Unit Six: Unbalanced Forces part 2

To depict the forces acting upon an object you would draw out a free body diagram. A free body diagram is a diagram showing all the forces exerted on an object with all its surroundings removed. The diagram consists mostly of a sketch of the object stated and arrows representing the forces exerted upon it. For example:

Here is a yoyo hanging from a string. It is at rest.

Here is the free body diagram of the forces:

The bottom picture is how you would depict forces that are balanced; here is an example of unbalanced:

·            A basketball is moving to the left with friction.

·            The normal force and the weight are balanced but there is no opposite force to balance out the friction.

·            So this is unbalanced.

We also learned how to calculate the acceleration of two objects on a pulley. Here is an example:

The blue box has the same tension and mass as the green box, the normal force and weight of the blue box are balanced but there is negative tension on the blue box as it moves towards the pulley. The green box's positive tension pulling up and the weight are balanced. The mass of the boxes are 12 kg. Now this is how you find the net force and the acceleration of the situation.

 

Let's start with acceleration, you know the mass and the Earth's gravitational pull (10 m/s² ). So now using the equation mBg + t2 - t1 = (mB + mA)a. mBg is the green box's weight, mB is the green box's mass, mA is the blue box's mass, t2 is the green box's tension, t1 is the blue box's tension, and "a" is the system's acceleration. So plugging in the variables, you'll get:

 

12kg(10 m/s²) + t2 - t1 = (12kg +12kg)a

 

Since the tensions are equal they cancel each other out and then you get:

 

120N = (24kg)a

Now to find the acceleration, you divide the mBg over the sum of mB + mA:

120N/24kg = a

a = 5 m/s²

And now, to find the system's net force you'll use the mass (12kg, because both objects are of same mass) and the acceleration you found above (5 m/s²) as your variables with the equation:

Fnet = ma

Fnet = (12 kg)(5 m/s²)

Fnet = 60 N

So that is how you find acceleration and the force net of a system.