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

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*alet P be the push forcef be the friction force opposes motion vthen Fnet = m*aimply P - f = m*aconstant 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 reachedlet mg is weight of mass, m under gravity glet D be drag force on mass musing Fnet = m*amg - D = m*aat terminal velocity, a = 0mg - D = 0

Jr. Member4 Re: how do you move an object at constant velocity?«  Reply #3 on: September 26, 2010, 10:00:52 pm »

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In the case of a horizontal motion, if the push force=resistive force, will the ball even move, let alone move at constant

velocity?  

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5 Re: how do you move an object at constant velocity?«  Reply #4 on: September 27, 2010, 07:53:44 am » posted from:SINGAPORE,SINGAPORE,SINGAPORE

Quote from: 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? 

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*aP - f = m*a0 = abut 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 >0if P =0then -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*aP- f = 0P = 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) 

« Last Edit: September 27, 2010, 08:07:48 am by lookang »

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6 Re: how do you move an object at constant velocity?«  Reply #5 on: September 27, 2010, 09:03:43 am » posted from:Taipei,T'ai-pei,Taiwan

If an object is at rest ( )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   with respect to you. You saw another observer is moving with velocity 

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   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   will be seen as at rest for another observer which is moving at the same velocity  .

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

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leeyirenJr. Member

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7 Re: how do you move an object at constant velocity?«  Reply #6 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? 

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8 Re: how do you move an object at constant velocity?«  Reply #7 on: September 28, 2010, 09:48:21 pm » posted from:,,Taiwan

Think about Newton's Law  and try to understand what it means:

Posts: 3054You need an external force if you want to change

momentum  .

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

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9 Re: how do you move an object at constant velocity?«  Reply #8 on: September 29, 2010, 04:01:50 pm » posted from:SINGAPORE,SINGAPORE,SINGAPORE

Quote from: 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? 

sound good to me

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acceleration is zero then the force must be zero, but

assuming you have an object moving with a constant

velocity of say 2 m/s, and that object strikes you, then

obviously some sort of 'force' would be felt by you, so

my question is what do you call that 'force' since it

actually is not a 'force' and is there an equation to

calculate it?

newtonian-mechanics

share improve this question

   

An object will slow down or change direction upon hitting something else. To test this, you could do an experiment where you throw something at a wall or any other object. You should see the thrown object slow down or change direction upon impact. Thus the force really is non-zero.

    The "force" (actually called an impulse) depends on what it strikes.

   

Why do you feel it is "actually not a force"? Is it because you feel the force doesn't cause any acceleration to the body it hits? That is not because there is no force, but because all the forces acting on that body cancel out. –  udiboy1209 Nov 15 '13 at 14:26

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2 Answersactive oldest votes

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eaccepted

Some students learning physics for the first time mistakenly think that objects that are

accelerating have force.

Force is not a property possessed by an object, but rather something you do to an object that

results in the object accelerating (changing its speed), given by the equation F = ma.

That is, Forces cause acceleration, not the other way around. This means that if you observe

an object accelerating, then it implies a force is acting on the object to cause such an

acceleration.

In this case, as the object strikes the hand, your hand applies a force to the ball causing it to

slow down (decelerate), and the ball applies an equal and opposite force to your hand causing

it to accelerate ever so slightly (Newton's third law), which is detected by your sensory

neurons.

share improve this answer answered

Mew

2,552730

   'the ball applies an equal and opposite force to your hand causing it to accelerate ever so slightly' by 'it' do you mean the hand is accelerating ever so slightly? –  Hani Sayegh Nov 16 '13 at 4:56 

    Yes that is correct. –  Mew Nov 16 '13 at 6:34

   

Lets just say the ball moving with constant velocity hits a wall, then according to you the wall must apply a force, but if the wall were to apply a force it must accelerate, but surely the wall is not accelerating? So what exactly is happening? How is the wall exerting a force? –  Hani Sayegh Nov 20 '13 at 1:10 

   

@HaniSayegh, the wall accelerates backwards, and the ball accelerates forwards (Newton's 3rd law). The wall doesn't have to accelerate forwards to apply a force. Acceleration results from force, you don't need forward acceleration to produce a force. –  Mew Nov 20 '13 at 2:09

   

@HaniSayegh, the wall doesn't appear to accelerate backwards because the acceleration is small, because of increased mass of the wall and supporting structures which provide an opposing force on the wall to resist its acceleration. (F=ma, so large mass means less acceleration). Locally however just where the ball hits the wall, you can see the wall move inwards a little bit with a slow motion camera. –  Mew Nov 20 '13 at 2:11

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vote0down vote

When two objects (of mass m1 and m2) collide with relative speed vrel then there is an

exchange of momentum (called an impulse) of magnitude

J=(ϵ+1)vrel1m1+1m2

where ϵ is the coefficient of restitution and it describes if the objects bounce or stick.

This impulse changes the speed of the object by Δv1=−Jm1 and Δv2=Jm2.

The actual force cannot be found from this as it changes rapidly with time, but an average

force can be computed if you know that the impact takes Δt time to occur.

Faverage=JΔt

PS. The definition of the impulse is J=∫F(t)dt.

share improve this answer answered 

ja72

8,52611541

    Why the -1? No comment left. –  ja72 Nov 16 '13 at 0:11