# Dynamic Equilibrium -- Acceleration of a rock thrown from a bridge

• Dman0500
In summary, the acceleration of a falling rock is equal to the gravitational acceleration ##\vec{g}##. The only force acting on the rock is its weight, which is equal to ##m\vec{g}##. This follows from Newton's first law, which states that the net force is equal to the mass times the acceleration. When something is thrown upward or downward, the acceleration is still just ##\vec{g}##, but the initial velocity may have a different direction. It is helpful to draw a diagram and use ##\vec{F} = m\vec{a}## to understand these concepts.
Dman0500
Homework Statement
A rock is thrown from a bridge at an angle 30° below horizontal. Immediately after the rock is released, is the magnitude of its acceleration greater than, less than, or equal to g? Explain.
Relevant Equations
Conceptual
I know the acceleration of the rock is equal to g, but why. If we neglect air resistance, what is actually making the rock fall? Wouldn't it be that g overcomes the acceleration of the y plane at some point so the rock starts coming down or in this case accelerate more by throwing below 0 degrees?

What are forces on the rock?

I suppose just g and a(y-axis)

Dman0500 said:
I suppose just g and a(y-axis)

There is a weight force, of magnitude ##mg##, acting downward. That is the only force! "##a##" is not a force, it is an acceleration!

What is Newton's second law?

Alright I get it now!
alright so when something is thrown upward, there is still only g?

Dman0500 said:
Alright I get it now!
alright so when something is thrown upward, there is still only g?

Yes, something undergoing free-fall (only force is the gravitational ##m\vec{g}##) will accelerate at ##\vec{g}##. Make sure you are clear to distinguish forces from accelerations!

The result follows from Newton's first law; the net force is ##m\vec{g}##, and Newton tells us that ##m\vec{g} = m\vec{a}##. So ##\vec{a} = \vec{g}##.

If you throw something downward, it has an initial downward component of velocity. However the acceleration is still just ##\vec{g}##!

Lnewqban
Make way more sense thank you

berkeman and etotheipi
No problem. If you are ever in doubt about similar matters, draw a diagram! Draw the forces on the diagram, and then use ##\vec{F} = m\vec{a}##.

## 1. What is dynamic equilibrium?

Dynamic equilibrium refers to a state of balance in a system where the forces acting on an object are equal and opposite, resulting in a constant velocity or no acceleration.

## 2. How does acceleration affect a rock thrown from a bridge?

Acceleration is the rate of change of an object's velocity. When a rock is thrown from a bridge, it experiences a constant acceleration due to the force of gravity pulling it towards the ground.

## 3. What factors affect the acceleration of a rock thrown from a bridge?

The acceleration of a rock thrown from a bridge is affected by the initial velocity of the rock, the force of gravity, and air resistance. Other factors such as the shape and weight of the rock can also play a role.

## 4. Can the acceleration of a rock thrown from a bridge change?

Yes, the acceleration of a rock thrown from a bridge can change if there is a change in the forces acting on the rock. For example, if the air resistance increases, the acceleration will decrease.

## 5. How is dynamic equilibrium related to the acceleration of a rock thrown from a bridge?

In a state of dynamic equilibrium, the forces acting on an object are balanced, resulting in no net force and therefore no acceleration. However, when a rock is thrown from a bridge, it is not in a state of dynamic equilibrium as the force of gravity is constantly acting on it, causing it to accelerate towards the ground.

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