# Homework Help: Newtons 3rd law+Horse Cart System+Action/Reactonhelp needed

1. Feb 9, 2006

### x0xgreenxeyezx0x

Newtons 3rd law+Horse Cart System+Action/Reacton..help needed ASAP!!!!!!!

im a sophmore in the ib program and i'm in conceptual physics

i have a test tommorow on newtons third law and all the stuff that goes along with it.. so i need an answer on this pronto!

first of all.. if you are standing still.. is the considered an action/reaction force? i thought it was.. because the earth exerts a force up on you and you exert a force down on the earth..? but for some reason, my friend said it wasn't.. and i dont know..?

then i have a question on the horse-cart system.. idk if you know of this but its an example found in alot of textbooks.. and it shows a horse attatched to a cart and the horse says 'I cannot accelerate this cart because the forces are equal and opposite of me and the cart and will cancel to zero'.. well i know thats not true.. but why.. and i also need to know..
a)what makes the cart accelerate
b)what makes the horse accelerate
c)what makes the horse-cart system accelerate

2. Feb 9, 2006

### AzMaphysics

well the standing still question is an action reaction pair and the other big question i think i know but, im not very good at describing stuff so sorry

3. Feb 9, 2006

### arildno

Strictly speaking, there are two action/reaction couples in the standing still scenario:
1. Gravity: Earth pulls you down, you pull the Earth up. Lots of motion here if it weren't for:
2. Normal force couple: The ground resists your hurtling down to the Earth's C.M, pushing you up, while you push the ground down.

Net result: both you and Earth at rest, with some stress contained in both of you.

Horse+cart:
a) Cart accelerates due to the horse's action upon it.
b) Horse accelerates due to the ground's action upon it.
c) Horse+Cart accelerates due to the ground's action upon them.

Last edited: Feb 9, 2006