Silly question -- Perpendicular forces have no effect on speed?

[Curious_Student]

If route perpendicullar forces supose to have no affect, why isn't this the case when somebody tries to cross a river? the motion is perpendicullar when water speeds aprox. at the same direction.. what am i missing?

 

[phinds]

If route perpendicullar forces supose to have no affect, why isn't this the case when somebody tries to cross a river? the motion is perpendicullar when water speeds aprox. at the same direction.. what am i missing?

Uh ... HUH ? Can you rewrite that in understandable English? Even better would be to include a diagram of what you are talking about.

 

[jbriggs444]

If route perpendicullar forces supose to have no affect, why isn't this the case when somebody tries to cross a river? the motion is perpendicullar when water speeds aprox. at the same direction.. what am i missing?

A force perpendicular to an objects existing velocity has no effect on the object's speed.

If a ferry is crossing a river purely crosswise, the downstream force of the river has no direct effect on the ferry's speed.

If a swimmer is crossing the river, stroking directly for the far shore, any downstream force from the river would not be at right angles to the swimmer's resulting velocity. The swimmer speeds up as a result. This leads to a velocity that is the vector sum of the swim speed plus the downstream flow speed.

 

[Curious_Student]

A force perpendicular to an objects existing velocity has no effect on the object's speed.

If a ferry is crossing a river purely crosswise, the downstream force of the river has no direct effect on the ferry's speed.

If a swimmer is crossing the river, stroking directly for the far shore, any downstream force from the river would not be at right angles to the swimmer's resulting velocity. The swimmer speeds up as a result. This leads to a velocity that is the vector sum of the swim speed plus the downstream flow speed.

Thanks.
So if a swimmer starts at certain velocity and crosses a river when his motion is perpendicular to the water's flow, his velocity will remain the same all along and he wouldn't need to spend additional energy to keep on constant velocity. Do we agree?

Here, is it a right assumption? (ATTACHED)

 

[Stephen Tashi]

his velocity will remain the same all along and he wouldn't need to spend additional energy to keep on constant velocity.

Velocity is a vector quantity. I think your question concerns the component of the swimmer's velocity that is in the direction pointing directly across the river. Theoretically, forces on the swimmer perpendicular to that component do not affect that component of velocity. However, such forces affect both the (total) velocity vector of the swimmer and his speed, which is the magnitude of the (total) velocity vector. If "speed" refers to only to the magnitude of the component of the swimmers velocity vector that points directly across the river then forces perpendicular to that component do not affect that speed.

 

[PeroK]

Thanks.
So if a swimmer starts at certain velocity and crosses a river when his motion is perpendicular to the water's flow, his velocity will remain the same all along and he wouldn't need to spend additional energy to keep on constant velocity. Do we agree?

Here, is it a right assumption? (ATTACHED)

Velocity has a direction; and, velocity is measured relative to something. The swimmer's velocity relative to the riverbank is the vector sum of the velocity of the river plus his velocity relative to the river.

In theory, the swimmer's ability to swim across the river is not affected by the perpendicular flow of the water. If it takes him 30 seconds to swim across still water, then it should take him the same 30 seconds to swin across a flowing river. In the second case, of course, he ends up downstream.

If the swimmer does not want to be carried downstream, but wants to swim directly across the river relative to the banks, then he does need to use more energy. He must aim to swim upstream at at angle so that his velocity relative to the bank is straight across. Then it will take him longer than 30 seconds. And, of course, if the river is flowing faster than he can swim, then he cannot prevent himself being carried downstream.

 

[Curious_Student]

And, of course, if the river is flowing faster than he can swim, then he cannot prevent himself being carried downstream.

But still as said, it has no affect on his velocity, as long as he swim towards the other bank at the same speed he should get there at the same time not depended on how speed is the water flow, it will only affect the horizontal dislocation.

won't you agree?

 

[PeroK]

But still as said, it has no affect on his velocity, as long as he swim towards the other bank at the same speed he should get there at the same time not depended on how speed is the water flow and only affect the horizontal dislocation.

won't you agree?

The river flow has no effect on the time it takes to cross ther river. But, the swimmer's velocity is different because he is moving downstream as well as across the river. The component of his velocity in the direction across the river is unaffected.

 

[Curious_Student]

Thanks man.

 

[Mister T]

If route perpendicullar forces supose to have no affect, why isn't this the case when somebody tries to cross a river?

Because the force exerted on him by the river is not perpendicular to his velocity except at the instant of time when he first starts. When a force is constant in magnitude but always perpendicular to an object's velocity, you end up with uniform circular motion, and the object's speed is constant. But note that the force is always changing direction.

 

[rcgldr]

If the net force on a swimmer is perpendicular to the swimmers velocity, the swimmer will be traveling in a circle with respect to the water. With respect to a bank on the river, the path will be some type of trochoid.

 

[sophiecentaur]

Without a proper diagram, I wonder how many contributors to this are actually talking of precisely the same situation. From the start there has been confusion between velocity and speed with occasional bursts of 'the right answer'.
Why did no one satisfy @phinds ' request for a diagram in post #2?