Why is the net displacement orthogonal to gravity in projectile motion?

[danilo_rj]

A body of mass m is thrown at an angle alpha to the horizontal with the initial velocity v0. Find the mean power developed by gravity over the whole time of motion of the body, and the instantaneous power of gravity as a function of time.

I didn't understand why delta r is orthogonal to mg. Where delta r is the net displacement of the body during time of flight.

 

[Doc Al]

Where does the body begin and end its motion? What's the direction of that displacement?

 

[danilo_rj]

what is said in the problem is only this, there is no picture.

 

[Doc Al]

You are given all the information needed to answer my question.

 

[danilo_rj]

Ok. So I have to suppose that the beginning and the ending of the motion is at a same plane.

 

[Doc Al]

Exactly.