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A horizontal rope is tied to a 50.0kg box on frictionless ice. What is the tension in the rope if,

Athe box is at rest
Bthe box moves at a steady 5.0m/s
Cthe box has v_x=5.0m/s and a_x=5.0m/s²

Verified Answer :

1) 500N2) 500N
3) 750N

AL2006

A horizontal rope is tied to a 50.0kg box on frictionless ice. 

Athe box is at rest; 
     zero acceleration ==> no net force on the box ==> no tension in the rope

Bthe box moves at a steady 5.0m/s;
     zero
acceleration ==> no net force on the box ==> no tension in the rope



C. the box has v_x=5.0m/s and a_x=5.0m/s²

     Finally some acceleration !     F = m A
     Net force on the box is (mass) x (acceleration)
                                         = (50 kg) x (5 m/s²)
                                         =        250 Newtons.

The only thing in the vicinity that can exert force on the box is the rope,
so the tension in the rope must be 250 N, and somebody or some thing
must be pulling on the other end in order to maintain the force and the
acceleration of the box.