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We've been studying momentum in class this week. As I was putting together some problems for students to use as review, I searched the internet to see if I could find a ready-made collection of problems for high school students that an instructor was willing to share. One of the first hits I found contained this problem:
A bus of mass 19,000 kg driving at 17 m/s runs into a brick wall and comes to a complete stop. What is the force of impact on the bus?*I quickly decided that I'd better not use this problem set--something is not right here! Can you see what I mean? Why is this confusing or misleading?
(*) I changed the nouns and numbers in this problem to protect the original author from ridicule. (And also so that a quick Goolge search doesn't immediately turn up this exact question as posed by a frustrated student.)
It seems as if the force of impact that is calculated should be on the wall, not the bus. Since the wall is stationary the impact should be measured for the wall, not the bus.
ReplyDeleteWhat is misleading about this problem is it asks for the impact on the bus when actually you would be calculating the impact on the wall. Also the mass of the wall is not given so finding the actual force on the wall is again harder.
ReplyDeleteI agree with Taylor F about that this problem is misleading by it asks for the impact on the bus when it actually wants you to calculate the impact on the wall. She is right that the walls mass is not given so you can't actual find the force.
ReplyDeleteMason Smeznik
I agree with all of the comments. Because the wall is stationary, the impact should be measured on the wall. So that is why the question is misleading. Also the mass of the wall is not given, so the problem would definately be difficult to solve.
ReplyDeleteI also agree. Without the mass of the wall, the problem is very difficult to solve.
DeleteI agree with them too. Something out of the ordinary I wish buses were the color purple! That would light up my day!
ReplyDeleteSomething else out of the ordinary Yale High School should not hire a bus driver who runs into a wall! Just saying.
DeleteMason Smeznik
I agree with what everyone has said. The wall is stationary, the force of impact should be measured on the wall not the bus.
ReplyDeleteInteresting comments, but I think we're still missing a major problem here. Forget the word "impact." What about the force on the bus? Can we calculate that?
ReplyDeleteI agree with everyone that the wall is obviously standing still, and we do not know its mass. This makes this problem difficult to solve. Also though I'm not sure whether this is right or not but when the bus collides into the wall shouldn't the impact force the bus backwards, not make it come to a complete stop right there.
ReplyDeleteI agree with what everyone else, without the mass of the wall the problem would be unsolvable because you would have 2 unknown variables. The force cannot be calculated with the information given in the problem
ReplyDeleteHalie Foulks
I think we're barking up the wrong tree here a little bit. Think of it like this: one instant, the bus is moving forward as it drives down the road. The next instant, the bus has stopped moving. For that to happen, there *must* have been a force that acted on the bus (think Newton's First Law). So, asking us to calculate the force on the bus in not an inherently bad question.
ReplyDeleteSo an object in motion will stay in motion unless on outside force acts on it. So the problem is staying true to Newton's first law because the brick wall (outside force) causes it to come to a stop.
ReplyDeleteThe school bus would impart all of it's force to the wall. Force equals mass times acceleration
ReplyDeleteI think that since Newtons 1st Law states that an object in motion will stay in motion,unless acted upon by an external force,(which is the wall)the force on the bus must be negative to have the bus stop completely.
ReplyDeleteSo instead you would be solving for the impact of the bus on the wall? So because of newtons first law stating that an object in motion will stay in motion, I get how the problem would be misleading. But I could honestly say that I would have got this problem wrong.
ReplyDeleteOlivia Vinckier
I couldn't seem to find the force, i agree with the other people that the mass of the wall would be very helpful. It would be very easy however to find the momentum of the bus, just the mass times the velocity, which would equal 323,000. Also, I agree that the bus is following newton's first law because the outside force is the wall. to find the force you would have to do (F delta t = delta p) If it starts at rest you would know one of the times is 0, you would also onlu have one value for p which is the momentum of the bus.
ReplyDeleteVery good, iloveweekends. You are the first person to notice that this would be a simple problem (to be solved with the impulse-change in momentum theorem) if we were given the time of the interaction. Without the time, this problem is not solvable.
ReplyDeleteWhat might a reasonable estimate be for the time?
Thanks, everyone, for the comments this week.
ReplyDelete