1. Suppose that a boy is pulling(Force: P) a train of 4 toy cars(The boy and toy cars are linked with a string) at a constant velocity.
The mass of each car is m kg. If friction is negligible, what is the tension of the string?
Is it equal to force P? That's my guess.....

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## Answers and Replies

no man.....apply the free-body diagrams....and see for yourself...the tension varies in evry wire...

The tension force should be zero because you are neglecting friction, and the cars are moving at constant velocity.

Doc Al
Mentor
If the boy pulls with a force P and there is no friction, how is it moving at constant velocity? (Unless P = 0, of course. )

Gonzolo
ArmoSkater87 said:
The tension force should be zero because you are neglecting friction, and the cars are moving at constant velocity.
With constant velocity, we can safely change "should be" to "is". The boy only needs to pull to get it started from zero mouvement and then to keep it going if there is friction. That's why wheels were invented. And that's why I'm confident Summer will be back next year.

Doc Al said:
If the boy pulls with a force P and there is no friction, how is it moving at constant velocity? (Unless P = 0, of course. )
It was a riddle yeah! P=0

Wouldn't there always be tension in the string? I mean, it cannot just float in midair, can it? If we neglect gravity completely, as zero friction would seem to suggest, then there would be no tension in the string, but otherwise there would have to be some tension in the string, otherwise it wouldn't be real! I am thinking specifically of a hanging cable and how (if I remember correctly) it hangs in the shape of a parabola under the influence of gravity and no other forces. Hmmm, so how do you make the string have no tension? Maybe you would just place it on a horizontal, flat, frictionless surface too! :uhh: