The sand bag placed on the rough ground is pulled by the spring scale horizontally and the spring scale is pulled by the force F. while the sand bag has the constant velocity, the value read from the spring scale is 2 N. Which is correct?

1. The sand bag is pulled with the resultant force of 2 N
2. The kinetic friction is 2 N
3. The kinetic friction is less than 2 N
4. The difference between force F and the kinetic friction is 2 N

What do you think is the correct answer (or answers), and why? Give it a try and we'll help you through it.

I think #2 is.
As the sand bag has a constant velocity, the tension in the spring equals to the kinetic friction which is 2N, but I'm not sure whether the force F equals to the tension or not.

You are correct about #2 - the 2N force as measured by the spring scale is what the bag "feels" itself being pulled by, and it must exactly equal the drag due to friction or else the bag's acceleration would not be zero. If you consider the forces acting on the spring it's being pulled by 2N and it's being held back by the sand bag by that same amount.

But how about answer #1? I'm not sure what they mean by "pulled by a resultant force" - if they mean the pulling force on the sand bag is 2N then #1 is true, but if they mean that the sum of forces on the bag is 2N then it's false.

Hmm.. I think it's the sum of forces since #1 is correct lol

Thank you very much!