A problem from college physics course :-(

[Vikas]

"A race car accelerates from 10 m/s to 50
m/s in a distance of 3 m. The mass of the
car is 2500 kg. If there is a force of
resistance of 120 N acting against the car,
find the average value for the force
propelling the car. "

Can we all agree that this professor probably bikes to the university and has no idea about how much a typical car (or race car) weighs? This is pretty sad state for a physics professor who is not in habit of noticing something odd about the numbers. By the way, this is one of the top ranked University in Boston :-( With these type of professors, how do we expect students to excel in real life?

 

[yvon_la]

Cadillac 1970s?

 

[Vikas]

I don't think even Cadillac Race Car in 1970 weighed that much!

 

[Vikas]

10 m/s to 50 m/s in 3 meter

This translates to 22 mph to 110 mph in about 10 feet! Don't we all want to drive that race car even if it weighs 5500 lbs?

 

[Rand]

The name of the racecar is Yugo..
one that just got rear ended by a big truck

the reason it weighs 2500kg is because it was on a trailer with a load of metal scrap to the scrapyard.

 

[KevGuy]

 

[Rick in PA]

You'd need something like this:

 

[Kuato]

Originally Posted By: Vikas
"A race car accelerates from 10 m/s to 50
m/s in a distance of 3 m. The mass of the
car is 2500 kg. If there is a force of
resistance of 120 N acting against the car,
find the average value for the force
propelling the car. "

Can we all agree that this professor probably bikes to the university and has no idea about how much a typical car (or race car) weighs? This is pretty sad state for a physics professor who is not in habit of noticing something odd about the numbers. By the way, this is one of the top ranked University in Boston :-( With these type of professors, how do we expect students to excel in real life?


It's an exercise, he probably chose the numbers for some instructional purpose.

Could be an interesting discussion in class to point out the errors and/or impossible acceleration.

 

[4wheeldog]

You need a treadmill to solve that problem.

 

[KevGuy]

The bottom line is that the force of resistance on the car is negligible considered to the overall force applied to the vehicle ( 1 million N compared to 120 N). The simple approach would be to just calculate the acceleration then use F = ma and the answer is good enough.

 

[spackard]

Well, I remember a psychology teacher at a junior college commenting once how he'd been shocked pretty bad once by the 12V in his car, so he thought being shocked by 120V wall socket would probably be fatal.

 

[KevGuy]

Originally Posted By: spackard
Well, I remember a psychology teacher at a junior college commenting once how he'd been shocked pretty bad once by the 12V in his car, so he thought being shocked by 120V wall socket would probably be fatal.



12 V in the car is DC
120 V wall socket is AC.

 

[Vikas]

what does DC/AC has to do with the shock? Since you just solved the physics problem *correctly*, I am giving you benefit of doubt and assume that you can explain why AC is fatal and DC is not.

 

[KevGuy]

Originally Posted By: Vikas
what does DC/AC has to do with the shock? Since you just solved the physics problem *correctly*, I am giving you benefit of doubt and assume that you can explain why AC is fatal and DC is not.


Not sure what glue you are sniffing, but where in my post did I say that AC is fatal and DC is not?

 

[brages]

12v won't shock you unless you put your tongue in the cigarette lighter socket.

You can stick a finger in there or grab both sides of the battery without feeling a thing.

 

[spackard]

Later I guessed he (the psychology teacher) had probably been shocked by a 40kV primary ignition wire. I don't know where he got the idea that was 12V. The label on the battery maybe?

 

[raytseng]

Put your money where your mouth is and write a snarky out of the box answer

such as 38.4 fuptons with the reasoning that apparently you're in a sci-fi alternate universe or a dream world where apparently the car is powered by space-time folding and not classical mechanics.

 

[Kuato]

It's not volts that kills you, it's amps.

 

[Vikas]

Quote:

12 V in the car is DC
120 V wall socket is AC.


So what were *you* implying by that reply?

 

[A_Harman]

Originally Posted By: KevGuy

400m/s^2 * 1G / 9.8m/s^2 = 40.8G's.

Seems like that would be fatal to the occupant of the car. Doesn't the heart rip loose from its moorings in the rib cage at 40 G's?