General physics question that is stumping me...

[James3shin]

If a block is placed on a frictionless ramp at height 'h' and the block slides down the ramp onto a frictionless plateau, why does the block not accelerate along the plateau?

I'm thinking that all of the potential energy at height 'h' was converted into kinetic energy at the foot of the ramp, thus there is no more acceleration and no force.

 

[TheInternet1980]

It does accelerate along the plateau. noob.

There are implied parenthesis in your equation.

 

[Ronstang]

Once there is no force acting on the block it is not going to accelerate. You answered your own question.

 

[rcpratt]

There is acceleration as it goes down the ramp. There is no acceleration (deceleration, really) because there is no friction on the plateau.

 

[Miramonti]

I don't know physics, but if the force of gravity is driving it's acceleration down the ramp, I'd assume the acceleration would stop as soon as its no longer going downward and is on a level surface, frictionless or not. I don't think that's how it would be written in the text book tho.

 

[NL5]

It does accelerate along the plateau. noob.

There are implied parenthesis in your equation.

Too funny!

 

[James3shin]

I'm getting hung up on the vectors. The mass sliding down the ramp has a vector that is parallel to the ramp. This vector has a vertical and horizontal component, why doesn't the horizontal vector continue to accelerate the mass along the plateau?

By the way I answered:
2

I think my first reasoning with conservation of energy is correct but I just can't rule out the horizontal vector for some reason.

 

[rcpratt]

I'm getting hung up on the vectors. The mass sliding down the ramp has a vector that is parallel to the ramp. This vector has a vertical and horizontal component, why doesn't the horizontal vector continue to accelerate the mass along the plateau?

The block accelerates in both directions on the ramp. The question asks whether it accelerates on the plateau. The only forces acting on the block on the plateau are the vertical acceleration due to gravity and the equivalent normal force in the opposite direction. No net force, no acceleration.

 

[James3shin]

Gah! I just can't figure out why all of the acceleration is done along the ramp...

 

[Ronstang]

Gah! I just can't figure out why all of the acceleration is done along the ramp...

You had better figure it out quick and understand it or you are going to fail physics.

 

[PieIsAwesome]

I'm getting hung up on the vectors. The mass sliding down the ramp has a vector that is parallel to the ramp. This vector has a vertical and horizontal component, why doesn't the horizontal vector continue to accelerate the mass along the plateau?

By the way I answered:
2

The mass's weight is a constant force that will always be acting on it in the same direction, completely vertical going downwards.

When the mass is in equilibrium, there must be some opposite force to cancel the weight out.

When the mass is on a flat surface, the normal force of the surface acting on the mass is vertical, is directed upwards, and has the same magnitude as the weight of the mass. The normal force cancels out the weight of the mass, and the mass is in equilibrium.

When the mass is on the inclined surface, the normal force is no longer vertical, but normal to the surface of the incline, and no longer cancels out the weight of the mass. The mass is no longer in equilibrium and accelerates.

On the incline, the normal force is less than on the flat surface. The normal force on the incline is equal to the component of the weight parallel to the normal force (or normal to the incline). This leaves the other component of the weight free to accelerate the mass. On the flat surface, the normal force is equal and opposite of the weight, there is no unbalanced force to accelerate the mass.

So what you need to see is that while the weight vector remains constant, the normal force vector does not, and this is what results in the mass accelerating or not.

 

[rcpratt]

There's no more horizontal force due to gravity since the plateau is flat...

Just think about it...things on a flat surface don't spontaneously move.

The explanation above is actually better. The normal force perpendicular to the surface is actually what causes the motion. It has a horizontal component on a ramp...it doesn't on a flat surface.

 

[AyashiKaibutsu]

I'm getting hung up on the vectors. The mass sliding down the ramp has a vector that is parallel to the ramp. This vector has a vertical and horizontal component, why doesn't the horizontal vector continue to accelerate the mass along the plateau?

By the way I answered:
2

I think my first reasoning with conservation of energy is correct but I just can't rule out the horizontal vector for some reason.

There is no horizontal vector of force once the block hits the plateau all of the force due to gravity is in the vertical vector at that point. The ramp is what breaks the force of gravity into a horizontal and vertical vector. A body in motion will stay at motion and a body at rest will stay at rest unless acted on by an outside force.

 

[SphinxnihpS]

When you shoot a rocket into the perfect vacuum of space (hey he has a magic ramp!), and the rocket fuel runs out and the engine quits, why does the rocket stop accelerating?

 

[SphinxnihpS]

Gah! I just can't figure out why all of the acceleration is done along the ramp...

Force of... gravity?

 

[Ghiddy]

I'm getting hung up on the vectors. The mass sliding down the ramp has a vector that is parallel to the ramp. This vector has a vertical and horizontal component, why doesn't the horizontal vector continue to accelerate the mass along the plateau?

By the way I answered:
2

I think my first reasoning with conservation of energy is correct but I just can't rule out the horizontal vector for some reason.

You have it backwards. There is a force vector of gravity, which points straight down, and then the normal force of the ramp, which points perpendicular to the ramp's surface.

Convert the gravity vector into vertical and horizontal components to get one vector parallel to the ramp, and one perpendicular to it. The latter is cancelled out by the normal force vector of the ramp's surface. You are left with a vector that points "along" the ramp, which is what accelerates the block down the ramp.

F=ma
You have m. You have a (-9.8m/s^2). F=ma is the magnitude of the original gravity vector before you broke it into "horizontal" and "vertical" components.

 

[James3shin]

Thanks everyone. I over-thought this problem and tried everything in my power to prove myself wrong.

 

[Ghiddy]

You have it backwards. There is a force vector of gravity, which points straight down, and then the normal force of the ramp, which points perpendicular to the ramp's surface.

Convert the gravity vector into vertical and horizontal components to get one vector parallel to the ramp, and one perpendicular to it. The latter is cancelled out by the normal force vector of the ramp's surface. You are left with a vector that points "along" the ramp, which is what accelerates the block down the ramp.

F=ma
You have m. You have a (-9.8m/s^2). F=ma is the magnitude of the original gravity vector before you broke it into "horizontal" and "vertical" components.

Above answers why the block accelerates while on the ramp.

To answer your original question, once the block is on the flat surface, you have two vectors, the gravity one, and the normal force. They both have equal magnitudes, and point in exactly opposite directions, so the net force is zero. If net force is zero, acceleration is zero.

Because the gravity and normal force point exact opposite to each other AKA act along the same axis, there is no need to break up gravity into vertical/horizontal components, so you don't end up with component vectors that re-guide the force from gravity into a direction other than straight down.

 

[DrPizza]

You keep saying "vector." There are a lot of vectors involved here. Force is a vector, acceleration is a vector, velocity is a vector. On the plateau, the only vector that's there is velocity. There's no force vector.

Along the incline, there are two forces acting on the object: the earth is pulling the object down, via gravity. And the slope itself is exerting a force against the object perpendicular to the surface of the slope. This is the normal force. Thus, there are two force vectors acting on the object. Since forces always occur in pairs (See Newton, equal and opposite) the force against the incline by the object is equal to the force against the object by the incline. You can draw a force vector straight down on the object representing the force due to Earth's gravitational pull on the object. Since vectors can be broken up into any number of component vectors, it's convenient to break this gravitational vector up into two components - one that's perpendicular to the surface, and one that's parallel to the surface. From there, it's just a little bit of geometry to find the component of the gravitational force which is parallel to the incline. Barring friction or drag from air, there is no force acting in opposition to this force. (The gravitational component perpendicular to the surface is balanced by the normal force that the surface exerts against your object.) Thus, the parallel component is the net force acting on the object. From there, it's a simple F_net = ma to find the acceleration.


Once your object is on the plateau, let's examine the forces acting on your object. Clearly, Earth still exists; at least until this Saturday when judgment day happens, and if your object has been a good object, some rapture voodoo happens and it'll suddenly start rising into the air, toward heaven. (This must suck for people in Australia to find out that the best they can do is go to hell, else fly to the U.S. prior to the Rapture.) Anyway, barring any magic forces acting on the object, we still have the force due to gravity (straight down), and the contact force that the surface is exerting against the object. And this time, perpendicular to the surface is straight up. There's no conceivable way you're going to add two vertical forces and result in a net horizontal force. The only possible direction for acceleration is either straight up or straight down. If the plateau were made of tissue paper, and the object made of a large chunk of lead, then the object is going to accelerate downward. Even though it would have a horizontal component of velocity, it will still accelerate in only one direction: vertically, downward.

 

[DrPizza]

(wow, I started that reply over an hour ago, momentarily distracted by caring for a llama that's in my dining room.)

 

[edro]

(wow, I started that reply over an hour ago, momentarily distracted by caring for a llama that's in my dining room.)

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