# Homework Help: Accelerating Beaker containing liquid

1. Nov 10, 2013

### andyrk

When a Beaker is accelerating in the x-direction, the water level gets sloped. Why does this happen? Is this situation analogous to the sloping pendulum in an accelerating cart? Similarly why doesn't the water level change in shape when the beaker is accelerated in y-direction? That is, the surface remains horizontal. Whereas in the case of an accelerating beaker the surface slopes. I think it has something to do with inertia but I think it also involves laws of motion. Can anyone help me understand it? Like in the case of a pendulum hanging from the roof of an accelerating cart, the pendulum aligns at angle θ to the vertical. This occurs because of inertia firstly and secondly so as to fulfil the newton's law's of motion, i.e the motion is in that direction in which there is force. So as the pendulum is moving in forward direction ( because it is the part of the cart) so the net force on it should also be forward along the direction of acceleration. So that is the reason why the pendulum aligns at an angle θ to the vertical. Because when we resolve the tension along the string it resolves into 2 components, Tsinθ and Tcosθ. The Tcosθ component equals the weight of the body and therefore all forces cancel along the vertical direction. The Tsinθ force is along the forward horizontal direction and there is no force to balance it. So Newton's second law is obeyed, i.e the net force is in the direction of net acceleration (in this case forward). So how to explain the slope of the liquid when the beaker is accelerated in the x-direction? Is the explanation analogous to that of the pendulum in an accelerating cart? If yes then how? I am not able to understand that.

Last edited: Nov 10, 2013
2. Nov 10, 2013

### Staff: Mentor

Yes.

Where is the y-direction in your coordinate system?
Right.

You can do the same analysis for the surface: There is no net force perpendicular to the surface (otherwise water would move in that direction and change the surface shape), the vertical forces have to cancel, and the horizontal force has to lead to the acceleration.

3. Nov 13, 2013

### andyrk

Can you explain it a bit quantitatively rather than qualitatively?

Is anyone there? Can anybody explain that using newton's laws of motion? Just like the bob hanging from the cart?

Last edited by a moderator: Nov 13, 2013
4. Nov 13, 2013

### Staff: Mentor

The quantitative part is exactly the same as for the pendulum.
Please don't make multiple posts in a row.