sand in particular has a way of reducing a four plus four crew cab
into an immobile tow vehicle, by its tire lubricating qualities.)

A good part of the problem concerns where a vehicle comes to a stop
when parked in a forward direction. It tends to stop with one or more
wheels on a bump - it might be a small piece of grit.
In this case, it is helpful to roll the car backwards a little.
(You will notice that after rolling back, it is *again* stationary,
so arguments concerning static vs rolling friction lose their effect).

The actual comparison is between pushing a vehicle over an excrescence,
in contrast to a shallow incline. The latter is usually preferable.

When the footing is loose, a rocking motion can enlarge a depression
and make the climb angle less severe.

Brian W

At 21:02 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


brian whatcott <inet@intellisys.net> Altus OK
Eureka!