Mechanics -- Problem Set #2

Marker's Comments

Take your best shot

Most people had the right idea. Folks had difficulties, though, in relating all the heights (water tower base, bullet hole, etc.) relative to one axis.

Fountain of (mis-spent) youth

Energy conservation is not useful to solve this problem, unless you keep track of heat generated -- kinetic energy changes to heat during the collision. Conservation of mechanical energy only gives us the speed of the water at the height of the can. If the speed of the water at the height of the can is zero, how can it hold the extra weight of the garbage can?!!

Huh? Hmmm...

Remember that this is a physics problem and not a geometry problem. Understanding the physics is much more important than trying to calculate the exact volumes of the containers. No-one among POPTOR participants tried to prove that the force exerted by the wall is equal to the differnce between the weight and the solution to part (a). You are expected to prove whatever you claim.

All aboard!

Train filling with grain: Most got the right frictional coefficient. Some also got the correct solution for the work done. Almost everyone that answered the question in the correct manner assumed that the grain was being dispensed in 1000 kg blocks and not poured continuosly at a rate of 1000kg/s. Maybe not many farmers & train-folk out there...

Java jump-up (eyow!!)

Some folks understood that they would need to work with the concept of the centre of mass. Others attempted using forces but did not get far. This was the least-answered question.

Weight! Bob!

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Question 1:
Total of marks 246
Overall Average 5.2
The number of students attempting question is 40
The average of those who attempted is 6.2

Question 2:
Total of marks 165
Overall Average 3.5
The number of students attempting question is 40
The average of those who attempted is 4.1

Question 3:
Total of marks 204
Overall Average 4.3
The number of students attempting question is 37
The average of those who attempted is 5.5

Question 4:
Total of marks 225
Overall Average 4.8
The number of students attempting question is 39
The average of those who attempted is 5.8

Question 5:
Total of marks 51
Overall Average 1.1
The number of students attempting question is 24
The average of those who attempted is 2.1

Question 6:
Total of marks 81
Overall Average 1.7
The number of students attempting question is 23
The average of those who attempted is 3.5