Confusion about Newton's laws, sum of forces equals zero

[LogarithmLuke]

I've lately began working with Newtons laws problems at school again, and I've already ran into a few problems.

When making calculations and solving problems, it is often nessecary to understand when forces are equal to zero, and when they are not. Since every force has an equal and opposite counter force, according to Newtons third law, it seems as though the sum of forces are always equal to zero, in both y and x directions.

The reason objects are still able to move though, the way i understand it, is because if let's say i push a book across the table, my mass is far greater than the books and therefore my acceleration will be negligible. So it makes sense to me that objects can move, but always using that the sum of forces equal zero when solving problems causes a lot of issues.

How do i know when the sum of forces in a direction equal zero, and when they do not?

 

[A.T.]

https://www.lhup.edu/~dsimanek/physics/horsecart.htm

 

[russ_watters]

How do i know when the sum of forces in a direction equal zero, and when they do not?

You are mixing together two separate issues. The forces you are summing are not the equal and opposite forces at every point, they are only one of each pair: only the forces applied to the object (not the forces the object applies back).

 

[LogarithmLuke]

You are mixing together two separate issues. The forces you are summing are not the equal and opposite forces at every point, they are only one of each pair: only the forces applied to the object (not the forces the object applies back).

So let's say I am pushing a car, using a force. The car pushes back at me with a force that has the same magnitude, but the force is directed at me. Therefore the sum of forces on the car are the force i use to push, friction and gravity as well as the normal force. The net force on the car will be zero if it has no acceleration or travels at a constant pace, but it is not automatically zero. Is my thinking right?

 

[russ_watters]

So let's say I am pushing a car, using a force. The car pushes back at me with a force that has the same magnitude, but the force is directed at me. Therefore the sum of forces on the car are the force i use to push, friction and gravity as well as the normal force. The net force on the car will be zero if it has no acceleration or travels at a constant pace, but it is not automatically zero. Is my thinking right?

Yep!