# Newton's 1st and 3rd laws

1. May 3, 2010

### mybrohshi5

1. The problem statement, all variables and given/known data

A painter is statically at rest on a ladder leaning against a wall. The painter argues that the force on the painter by the ladder must be equal and opposite to the force of gravity on the painter by the Earth. Evaluate the painters argument.

3. The attempt at a solution

the answer is: Newton's first law can be used to demonstrate the the painter is correct.

I agree with this but i also believe it should be Newtons third law too because that law states that for every action there is an equal and opposite reaction right?

Is Newtons third law not used to to demonstrate that the painter is correct because the question states "the force on the painter by the ladder must be equal and opposite to the force of gravity on the painter by the earth." If it would have stated "the force on the painter by the ladder must be equal and opposite to the force on the ladder by the painter." then would Newtons third law be used to demonstrate he is correct?

But the thing is i thought the force on the ladder by the painter is the force of gravity on the painter by the earth, and that is why i think Newtons third law should be used as well.

Thanks for any input on this :)

2. May 3, 2010

### D H

Staff Emeritus
No. Newton's third law is not adequate. Think of it in terms of skydiver who has just jumped out of a plane. The gravitational force exerted by the Earth on the person is equal and opposite to the gravitational force exerted by the person on the Earth. Yet the person accelerates downward. Why is that?

3. May 3, 2010

### mybrohshi5

I am not sure exactly why [ but i would like to know ;) ] but i think i get what you are trying to get at. The first law explains he is correct because the net force on him is zero which means his motion wont change (he is statically at rest).

The third law doesnt explain he is correct because (like the skydiver) even though there may be forces equal and opposite in direction, the body can still be accelerating (because one force is larger than the other?????)

4. May 3, 2010

### D H

Staff Emeritus
The third law, along with Newton's universal law of gravitation, very clearly says that the force exerted by the Earth on the person is equal and opposite to the force exerted by the person on the Earth.

Suppose the skydiver steps out of a hovering helicopter rather than an airplane. At the moment the skydiver jumps out the only force acting on the skydiver is gravity. The skydiver falls Earthward. That the skydiver exerts an equal but opposite force on the Earth does not affect the skydiver's acceleration with respect to an inertial frame. Instead, it makes the Earth accelerate (by a tiny, immeasurable amount) toward the skydiver.

As soon as the skydiver picks up some speed the skydiver will start meeting some air resistance. This drag force is directed upward and grows as the skydiver's downward velocity increases. Eventually the skydiver will reach what is called terminal velocity; the velocity no longer increases. Again using Newton's first law this can only mean one thing: that the upward drag force is exactly canceling the downward gravitational force.

However, that this drag force is equal but opposite to the gravitational force does not mean that the drag and gravitational force are third law pairs. For one thing, the drag force is not always equal but opposite to the gravitational force; drag is zero at the instant the skydiver jumps out of the helicopter. For another thing, third law pairs always acts on two different bodies. In the case of gravitation, the two bodies are the Earth and the skydiver. For drag, it is the atmosphere and the skydiver. That the air pushes the skydiver upwards means that the skydiver is pulling the air around him downward.

5. May 3, 2010

### collinsmark

What does Newton's second law tell you about the sum of all the forces involved?

Is the painter accelerating? If not, why not?

[Edit: this was posted as a hint, btw. ]

Last edited: May 3, 2010