[gigglez3576]

Two Objects of masses of 1 kg and 5 kg are connected together by a light string that passes over a frictionless pulley. What is the tension in the string? What is the acceleration of the block?

When I calculated this I found that the tension in the string was 0 N and that the acceleration was -9.8 m/s^2.

But I'm unsure if you can have 0 N of tension if the blocks are indeed pulling on the string. Please help?

[Unknown008]

You're right that there cannot be 0 N tension in a string. Maybe what you got as 0 N is the net force on the string, but let's see.

"Two objects of masses of 1 kg and 5 kg are connected together by a light string that passes over a frictionless pulley. "

Make a quick sketch, a pulley, to its right, downwards a mass of 5 kg, to its left downwards a mass of 1 kg.

The 5 kg mass exerts a force (W = mg = 5*9.8) on the string, and the 1 kg mass does the same (W = mg = 1*9.8)

Now, the string will try to resist this force, through tension.

Use F = ma on the right side.

F(net) = ma
W - T = ma

Where W is the weight of the object, m is the mass and a is the acceleration of the object. By logic, the 5 kg object will go down and pull the 1 kg object up. Which means that W is bigger than T and so, we get W - T. If it were the opposite, then you put in T - W

Can you fill that in?

49 - T = 5a

Now, you do the same on the 1 kg object. Remember what I just said? Here, the 1 kg object will be pulled upwards, meaning the tension in the string is greater that the weight of the object. Hence, you have T - W instead as the net force.

T - W = ma

a in both cases are the same since both objects are connected, they'll accelerate at the same rate. Fill in with the values of W and m for the 1 kg object.

Now that you've got 2 equations with to unknowns, you can get the tension in the string, T.

[ebaines]

If the tension in the string is 0 N, that means nothing is keeping the two objects from falling. It would be what you would expect if the string was cut. Try to visualize what happens here - do you think the behavior of the objects is the same as if the string was cut?

Let's start over again. First, consider the external forces acting on the system as a whole -that would be the force of gravity pulling down and the tension in the string pulling up. From : the difference in the force of gravity acting on the two masses () equals the total mass of the system ( 1 Kg + 5 Kg) times the acceleration of the system. Now you can calculate . Then consider the movement of just one of the masses, and again use . Here is the tension in the string pulling up minus the force of gravity pulling down, so . The mass is the mass of the object, and you calculated previously. Solve for .