Originally Posted by
Unknown008
Ok, let me restate my points:
1. Cart and horse are at rest, horse not pulling at all.
2. Horse starts to pull, unbalanced forces are generated and acceleration results.
3. Friction builds up as the speed of the cart increases.
4. When frictional force balances the pulling force of the horse, the acceleration becomes 0 and the velocity remains constant.
That's what I was saying from the beginning. I was not understanding why the Earth had to be considered in the problem, why we had to consider that the Earth pushed on the horse so that it moved if the forces were equal and opposite.
If you consider the horse and cart, without the earth, and that the only object the horse can pull or push on is the cart, the forces would be equal and opposite. If the horse and cart are attached, they are considered as one entity and ultimately would go nowhere if the horse exerted any forces on the cart. If the horse and cart were not connected, and considered as 2 entities, then any push or pull caused by the horse would cause the kart to either move towards or away the horse, but never in the same direction.
If you look at Newton's first law, we need a frame of reference, a frame of inertia. For a majority of the things we measure here, we measure it with respect to the ground I.E. Earth.
The horse doesn't just push on the cart to accelerate, it pushes AGAINST the earth to do so. In turn the earth pushes back against the horse with equal force.
and
The it must be true that
is small, but
is HUGE in comparison, so the
must increase to compensate for the difference, and
decreases to keep the relationship true for
