Why does the horse move forward if the cart pulls back equally?

[kasheee]

Summary:: Horse and Cart question. Help gratefully appreciated.

Referring to the power point screen twelve minutes in for the video link below, I think it is not the internal force of the cart exerting on the horse when pulling it back that is of relevance, but instead it is the relative size of the external force of the ground on the cart compared to the size of the ground force exerted on the horse is what needs to be considered to see if we have a net resultant force.

 

[berkeman]

Welcome to PhysicsForums. I have moved your thread to the Schoolwork forums for you.

Can you show us your FBDs for the horse and for the cart? I don't really want to watch the presentation if we can answer your question by looking at your FBDs. Thank you.

 

[haruspex]

Summary:: Horse and Cart question. Help gratefully appreciated.

Referring to the power point screen twelve minutes in for the video link below, I think it is not the internal force of the cart exerting on the horse when pulling it back that is of relevance, but instead it is the relative size of the external force of the ground on the cart compared to the size of the ground force exerted on the horse is what needs to be considered to see if we have a net resultant force.

At the point you are referring to, we are considering just the horse as the system. We must only consider forces acting directly on it.
The horizontal forces applied to the horse are the forward friction from the ground and the tension in the link connecting it to the cart.

 

[kasheee]

I see. So if I understand you correctly, as long as there is a resultant force on the horse as a system, and therefore ,as the cart is connected indirectly to the horse, the horse and cart system will automatically accelerate.

 

[haruspex]

I see. So if I understand you correctly, as long as there is a resultant force on the horse as a system, and therefore ,as the cart is connected indirectly to the horse, the horse and cart system will automatically accelerate.

As long as there is a net force on the horse the horse will accelerate. But we do not at first know if that is the case. If the cart had a brake applied the tension in the link would have been greater, maybe balancing the force on the horse.

Since the tension is initially unknown, it is more convenient to consider the horse and cart as one system. If we wish to find the tension, we must also consider them as separate systems.