Moving object acquiring mass on a frictionless surface

In summary: It was a long and complex discussion, but in summary, the question posed was whether a bucket on wheels moving at a constant velocity on a frictionless surface would slow down, speed up, or remain at that velocity as it gained mass due to the rain. The group initially believed that it would remain constant due to the lack of friction, but after considering the concept of inertia and the fact that the raindrops would have to be accelerated to have a horizontal velocity, it was concluded that the cart would slow down due to the extra force exerted on it. There was also a discussion about whether the rain was coming down vertically or at an angle, but ultimately it was determined that the cart would still slow down regardless of the angle of the rain.
  • #1
Terp
41
0
Hey all. My professor gave us this question in class the other day but I'm having a hell of a time reasoning out how the answer can be true. Firstly, the problem:

There's a bucket on wheels moving at a constant velocity on a frictionless surface. It's raining out so the bucket is gaining mass. Will it slow down, speed up, or remain at that velocity as it gains mass?


My gut tells me that on a frictionless surface that it'd remain constant, but he told the class that it will slow down and that it doesn't matter if the surface is frictionless or not.

It makes perfect sense to me that it'd slow down with friction (becuase as it gains mass, its normal force increases which increases the frictional force).

If anybody could help me reconcile this it'd be appreciated. Thanks a lot!
 
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  • #2
The energy of the moving bucket will not change. If the kinetic energy is constant, what happens to the velocity if the mass increases?
 
  • #3
Oh, I meant to add this to the post above. We have not yet discussed energy in the class so I'm guessing there's an explanation somewhere that has to deal with Newton's laws of motion...

Thanks for the reply :).
 
  • #4
I guess, even kinetic energy would decrease. And yes, explanation lies in some chapter before energy!
COLLISION!
As the rain drops get into the bucket, eventually we are looking at a case of inelastic collision. This is bound to decrease it's speed, ain't it? (Since the mass is increasing.)
 
  • #5
so, you see, your professor had something on his mind.. when he made that statement!
 
  • #6
Hey, Terp!
Do you go to the university of maryland? If you do, so do I, let me know.

Anyways, to answer your question...

So, the object is moving at a constant speed. It needs to force to keep it moving it just keeps going on this frictionless surface. Well, the raindrops are falling directly into the cart. The raindrops previously had no horizontal(x) velocity, and now the cart must accelerate them to have a horizontal(x) velocity. Well, the cart must exert a force on each raindrop in the horizontal direction to have it move that way. If the cart exerts a force on the raindrop, the raindrop exerts a force on the cart, the cart slows down.

Basically, since the cart has to push each raindrop from 0 velocity to some x velocity, it has to exert an extra force and slows down.


Did that help?
 
  • #7
I haven't got all that far in class yet, but it makes sense to me like this:
An object moving across a frictionless surface will maintain the same velocity due to its inertia. Inertia is dependent on an object's mass so it follows that if the mass increases, the degree at which the object resists change in velocity also changes. Since the amount of force necessary to get an object of a certain mass moving, it wouldn't be enough to move an object a certain mass +x. Thus it will have a negative acceleration!
 
  • #8
Hey, you seem to be really physics savvy, could you check out my post--maybe you could help me too? Its 'Analyzing Force vs. Time graphs'
 
  • #9
FrixioN911 said:
Hey, Terp!
Do you go to the university of maryland? If you do, so do I, let me know.

Anyways, to answer your question...

So, the object is moving at a constant speed. It needs to force to keep it moving it just keeps going on this frictionless surface. Well, the raindrops are falling directly into the cart. The raindrops previously had no horizontal(x) velocity, and now the cart must accelerate them to have a horizontal(x) velocity. Well, the cart must exert a force on each raindrop in the horizontal direction to have it move that way. If the cart exerts a force on the raindrop, the raindrop exerts a force on the cart, the cart slows down.

Basically, since the cart has to push each raindrop from 0 velocity to some x velocity, it has to exert an extra force and slows down.Did that help?
Ahh, that makes a lot of sense. And yeah, I do go to UMD I'm a junior in the civil engineering dept. I'm taking these general courses becuase I wasn't sure what I wanted to major in until this year, which was when I had to declare.
 
  • #10
Nowhere in the question does it say that the rain is coming vertically down. Your answer is dependent on this assumption. If the rain was coming in at an angle the answer would be different.
 
  • #11
Terp -- What class are you in and when? You might be in my class.

Tyco -- You're wrong. If the rain was coming down at an angle, it still has LESS horizontal velocity than the cart, so the cart will have to give it more horizontal velocity, or, the cart will still exert a hnorizontal force on the raindrop (the only force we care about in this problem). Hence, the cart will slow down. No matter what (within the bounds of this question).
The coefficient or proportionality that the cart would slow down compared to how many raindrops fall, may chage slightly due to an angle - but that has nothing to do with this question.
 
  • #12
It doesn't necessarily have less horizontal velocity than the cart.
 
  • #13
u are moving on a chair from above some one drops a cannon ball in ur lap. the cannon ball is moving in 1 direction u are moving in another to change the direction of the cannon ball it needs energy the only energy there is is your's so it takes ur energy and u slow down
 
  • #14
tyco. lol. For some reason I knew you'd say that just to try and prove a point. OK. you win, the cart might have to be moving 0.1 m/s or more... Or maybe it's a typhoon and the raindrops are shooting through your "cart". But then it wouldn't matter because the wind would pick up and since you were smart enough to put a sail on the back of your cart, your defying 12 laws at once. It is implied that the rain is falling vertically. ^_^
 

What is meant by a "frictionless surface"?

A frictionless surface is a hypothetical surface that has no resistance to motion. This means that objects can move across it without any force opposing their movement.

How can an object acquire mass on a frictionless surface?

An object can acquire mass on a frictionless surface through a process called accretion. This is when particles or objects come together and stick to the surface, increasing its mass.

Why is it important to study moving objects on a frictionless surface?

Studying moving objects on a frictionless surface allows scientists to isolate and understand the effects of other forces, such as gravity or air resistance, on the motion of the object. This can help in the development of technologies and theories related to motion and energy.

Can an object continue to accelerate on a frictionless surface?

Yes, an object can continue to accelerate on a frictionless surface as long as there are no other forces acting upon it. This is because there is no opposing force to slow down or stop the object's motion.

What are some real-life examples of frictionless surfaces?

Some examples of frictionless surfaces in real life include air hockey tables, which have minimal friction between the surface and the puck, and rollercoasters, where the tracks are often designed to minimize friction and allow for a smoother, faster ride.

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