Question

Throw a 15kg rock by applying a force of 300N up over .8 seconds. what is the rocks change in momentum?

Answer 1

The momentum change is about `120 (kg*m)/s`.

Explanation:

Throwing the rock that way is an application of an impulse. An impulse is defined by this formula:

`F_"net" * t = Deltap`

where `F_"net"` is the unbalanced force on the rock, t is the time that the force was applied, and `Deltap` is the change in momentum of the rock.

In this situation it is important to understand that `F_"net"` is not 300 N. The rock will be accelerated by the unbalanced force acting on it. A 15 kg rock has a weight of `15 kg*9.8 m/s^2 = 147 N`. Therefore `F_"net" = 300 N - 147 N = 153 N`

Using that in the impulse formula

`F_"net" * t = Deltap`

yields

`Deltap = 153 N * 0.8 s = 122.4 (kg*m)/s ~= 120 (kg*m)/s`

I hope this helps,
Steve