Power, force, velocity and tension...

[hello478]

The tension force exists because the motor is pulling with force F. We can look at it from the point of view of the wire or of the motor.
The motor, presumably, is winding the wire onto a spool. The force it exerts is $\vec F$, positive in the downslope direction. The velocity with which this is winding wire onto the spool is $\vec v$, also positive in the downslope direction. The power is $\vec F\cdot \vec v$, also positive.
From the wire's perspective, the force is positive upslope, but stationary, so the wire is not producing any power.

this is very difficult to understand
im sorry but can you please explain again in more simpler words...

 

[haruspex]

this is very difficult to understand
im sorry but can you please explain again in more simpler words...

Picture what is going on inside the block. There is a drum turning, hauling in the wire. It exerts a force F on the wire, causing it to be drawn onto the drum with speed v. Both F and v act down the slope. The power required to do that is Fv.

 

[BvU]

in more simpler words...

$\$

 

[hello478]

thank you!

 

[hello478]

Picture what is going on inside the block. There is a drum turning, hauling in the wire. It exerts a force F on the wire, causing it to be drawn onto the drum with speed v. Both F and v act down the slope. The power required to do that is Fv.

so which force am i supposed to use to calculate the velocity?

 

[hello478]

We don't care about whaat causes the motion.

You want to find speed of the object using $P=\vec F. \vec v$ and you have power of the Tension force so that F in above equation represents tension force.

why dont we care about it? how are we supposed to find the velocity then?

 

[BvU]

so which force am i supposed to use to calculate the velocity?

The $F$ in $P = \vec F \cdot \vec v$ is the one that does the work. The motor does the work and the only way it is exerting a force is in the form of tension in the cable. The $T$ you calculated in a .

why dont we care about it? how are we supposed to find the velocity then?

We don't care about what causes the motion: we are given that the speed is constant ($\Rightarrow F_{net}=0$) and we know the opposing force component along the slope ($-mg\sin\theta$ -- the minus sign is because its direction is to the left. g = 9.8 m/s²). So we know $F$ in #33 (aka The $T$ you calculated in a ).

(as you can see I also didn't bother to draw more than just $F$ in #33)

$\$

 

[MatinSAR]

why dont we care about it? how are we supposed to find the velocity then?

Earlier you've said :

yes, so then i would use the tension force and multiply with velocity, because it is the force causing upward motion? is this explanation correct?

And I disagree. You should use the force that is related to the motor power. It doesn't necessary for the force to cause upward/downward motion.

My interpretation of the question is apparently different from @haruspex . I can't understand why in his opinion the force is in the downslope direction. I think it is better for you to use the ideas of @BvU and @haruspex . I prefer not to contribute anymore. This way you don't get misled by different ideas.

 

[hello478]

ok i sort of get it now... thank you very much!