# Show that dA/dt is constant in time

• nns91
In summary, the particle has a constant velocity and its position vector sweeps out a right triangle that has a height of b. The area of the triangle is (1/2)*b*L/(2*m*v).
nns91

## Homework Statement

A particle is traveling with a constant velocity v along a line that is a distance b from the origin O. Let dA be the are swept out by the position vector from O to the particle in time dt. Show that dA/dt is constant in time and equal to (1/2)*L/m, where L is the angular momentum of the particle about the origin.

L= rmv

## The Attempt at a Solution

Any hint on how to do this problem ?

You'll need a nice diagram showing O, the distance b straight down from O to B. Off to the side a bit you need a bit of the path of the particle heading straight for B. Draw lines from O to each end of this bit of path, to form the triangle the question talks of. Label the area dA.

You must get an expression for the area of the triangle. You'll need the base and height, one of which is easy. For the other, think about how far the particle goes in time dt at its uniform speed. Just a bit of playing to get that dA/dt = L/(2m).

rho=1/3*pi*r^2
Trying to wrap my mind around this. Why one-third of the area of a circle?

Delphi51 said:
Why one-third of the area of a circle?
It's just nonsense--ignore it.

Delphi51 said:
Trying to wrap my mind around this. Why one-third of the area of a circle?

What you're working on seems to be Kepler's second law. It's used in the derivation of the Volume of a sphere, since this has to be determined to yield mass.

It's not the only way to approach, as there's no absolute. It's just the way I did it.

You might find it easier this way, though, since the derivations will come in handy when you start playing around with gravity and the constant yield vectors.

Doc Al said:
It's just nonsense--ignore it.

The BOMB!

Ok. Here is what I have been trying to do.

so L=mrv. Thus, r= L/(m*v)

The area of triangle is (1/2)*base*height. Height will be "b" so the base will just be r then ?

the I substitute as: (1/2)*b*(L/(m*v)) and get b*L/(2*m*v).

Am I going the right direction ?

Any further suggestion ??

nns91 said:
Ok. Here is what I have been trying to do.

so L=mrv. Thus, r= L/(m*v)
In general, L = r X mv = rmv sinθ. But r sinθ = b in this case.

The area of triangle is (1/2)*base*height. Height will be "b" so the base will just be r then ?
Imagine the particle sweeping out a right triangle that has height b. The position vector forms the hypotenuse, but what about the other side? Express that other side as a function of time. Then find the area of the triangle as a function of time.

(Draw yourself a diagram.)

## 1. What is dA/dt?

dA/dt is a mathematical notation used to represent the rate of change of a quantity A over time.

## 2. How do you show that dA/dt is constant in time?

To show that dA/dt is constant in time, you must first differentiate the quantity A with respect to time using calculus. If the resulting derivative is a constant value, then dA/dt is indeed constant in time.

## 3. Why is it important to show that dA/dt is constant in time?

Showing that dA/dt is constant in time is important because it indicates that the rate of change of the quantity A is not affected by time. This can provide valuable insights and information in various scientific fields, such as physics and economics.

## 4. Can dA/dt ever be negative or zero?

Yes, dA/dt can be negative or zero. A negative value for dA/dt indicates a decreasing rate of change, while a zero value indicates no change over time.

## 5. How is dA/dt related to other mathematical concepts?

dA/dt is closely related to the concepts of derivatives and rates of change. It is also used in various equations, such as the chain rule and the fundamental theorem of calculus, to solve problems involving rates of change.

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