Did This AP Physics Problem Analysis Get It Right?

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Homework Help Overview

The discussion revolves around an AP Physics problem involving a car and a truck, focusing on their velocities, momentum, and the implications of a traffic accident. Participants are analyzing the physics concepts related to motion, forces, and friction in the context of a collision scenario.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to explain their reasoning and calculations regarding the problem, including the use of momentum and forces. Some participants question the clarity of the problem statement and the calculations presented, while others suggest reconsidering the approach to finding deceleration from friction.

Discussion Status

The discussion is ongoing, with participants providing feedback on the original poster's calculations and reasoning. Some guidance has been offered regarding the proper use of equations and units, but there is no explicit consensus on the correctness of the original poster's conclusions.

Contextual Notes

Participants note the absence of the actual problem text, which may hinder the discussion. There is also mention of the original poster's uncertainty due to a recent absence from class, which may affect their understanding of the concepts involved.

WoodenFire
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Here is the actual physics problem. Below it I will try to explain exactly how I thought to figure it out. I would appreciate it if you corrected me If I made any errors. I am very sketchy on this one after being out for a week from class.

http://img74.imageshack.us/img74/2937/img012brz6.th.jpg
For some reason the picture isn't showing to most people... here is the imageshack direct link. http://img74.imageshack.us/my.php?image=img012brz6.jpg
**Note : the friction value is for the stretch of 9.6m in the picture.

u = .89, u = (Ff/Fn), 1714kg + 750kg = (2464kg)(9.8m/s/s) = 24147.2N, .89 = (Ff)/(24147.2), Ff = 21491N

F = ma, 21491 = 2464a, a = 8.71m/s/s

d = (1/2)(a)(t^2), 9.6m = 4.36(t^2), t = 1.48s

P = Momentum, (P)/(Ff) = t **I made this equation up because of the fact that momentum is Newtons x Seconds and dividing it by Newtons will give me time., (P)/(21491)=(1.48), P = 31806.7NS

Plugging into a triangle the 31806.7NS as the hypotinuse and 48.8degrees as the angle, I used cosine and sine to get the following information.
1) North = 20950.7NS
2) East = 23931.8NS

P = (M)(V)

Car : 20950.7 = (750)(V), V = 27.9m/s ~ 56mph
Truck : 23931.8 = (1714)(V), V = 13.96m/s ~ 28mph

My conclusion is that the car obviously didn't stop at the stopsign and was speeding while the truck kept just under the speed limit. The blame would entirely be on the driver of the car.

What do you think?
Due tomarrow!
 
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Hello WoodenFire, and welcome to PF!

It would be better you used the homework thread. Anyway, I can't see at which problem you actually stated what exactly the problem is (i.e. the text of the problem). Without it, I'm afraid no one will be able to help you.
 
What are you talking about? The question in the problem clearly states "Who was lieing in the accident report?". The car obviously didn't stop at the stop sign and was going over the speed limit while the truck was ~2mph below the 30mph speed limit. The blame would entirely be placed on the car.

I showed all of my work and how I did it.

I am sorry I thought this was the homework thread. :O
But I am just wondering, at the velocities I found, and the coefficient of friction slowing the inelastic truck+car bundle down, does it all work out that they travel exactly 9.6m? If it does, doesn't that mean I did it correctly? I really am not an expert on physics. This is the first physics course I have ever taken.

Have you tried solving the problem from the picture given and masses of the car and truck? It really doesn't take more than 10-15minutes if you can spare it! :P

Thankyou again for your help and welcoming me.
 
WoodenFire said:
Here is the actual physics problem. Below it I will try to explain exactly how I thought to figure it out. I would appreciate it if you corrected me If I made any errors. I am very sketchy on this one after being out for a week from class.

http://img74.imageshack.us/img74/2937/img012brz6.th.jpg
For some reason the picture isn't showing to most people... here is the imageshack direct link. http://img74.imageshack.us/my.php?image=img012brz6.jpg
**Note : the friction value is for the stretch of 9.6m in the picture.

u = .89, u = (Ff/Fn), 1714kg + 750kg = (2464kg)(9.8m/s/s) = 24147.2N, .89 = (Ff)/(24147.2), Ff = 21491N

F = ma, 21491 = 2464a, a = 8.71m/s/s

d = (1/2)(a)(t^2), 9.6m = 4.36(t^2), t = 1.48s

P = Momentum, (P)/(Ff) = t **I made this equation up because of the fact that momentum is Newtons x Seconds and dividing it by Newtons will give me time., (P)/(21491)=(1.48), P = 31806.7NS

Plugging into a triangle the 31806.7NS as the hypotinuse and 48.8degrees as the angle, I used cosine and sine to get the following information.
1) North = 20950.7NS
2) East = 23931.8NS

P = (M)(V)

Car : 20950.7 = (750)(V), V = 27.9m/s ~ 56mph
Truck : 23931.8 = (1714)(V), V = 13.96m/s ~ 28mph

My conclusion is that the car obviously didn't stop at the stopsign and was speeding while the truck kept just under the speed limit. The blame would entirely be on the driver of the car.

What do you think?
Due tomarrow!

Looks about right. You could have found the deceleration from friction by simply using

F = Ma = μMg
a = μg

You would do well to change your style a bit from what I call "stream of consciousness computing" to writing actual equations. Statements like

1714kg + 750kg = (2464kg)(9.8m/s/s) = 24147.2N

are pure nonsense. Kilograms do not equal Newtons. Get in the habit of writing equal signs only when relating quantities that are equal, such as

M = 1714kg + 750kg = (2464kg)
W = Mg = (2464kg)(9.8m/s/s) = 24147.2N
 
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