Is the force Exerted by a ball thrown at a wall regardless of the angle always perpendicular to the wall? Why? The observation of the ball going of in a different direction says but why does this happen?
 A: Sometimes to visualise where forces apply and what in direction the forces apply it is useful to think about the momentum of the objects in question. Loosely speaking:


*

*Momentum is conserved, so if two objects interact and one loses momentum, the other must gain it. (This is very similar to the idea in Newton's 3rd law - every action has an equal and opposite reaction).

*When the momentum of an object changes, the rate of change of the momentum is the force applied to the object. (This is Newton's 2nd law - force equals mass times acceleration).
Throw a ball directly (i.e. perpendicular) at a wall, the ball's momentum will change direction back towards the thrower, when it hits the wall. Therefore, there will be a force applied, from the wall to the ball, in the direction back towards the thrower. The wall also has a change in momentum due to statement 1 and therefore the ball also applies a force to the wall due to statement 2.
When throwing the ball at an angle to the wall, imagine the momentum of the ball resolved into motion perpendicular and tangential to the point of impact on the wall. We've already looked at the perpendicular motion so now look at the motion of the ball tangential to the point of impact on the wall. If the momentum of the ball changes tangentially then you can identify a force in the direction of the changing momentum. This will only happen if the ball interacts with the wall because it's not perfectly smooth (i.e. friction) or the wall's not straight. (You can add the parallel and tangential forces by vector addition into a single force which won't necessarily be perpendicular to the wall if you like)
This approach also helps with another question commonly asked when people first encounter Newton's third law. 'What happens if instead of hitting the wall the ball hits and breaks a window and falls through the other side. Where is the equal and opposite reaction then? Tracing through the changes in momentum will again answer this question and if you can do that you will have got the concept.
