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Physics: Post your doubts here!

princess Anu

princess Anu

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Q10: How do we estimate this? any logic?:S
Screen Shot 2015-06-08 at 9.28.24 AM.png

Q4: Should I use 50N to find the power & hence the work done OR the horizontal component of 50? I think we shud use 50N because the question asks TOTAL WORK DONE( not the NET WORK DONE) ,right?

Screen Shot 2015-06-08 at 9.27.13 AM.png


Screen Shot 2015-06-08 at 9.28.02 AM.png

Help please
 
qwertypoiu

qwertypoiu

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princess Anu said:
Q10: How do we estimate this? any logic?:S
View attachment 54796

Q4: Should I use 50N to find the power & hence the work done OR the horizontal component of 50? I think we shud use 50N because the question asks TOTAL WORK DONE( not the NET WORK DONE) ,right?

View attachment 54798


View attachment 54797

Help please
Click to expand...
Estimating requires us to know roughly the physical properties of evereyday objects.
It's the head they're asking. Stress = force/area.
Force = mg. G we already know. Mass of our head might be difficult to guess, but we can make rough guesses. It can't be something large like 50kg, that's nearly whole body mass. Yet, the head is a significant part of our body, I'll say 5kg.
So force is around 50N.

Now for area. It's the cross sectional area of the neck we want, and I'll be honest I'd just grab a ruler and estimate the diameter of my neck. Which is around 15cm or 0.15m.
Area = pi*(0.15^2)/4 = 0.0177m^2.
Stress = 50N/0.0177m^2 = 2825Pa. You can see there is no answer anywhere close to this except for C. They do this deliberately so that even poor guesses at our raw measurements would still allow us to achieve the marks. The real pitfall in these questions is actually forgetting to convert units. FOr example, if, instead of 0.15, we used 15 as our diameter, that would give us 0.3Pa.


Work done should be measured in terms of the direction it is actually done in. In this case, horizontally, to the right.
In 10s, it moved 5m.
Work done = 50cos(30) * 5 = 216.5J = 220J


The last question is about terminal velocity. An object, when released from a great height, has a constant weight, and a resistance to motion that increases with its speed. Acceleration only decreases to 0 in this case, there is no increase. Velocity increases from zero to maximum, so answer is C.
 
princess Anu

princess Anu

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Thank u :)
qwertypoiu said:
Estimating requires us to know roughly the physical properties of evereyday objects.
It's the head they're asking. Stress = force/area.
Force = mg. G we already know. Mass of our head might be difficult to guess, but we can make rough guesses. It can't be something large like 50kg, that's nearly whole body mass. Yet, the head is a significant part of our body, I'll say 5kg.
So force is around 50N.

Now for area. It's the cross sectional area of the neck we want, and I'll be honest I'd just grab a ruler and estimate the diameter of my neck. Which is around 15cm or 0.15m.
Area = pi*(0.15^2)/4 = 0.0177m^2.
Stress = 50N/0.0177m^2 = 2825Pa. You can see there is no answer anywhere close to this except for C. They do this deliberately so that even poor guesses at our raw measurements would still allow us to achieve the marks. The real pitfall in these questions is actually forgetting to convert units. FOr example, if, instead of 0.15, we used 15 as our diameter, that would give us 0.3Pa.


Work done should be measured in terms of the direction it is actually done in. In this case, horizontally, to the right.
In 10s, it moved 5m.
Work done = 50cos(30) * 5 = 216.5J = 220J


The last question is about terminal velocity. An object, when released from a great height, has a constant weight, and a resistance to motion that increases with its speed. Acceleration only decreases to 0 in this case, there is no increase. Velocity increases from zero to maximum, so answer is C.
Click to expand...
 
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