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Physics: Post your doubts here!

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Why cant the answer be A?
Is it B?
If so its because the moment has to be kept constant to keep the bar in equilbrium so if x is increased then F should decrease therefore F and x would be inversely proportional
 
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Yes in this case the forward force will be taken as resultant force. ( when assuming zero resistance)
But in the case if acceleration is to be zero it will be only be if the resultant force is zero which is not in this case thus option D is not correct.
And the points you stated above are correct.

Surely you will need to take into account the changing mass of the rocket. In this case Newton's Law becomes F = d/dt(mv), the reducing mass of the rocket will imply an increasing value of v since momentum must be constant as force is constant.
 
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I understand why option A and D are wrong. However, I dont get how option C is justified.

The er says under conditions where mass keeps reducing, the bodys speed can change and it can accelerate. My question is, when a body is moving with constant velocity (zero resultant force) and even if its mass starts reducing, resultant force still is 0 and hence acceleration also is zero even if mass is lost. Speed shouldnt change.
 

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I did. Still having this confusion.
The main thing you have to focus on here is that not the ' Speed ' remains unchanged but the ' Velocity ' remains unchanged. For the acceleration to be zero the change in velocity should be zero not speed.
 
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The main thing you have to focus on here is that not the ' Speed ' remains unchanged but the ' Velocity ' remains unchanged. For the acceleration to be zero the change in velocity should be zero not speed.
The question implies there is 0 net force (by saying all external forces on a body cancel out which means it must be moving with constant velocity and 0 net force). When the net force is 0, by N2L there shouldnt be any change in speed as fnet=ma. Fnet=0 and 0=ma for even a reducing mass gives 0 acceleration just due to 0 net force. With 0 net force f=ma shouldnt work as an explanation for change in speed even if mass is changing.

But, if there is net force, then fnet=ma can explain how change in mass causes the body to accelerate/change its speed

My question is, with 0 net force how can change in mass cause a body to change its speed?
 
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A2 op-amp question.
This is from the cie application booklet:
upload_2019-2-25_18-54-26.png

Can somebody explain how D2 blocks the back emf from affecting the op-amp exactly? Thanks.
 
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Need help understanding this question. D is the answer.
I assume that u already know that gravity is always acting on the object through the motion ... therefore at its Max height u will have g acting downwards .... now let's study the horizontal component of the velocity, u will find out that the horizontal component throughout the journey stays constant even at its Max height which implies that air resistance is acting to balance the force , hence the acceleration of the horizontal component is zero n as u know air resistance act on opposite direction which in this case it's to the right as the object is moving to the left ...
 
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Actually my logic was wrong .... it's only g that acts on the object throughout the journey .... so ur answer maybe wrong ....
 
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Need help understanding this question. D is the answer.
I hope the examination report help understand the question:
Question 12
When a body is moving through air to the left, there will be a drag force acting on it to the right, as well as its weight. The answer here is D, but both A and C were more popular responses.
 
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In this question, is the journey of the object between X and Y considered as projectile motion? As it follows a parabolic path.
 

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In this question, is the journey of the object between X and Y considered as projectile motion? As it follows a parabolic path.
Yes it is considered a projectile for the motion is without air resistance and it's a parabolic path.
 
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