From: pauld@exploratorium.edu
Date: Tue Nov 04 2003 - 20:56:26 PST
Message-ID: <2543.209.239.173.234.1068008186.squirrel@www.exo.net> Date: Tue, 4 Nov 2003 20:56:26 -0800 (PST) Subject: Re: pinhole Re: forces/friction From: pauld@exploratorium.edu
Hi Dave
The brick wall exerts a force on the car.
The action/reaction pair to this force is the force of the car on the wall.
Paul D
> Ron-
> I loved your response to Jennie's questions, and it brought up a point
> that I've
> always wondered about. Let's say that you are driving a car and turn off
> the engine
> so that there is no forward force due to the engine, and the only force at
> work is
> the force of friction which acts in the backwards direction.
>
> Now... what if this "coasting" car hit a brick wall. Certainly we would
> describe a
> force being applied to the brick wall by the car, yet we just said that
> there is no
> force in the forward direction.
>
> I imagine my confusion arises from a misunderstanding of the relationship
> between
> force and momentum. Could someone help me out?
>
> -Dave
>
>
> --- Ronald Wong <ronwong@inreach.com> wrote:
>> Recently Jennie said:
>>
>> >...i'd love some help.
>> >...if i am driving a car at a constant velocity, and the engine
>> > is exerting a force of 100N, then the force of friction balances
>> > that out with 100N the other way.
>>
>> Right.
>>
>>
>>
>>
>> > now, let's say, i suddenly stop the car mid-motion and turn
>> > off the engine. the car will slow down and eventually
>> > stop because of the force of friction, which is counteracting
>> > the car's "desire" to keep moving because of its inertia, right?.
>>
>> Right.
>>
>>
>>
>>
>> > But how do we describe the forces on the car during its slow-down?
>> > My student asked me if the only force on the car were friction
>> > in the opposite direction, why the car wouldn't move backwards.
>>
>> That was a good question on the part of the student. It brought out the
>> fact that the student didn't understand the nature of friction.
>>
>> My approach to physics teaching was to determine what the students knew
>> about a topic BEFORE I introduced it to them. In this way they could
>> integrate what they already knew about nature with the things that they
>> were about to learn. The material seemed to stick with them a lot longer
>> than if I simply introduced them to the topics of kinematics, dynamics,
>> etc. as they are so often found in physics textbooks.
>>
>> I would frequently do this by means of a simple demonstration or two.
>>
>> For instance: I would place a book on a table and ask the students,
>> "What
>> forces are acting on the book as it lies on the table?". Some would say
>> none because it wasn't moving and I'd give them an initial nod of
>> approval
>> because, in one sense, they were absolutely right (a point we return to
>> later in the presentation). Inevitably, someone would mention the force
>> of
>> gravity (if not, I'd do a second demonstration that quickly got their
>> grey
>> cells going - I'd drop something on the floor).
>>
>> Now that they had a force acting down on the object, the next question
>> was:
>> "Why doesn't it fall down?". They quickly drew the conclusion that the
>> table was pushing up on the book (i.e. "Holding it up"). This lead to a
>> series of activities where the students pushed one of their fingers down
>> on
>> their desks and noticed the effect that this had on the tip of their
>> finger. They then pushed the finger of one hand against that of the
>> other.
>> This was followed by other activities which, with time and patience,
>> revealed to them the properties of Newton's 3rd Law.
>>
>> When we took these observations and applied them to the book sitting on
>> the
>> table, the students suddenly realized that it sat there NOT because
>> there
>> was no forces acting on the book but because the forces acting on the
>> book
>> were balanced (Newton's 1st law had already been covered earlier through
>> a
>> process similar to the one they were presently engaged in so this served
>> as
>> a review in one sense). The nice part is that I didn't have to tell them
>> that and all I had to do thereafter was to remind them about these
>> discoveries whenever they forgot.
>>
>> The next question, of course, was, "How do we set this book in motion
>> along
>> the table?". Everybody knew that one. If they didn't give me a direction
>> for the "unbalanced" force, I would initially push down on the book.
>> After
>> a little moaning and groaning on their part, they would tell me to
>> direct
>> the unbalanced force in the horizontal direction. Doing this gave me an
>> opportunity to show that an "unbalanced" force changed the velocity of
>> an
>> object in the direction of the "unbalanced" force (i.e. accelerated the
>> object in the same direction as the unbalanced force) - laying down the
>> groundwork for the Newton's 2nd Law.
>>
>> At this point we get into friction because the next question I asked
>> was,
>> "What happened to the book after it left my hand?". Everybody knew the
>> answer to that one too - it slowed down (it decelerated).
>>
>> They also knew the answer to the next question: "Why?" - FRICTION!. They
>> even were able to tell me which way the frictional force must have acted
>> to
>> slow the book down.
>>
>> So, an "unbalanced" force acting in the direction of motion accelerates
>> an
>> object and later the unbalanced force of friction acting in the
>> direction
>> opposite to the direction of motion slows it down and stopped it
>> (re-inforcing the initial ideas of Newton's 2nd Law introduced earlier).
>>
>> It was clear to them that friction opposes motion.
>>
>> It was opposing the motion even as I was accelerating the book with my
>> pushing force.
>>
>> Thus there were two forces acting during the acceleration phase of the
>> demonstration. The unbalanced force was NOT the "unbalanced" force
>> mentioned in the earlier paragraphs - the pushing force - but the
>> difference between the pushing force and the smaller (a conclusion the
>> students make - not I), frictional force.
>>
>> Once the book left my hand I was no longer pushing on it and only the
>> frictional force remained. The unbalanced force acting on the book was
>> now
>> only the frictional force and, since it was opposing the motion of the
>> book, it caused the book's velocity to change until it was reduced to
>> zero.
>>
>> At that point the book was at rest. It was once again simply lying on
>> the
>> table and we were back to square one where only the force of gravity and
>> the table's reaction force were acting on the book. Just as before,
>> there
>> was no frictional force. All these points would be made by the students
>> -
>> not I.
>>
>> That's why the "..the car wouldn't move backwards". The frictional force
>> brought it to rest and, once it came to rest, the frictional force
>> ceased
>> to exist. There was no longer any unbalanced forces acting on the car so
>> it
>> just sat there.
>>
>>
>>
>>
>>
>> > is there still the force of the car engine acting in the forward
>> > direction even when the car shuts off?
>>
>> One of the questions I asked during the demonstration was "Why does the
>> book keep moving after I was no longer pushing it? Why didn't it just
>> stop?"
>>
>> Earlier in the course, the class had considered
>> Galileo's speculations involving a ball rolling
>> down one inclined plane and up another in which
>> the latter became less and less inclined. A series
>> of demonstrations followed that confirmed the
>> conclusions that they had arrived at earlier after
>> reflecting on Galileo's thoughts on the matter
>> and allowed me to put them in a frame of mind
>> where they could see the connection between
>> uniform velocity and balanced forces. At that
>> point I could bring up a new concept - Newton's
>> 1st Law (the Law of Inertia).
>>
>> Their answer to the question was immediate - INERTIA! There is NO force
>> acting in the forward direction. Only the tendency of a body to maintain
>> it's state of motion ("'desire' to keep moving" as you said) is
>> responsible
>> for the car's continued movement in the forward direction. That's the
>> law.
>>
>>
>>
>>
>>
>>
>>
>> > (and even so, if the forces were balanced before, it seems the
>> > net force would still be in the direction of the friction force)?
>>
>> When the driving force produced by the engine was balanced by the
>> frictional force, the net force was zero. That's why the car moved with
>> uniform velocity down the road - neither speeding up, or down, or
>> changing
>> directions. Once you turned off the driving force, only the frictional
>> force was left to act on the car. Now we have a net force and, since it
>> is
>> the frictional force, the net force acts in the direction of the
>> frictional
>> force. Since this is opposite to the direction of motion, the car slows
>> down.
>>
>>
>>
>>
>>
>>
>>
>> > is it the ground exerting a forward force on the car?
>>
>> The ground exerts an upward force on the car perpendicular to the car's
>> motion so it can't contribute to a horizontal force in either direction.
>> It
>> does play a role in determining the size of the frictional force and
>> thus
>> the rate at which the car slows down.
>>
>> The important thing to remember is that there is NO forward force on the
>> car "...when the car shuts off".
>>
>> Now Aristotle would vehemently argue otherwise
>> - so you are in very good company when you
>> expressed the idea that there should be a force
>> in the forward direction to account for the
>> continued motion in that direction. In fact,
>> earlier in my physics course my students would
>> also have agreed with you. They had been shown
>> examples where motion could only be sustained
>> by the application of a force - something that
>> they were very familiar with. But this was
>> before we considered Galileo's thought
>> experiments, created a few of our own, and
>> looked at a system where very little friction
>> was present (an air track) - all of this part
>> of a prelude to Newton's 1st Law.
>>
>>
>>
>>
>> > i guess a similar example would be rolling a ball and letting go...
>> > what other force is at work as the ball is being slowed down by
>> > friction in the opposite direction?
>>
>> None. Friction is the only force acting on the ball in the horizontal
>> direction. As a result, it is an unbalanced force and, because it
>> opposes
>>
> === message truncated ===
>
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