How to Solve Dynamics Exercise Involving Force Representation?

[haruspex]

If B hasn't moved yet, does this mean he has only two forces? ( The weight and contact force?)

No, it just means the forces are still all in balance, so B is not yet accelerating.
Vertically, the gravitational force balances the normal force from the ground, while horizontally the compression in the spring balances the static friction force.

In q5, we are told it is about to move. This means the static friction force has reached its limit.

 

[haruspex]

In my analysis, block A moves 1 cm then stops. Block B does not move.

Are you assuming the applied force is constant? Since we are told A moves at a constant speed, it must be increasing.

 

[PeroK]

Are you assuming the applied force is constant? Since we are told A moves at a constant speed, it must be increasing.

Which is physically impossible, without yet more unstated assumptions.

If the force on A is changing in some unspecified way, how can you calculate it in part 5? The applied force on block A could be anything by the time block B moves.

I don't understand trying to make sense of an ill-conceived, ill-posed problem.

 

[jbriggs444]

Which is physically impossible, without yet more unstated assumptions.

If the force on A is changing in some unspecified way, how can you calculate it in part 5? The applied force on block A could be anything by the time block B moves.

I don't understand trying to make sense of an ill-conceived, ill-posed problem.

We know that block A is moving at constant speed at [just prior to anyway] the time in question. So we know its acceleration. So we can write down a force balance. All of the other forces on block A are knowable. So we can solve for $\vec{F}$ at that time. [Technically a one sided derivative, but we can let that slide]

 

[PeroK]

We know that block A is moving at constant speed at the time in question. So we know its acceleration. So we can write down a force balance. All of the other forces on block A are knowable. So we can solve for $\vec{F}$ at that time.

No we don't. It only moves at constant speed for 2 cm. Block B doesn't move at that point.

 

[jbriggs444]

No we don't. It only moves at constant speed for 2 cm. Block B doesn't move at that point.

OK, I'll concede that.

 

[berkeman]

Thread is closed temporarily for Moderation, bruh.

 

[Mark44]

[+ I am one of those people who, if they see the solution directly, take a certain amount of time to find the steps of this solution. It is not a requirement that finding the solution directly is a bad thing and makes the student lazy.]

Being unable to find the solution directly is not a good thing. There's really not much learning going in when someone has to see the solution and then reverse-engineers the steps to it.

And let you know, I also don't have the time. I still have to study mathematics, chemistry, and Computer science as well.

Maybe you have bitten off more than you can chew.

 

[berkeman]

After a Mentor discussion, the thread will remain closed. Thank you to all who tried to help this poster.