What is the Net Force on a Rock Thrown Vertically Into the Air?

[holly]

The Question: If a rock is thrown vertically into the air, at the very top of its trajectory the net force on it is:
a) its weight b)less than its weight c)more than its weight.

I think it is c, More than its weight, because it has its weight plus the 9.81 bearing down on it. Study group buddy says no, it is less than its weight, because it has power from being thrown. Study buddy #2 says no, it is simply its weight.

So I am stuck...three tiny brains, three answers...what is real answer? Thanks for any hints.

P.S. our tests are drawn from our homework, we do not get credit for turning homework in, so I am not fishing for answers in order to get a better grade, but to learn so I can do the tests. Thanks

 

[ShawnD]

Draw a FBD.

When it's at the top, what forces are acting on it? Only gravity is.
The net force is the weight.

 

[M98Ranger]

The correct answer is b) less than its weight. When the rock is at the very top of its trajectory, it is momentarily at rest and experiencing zero acceleration. Therefore, the net force on the rock must be equal to zero. This means that the upward force from the throw (which is less than the rock's weight) must be equal to the downward force of the rock's weight. This results in a net force that is less than the rock's weight.