# Newton's Law Homework Question

1. Aug 2, 2009

### anti_physical

Newton's Law Homework Question!!

Question: Peter and John are playing a game of tug-of-war on a frictionless, icy surface. Peter weighs 539 N and John weighs 392 N. During the course of the game, John accelerates toward Peter at a rate of 3.0m/s^2.

a) What is the magnitude of the force that Peter exerts on John?
b) What is the magnitude of the force that John exerts on Peter?
c) What is the magnitude of Peter's acceleration toward John?
d) Sarah decides to john the game as well. Now Peter pulls on Sarah with a force of 45.0 N [E], and John pulls on her with a force of 25.0 N [N]. What is Sarah's resultant acceleration if she weighs 294 N?

--I just don't seem to understand what they're actually asking me to look for in parts a-c. What exactly does magnitude mean? Would I simply see the difference of their forces (by subtraction?)
--Also, because its tug-of-war, that would mean that we could assume one is pulling towards the W and the other the E, correct? Why would Peter accelerate TOWARD John then??

F=ma

2. Aug 2, 2009

### jgens

Re: Newton's Law Homework Question!!

Think in terms of Newton's 3rd Law: If A exerts a force on B, then B exerts a force on A of equal magnitude and opposite direction.

3. Aug 2, 2009

### cepheid

Staff Emeritus
Re: Newton's Law Homework Question!!

Hi anti_physical, welcome to PF.

I'm sure you're familiar with the everyday sense of the word magnitude, meaning the degree or extent of something, its amount, its greatness (or smallness). The magnitude of something could be a measure of the strength of its effect or perhaps of its proportions.

In physics, many quantities are vector quantities. This means that they have to be described using two numbers: a magnitude and a direction. Forces are an example of a vector quantity. They have both a magnitude (which describes how strong or weak the pulling is) and a direction (which describes in what direction the pulling is occurring). A force of three newtons would be greater (in magnitude) than a force of two newtons.

So, based on what I've said, I think you can see that an equivalent way of stating the question in part a would be, "with how much force (in newtons) is Peter pulling on John?"

You are correct that because it is a tug of war, Peter and John are pulling in opposite directions. The reason why Peter is accelerating towards John is because John is *winning.* ;)

4. Aug 2, 2009

### anti_physical

Re: Newton's Law Homework Question!!

That being said, would I simply use subtraction to show that Peter has 147 N of magnitude of force on John??

I'm not sure how I'd complete the magnitude of force of John on Peter though. Negative value of force?

Sorry everyone! Physics proves to be quite difficult for me! Thanks so much though!

5. Aug 2, 2009

### Staff: Mentor

Re: Newton's Law Homework Question!!

(1) No subtraction needed.
(2) Why would you subtract their weights? You need the force with which they pull on the rope, which hasn't much to do with their weights. (They pull horizontally; weight acts vertically.)

Hint: Use Newton's 2nd law to find the force.

Don't forget Newton's 1st law.

6. Aug 2, 2009

### Lok

Re: Newton's Law Homework Question!!

A) with what force is john pulled by peter
B) with what force is peter pulled by john
C) What is peters acceleration towards john

Magnitude would imply that it's an average as a tug of war does not produce a constant acceleration as the two pull in different sequences. Just answer them as above...

The second question ... no sexual attraction definitely. But one of them is the standpoint so their acceleration is not measured relative to the ground but relative to one another. Because they both exert a force they both feel an acceleration.

7. Aug 2, 2009

### anti_physical

Re: Newton's Law Homework Question!!

I think I figured it out!

I changed their weight in N into kg (mass) and used their masses to find out their individual forces. Peter's force is 165 N and John's is 120 N.

For part c) the only thing I can think of is that Peter doesn't have an acceleration because he isn't moving. He will have a negative acceleration if anything, but I'm not quite sure how that would be calculated.

8. Aug 2, 2009

### Lok

Re: Newton's Law Homework Question!!

Both are moving because it is a frictionless icy surface. :P

Still in disagreement to things said above Peter is still winning :P

9. Aug 3, 2009

### Staff: Mentor

Re: Newton's Law Homework Question!!

You are given John's acceleration. Don't assume both have the same acceleration. (But Newton's 1st law will tell you something about the forces.)

10. Aug 3, 2009

### Lok

Re: Newton's Law Homework Question!!

Imagine that you are pulling a car on ice. Both of you are moving but you are moving faster as you are the lighter one.