Physics: Post your doubts here!

[Hamnah Zahoor]

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.

 

[Clark20]

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?

 

[Clark20]

it can be true but is NOT always.

that is why the questions says 'MUST'

Thank you. I understand why C is wrong.

 

[Alis alis]

how to do this one?

 

[Ebrahim12]

A2 op-amp question.
This is from the cie application booklet:


Can somebody explain how D2 blocks the back emf from affecting the op-amp exactly? Thanks.

 

[Clark20]

Need help understanding this question. D is the answer.

 

[Emmris]

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 ...

 

[Emmris]

Actually my logic was wrong .... it's only g that acts on the object throughout the journey .... so ur answer maybe wrong ....

 

[Hamnah Zahoor]

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.

 

[Emmris]

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.

https://www.google.com/search?q=for...AQ&biw=360&bih=511&dpr=2#imgrc=1Mi7aLR1qrdwYM

It says dat only g acts on the object ....

 

[Hamnah Zahoor]

https://www.google.com/search?q=for...AQ&biw=360&bih=511&dpr=2#imgrc=1Mi7aLR1qrdwYM

It says dat only g acts on the object ....

The site you mentioned is for projectile motion in which we assume that air resistance is negligible or equal to zero while doing our calculations.
However, in the question mentioned above it does not states that the the air resistance was zero or it is projectile motion.

 

[Clark20]

Thank you.

 

[Clark20]

In this question, is the journey of the object between X and Y considered as projectile motion? As it follows a parabolic path.

 

[Hamnah Zahoor]

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.

 

[MShaheerUddin]

Can anyone draw b(i) and b(ii) for me?

Thanks

 

[Clark20]

I have one question related to Dynamics. Suppose you throw a ball at an angle in air, or release it from rest so it falls under free-fall or you throw the ball upwards, there can be only 2 forces acting (air resistance and weight) on the ball?

Is there no type of applied force there in the downward direction which opposes air resistance when you throw and release the ball in the downward direction from top?


And an applied force in the direction of the object similarly which opposes air resistance in the case it is released at an angle? And even when you throw an object upwards, there is no applied force in the upward direction, but only weight and air resistance acting on the object?

(By applied force I mean the force along the direction in which the object is travelling.)

 

[A*****]

I have one question related to Dynamics. Suppose you throw a ball at an angle in air, or release it from rest so it falls under free-fall or you throw the ball upwards, there can be only 2 forces acting (air resistance and weight) on the ball?

Is there no type of applied force there in the downward direction which opposes air resistance when you throw and release the ball in the downward direction from top?


And an applied force in the direction of the object similarly which opposes air resistance in the case it is released at an angle? And even when you throw an object upwards, there is no applied force in the upward direction, but only weight and air resistance acting on the object?

(By applied force I mean the force along the direction in which the object is travelling.)

You are right that there are only 2 forces acting on the ball in any case i.e. the weight of the ball and air resistance. The weight ALWAYS acts downwards but air resistance acts in the OPPOSITE direction of the ball's motion.
When you release it from a point above ground, the ball moves downwards so the air resistance acts upwards and weight downwards.
When it is thrown upwards, both the weight and air resistance act downwards.
When thrown at an angle, weight acts downwards and resistance is changing direction every instant as the ball changes direction.

 

[Clark20]

You are right that there are only 2 forces acting on the ball in any case i.e. the weight of the ball and air resistance. The weight ALWAYS acts downwards but air resistance acts in the OPPOSITE direction of the ball's motion.
When you release it from a point above ground, the ball moves downwards so the air resistance acts upwards and weight downwards.
When it is thrown upwards, both the weight and air resistance act downwards.
When thrown at an angle, weight acts downwards and resistance is changing direction every instant as the ball changes direction.

Thank you

 

[Clark20]

Need help understanding this question.

 

[Clark20]

see solution 152 at
http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-26.html

I did. Still I dont quite get it.