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Misc => Physics Questions => Topic started by: leeyiren on September 25, 2010, 10:40:22 am



Title: how do you move an object at constant velocity?
Post by: leeyiren on September 25, 2010, 10:40:22 am
Hi prof,

I want to move an object at constant velocity, say 1m/s

To move an object, I have to apply a force.

But F=ma

Hence every time I apply a force there will be acceleration.

So how do you move an object at constant velocity?

Thx  :P

PS: Why are there so few ppl are posting questions in this section?


Title: Re: how do you move an object at constant velocity?
Post by: Fu-Kwun Hwang on September 25, 2010, 08:56:16 pm
If there is no external force acting on an object, the object will move at constant velocity.
If there is external force acting on the object, you just need to apply force to cancel the external force.


Title: Re: how do you move an object at constant velocity?
Post by: lookang on September 25, 2010, 09:13:43 pm


To move an object, I have to apply a force.
actually, if object is already in uniform motion, no external force is require. or simply Newton 1st Law
to change the motion of an object, yes a force is needed or simply Newton 2nd Law, Fnet = m*a


But F=ma
Hence every time I apply a force there will be acceleration.
So how do you move an object at constant velocity?
if we look at Fnet = m*a
let P be the push force
f be the friction force opposes motion v
then Fnet = m*a
imply P - f = m*a
constant v imply a = 0,
therefore, it is possible if P - f = 0

how can we relate this to real life situation?
when a ball falls under gravity from a tall building for a long time, terminal velocity is reached
let mg is weight of mass, m under gravity g
let D be drag force on mass m
using Fnet = m*a
mg - D = m*a
at terminal velocity, a = 0
mg - D = 0



PS: Why are there so few ppl are posting questions in this section?
no everyone leaves a question,but there many views.  ;D


Title: Re: how do you move an object at constant velocity?
Post by: leeyiren on September 26, 2010, 10:00:52 pm
In the case of a horizontal motion, if the push force=resistive force, will the ball even move, let alone move at constant velocity?  ???


Title: Re: how do you move an object at constant velocity?
Post by: lookang on September 27, 2010, 07:53:44 am
In the case of a horizontal motion, if the push force=resistive force, will the ball even move, let alone move at constant velocity?  ???
The original starting condition determines the outcome of the motion.

your answer is true if the initial velocity = 0.

however, it is agreed by the physics community that if the initial velocity of the object is non-zero, it will continue in the state of motion due to the evidences of data with real life experiments.

so even if the push=resistive force at the beginning,

using Fnet = m*a
P - f = m*a
0 = a
but v is the starting velocity, thus it could be still traveling at some speed without any direction change  (this will happen but not likely to be observed because it is an idealized motion)

to extend this knowledge to real life situation, you could relate this to a ice sledge on very smooth ice horizontal ground.
when the ice sledge is already moving at constant velocity, say v=10 m/s

ideally does not require any pushing force to continue at constant v

in the presence of air resistance, P - f  = m*a where f >0
if P =0
then -f = m*a, therefore, it will -ve = a and it will decelerate. (this is observed in everyday life motion)

in this case, to maintain v = constant,
P - f = m*a
P- f = 0
P = f so a small Push P of equal magnitude to f is required for Constant v (this is observed in everyday life motion like the motion of say a bicyle in constant v on horizontal ground)
 


Title: Re: how do you move an object at constant velocity?
Post by: Fu-Kwun Hwang on September 27, 2010, 09:03:43 am
If an object is at rest ($\vec{u}=0$)at one reference frame, e.g. with respect to you.
I think you would accept that the NET force acting on the object is zero.
 
Assume there is another observer which is moving with constant velocity $\vec{v}$ with respect to you.
You saw another observer is moving with velocity $\vec{v}$

Let's back to the previous object, the other observer should find there is no NET force acting on that object.
And that object is moving with $\vec{u'}=-\vec{v}$ with respect to that observer.

From physics point of view: The physics law is the same for all inertia frames, so they all find the same force (or no net force).

An object moving with velocity $\vec{v}$ will be seen as at rest for another observer which is moving at the same velocity $\vec{v}$.

Assume you site in a train which is moving at constant velocity $\vec{v}$ with respect to ground:
 For all observer on the ground, you and other people in the train moving with constant velocity $vec{v}$
However, for observer in the same train, all the others are not moving at all ,right? ;)


Title: Re: how do you move an object at constant velocity?
Post by: leeyiren on September 28, 2010, 07:13:00 pm
does it mean that to move a resting object at constant velocity. I have to first apply a force greater than the resistive force for it to accelerate to a specific velocity, after which I can apply a force equal to the resistive force to make it constant velocity. Accurate enough? :-\


Title: Re: how do you move an object at constant velocity?
Post by: Fu-Kwun Hwang on September 28, 2010, 09:48:21 pm
Think about Newton's Law  and try to understand what it means:

You need an external force if you want to change momentum $\vec{P}=m\vec{v}$.

When there is no net force, the object move with constant velocity.


Title: Re: how do you move an object at constant velocity?
Post by: lookang on September 29, 2010, 04:01:50 pm
does it mean that to move a resting object at constant velocity. I have to first apply a force greater than the resistive force for it to accelerate to a specific velocity, after which I can apply a force equal to the resistive force to make it constant velocity. Accurate enough? :-\
sound good to me