resisted motion (1 Viewer)

[kr73114]

How do you know when it is mg + kv or mg- kv or -mg-kv or whatever...i dont get it? I know it has something to do with the direction of forces or something.

 

[Shadowdude]

Draw a diagram. You'll most often find it's mg - kv because it's resisted motion and the 'kv' is meant to counter the force of gravity.

 

[aya-chan]

mg is the force due to gravity, and kv is the resistance force. So, mg would be in the downward direction, whereas kv is in the direction opposite to the motion of the object.


So, take for example this object, moving downwards with velocity v, shown by the v arrow:

It is being affect by gravity, which produces a force 'mg' downwards. Since it's moving downwards, the resistive force is in the opposite direction - so the resistive force 'kv' is upwards. So the total force downwards is, mg - kv.


Now, if it were to go upwards:

So here, gravity is still acting downwards, but in this case the object is going upwards. Hence, your resisting force is in the opposite direction - downwards. So going downwards, you'd add the forces. i.e. the total downward force = mg + kv


Hope that helped!

 

[Shadowdude]

Moral of the story: Draw a diagram.

 

[kr73114]

mg is the force due to gravity, and kv is the resistance force. So, mg would be in the downward direction, whereas kv is in the direction opposite to the motion of the object.


So, take for example this object, moving downwards with velocity v, shown by the v arrow:

It is being affect by gravity, which produces a force 'mg' downwards. Since it's moving downwards, the resistive force is in the opposite direction - so the resistive force 'kv' is upwards. So the total force downwards is, mg - kv.


Now, if it were to go upwards:

So here, gravity is still acting downwards, but in this case the object is going upwards. Hence, your resisting force is in the opposite direction - downwards. So going downwards, you'd add the forces. i.e. the total downward force = mg + kv


Hope that helped!

thanks that did help...btw when is it -mg-kv?

 

[aya-chan]

It'd be the second case, if you wanted to find the force in the upward direction.