A boy with a mass of 30 Kg pulls a cart with a mass of 100kg towards himself by a rope. With that force does he have to pull on the rope to accelerate the cart at 2.0m/s/s? With what force must his feet push on the ground to keep him from moving towards the cart? If there is no friction between his feet and the ground. What is his acceleration?

Question

theeric · Answer

This looks challenging. Have you ttempted a solution yet?

anonymous · Answer

yea i found the first part of the question
its 200N [towards himself]. I just need help with the second part

theeric · Answer

Okay!
" With what force must his feet push on the ground to keep him from moving towards the cart?"

Let's start. What is the force on this boy from the rope?

anonymous · Answer

He pulls the rope with 200N of force

theeric · Answer

And so it pulls him 200N also! 200N [behind him].
So to stay still (no acceleration (or velocity)), the net force must be zero.

So if there is a 200N force pulling him back, and the force on his feet is the only other force... What must that force on his feet be for a zero net force?

anonymous · Answer

200N?

anonymous · Answer

is Newtons 3rd law being applied here?

theeric · Answer

Yep and yep!

anonymous · Answer

oh okay thanks

theeric · Answer

The third part of that problem is a little trickier, I think.

theeric · Answer

"If there is no friction between his feet and the ground. What is his acceleration?"

theeric · Answer

But not too tricky, if I'm correct :)

anonymous · Answer

Fnet y=200N 
so i can just use fnet=ma equestion for that

anonymous · Answer

equation*

theeric · Answer

Yep! With his 30kg mass.

theeric · Answer

Congrats! :)

anonymous · Answer

thanks for ur help

theeric · Answer

My pleasure! You did most of it! I pretty much said something and you realized, "hey, third law." And now you got it.

The rope was an internal force. You pull 200N on it, and it does the same to you in the other direction.