Question

If the pulley is being held by a force sensor, assuming pulley and string are massless, the force would measure the combination of tension in both sides of the rope. Assume that you are lightly holding the string on top of the pulley to prevent movement. (Your finger doesn't add weight, it just holds the string still). How does the force measured change from this moment (string held still) to when the system is able to freely accelerate? Explain why.

Answer 1

Tipically, we allways have a hanging mass on one end of the rope. When the string is held still the tension in it must balance the weight of the hanging mass. If the system is then released, and the counterweight is not enough, the hanging mass will experience an acceleration (and with it the counter weight and the rope also). In this case the tension on the rope must be less than the weight of the hanging mass, so that the net force can provide the acceleration, this is because of Newton's 2nd Law:

Before releasing the system:

After releasing the system: