Projectile motion explanation I’m studying projectiles at the moment and I am sure this is a very simple question, but can someone explain if I have a light object (a tennis ball) and a heavier object (a similar size solid steel ball) and launch them at the same initial velocity and the same angle, will the range be the same? Any why? (Neglecting air resistance etc)
 A: It seems you haven't yet learnt about forces. Need not worry! 
In projectile kinematics the motion of a particle is based on certain parameters. And acceleration is one of them. In vertical projectiles the gravitational pull by the earth $mg$ acts on the body. 
'Favourite Man' Newton now comes into the scene and states in its second law that 
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object (considering mass is constant).
So we get
 $ \vec {a}=\frac {\vec {F}}{m} $
In your car we will have $F=mg$. This will leave us with the value of acceleration as $g$ irrespective of the mass(since masses will get cancelled out). So both lighter and heavier particle will have the same range. 
A: A way to answer is just to write down the equation of motion and let you see that mass is not involved. However, I think that your question may lie on a more deep level. It seems stupid but, perhaps, I can in turn ask you why not? Let me explain, and sorry for the length of the answer.
Let's analyze the motion of a projectile on the Earth. We have an initial velocity with some angle with respect to the ground. At each point, the object will feel the gravity and so, instead of keeping going straight along the direction of the velocity, it will fall down. The motion is easy described by splitting the velocities in two components, the horizontal and the vertical. As for the first, the motion is a constant-speed one, while the second one is uniformly accelerated. The combination of the two defines the trajectory which turns out to be a parabola. Notice that in all this description I never said that the mass cannot have some kind of role and, indeed, so far the light and the heavy object may, in principle, follow two different parabolas and/or fall down at different distances.
But they do the same path and the reason are two:


*

*The velocity is the amount of space done in a certain time unit. To define it you just need the position of an object in two points and divide it by the time spent to move, and nothing else. This means that all other properties of your object have no role in what velocity describes of the motion of the projectile in your example. Both the light and heavy object have the same velocity, so at this level there's no difference between them at this stage. Formally and logically, this just prove that the horizontal motion, which involves only the concept of velocity, will be the same.

*The gravity acceleration does not depend on the mass. That's a fact, we observed this during the centuries and this is implemented in our theories. In our example, this means also that both the heavy and light object behave exactly in the same way from the point of view of gravity. The acceleration term enters in the description of the vertical motion of or projectile. Then, we now found that also the vertical motion will be the same.
If you write down the equation of motion, you will find exactly what said above.
Now I want to reverse the question and try to answer. Why not? I think that this kind of doubt is due to an erroneous application of our every-day experience (and this is perfectly normal) in which we are forced to merge our observation with other phenomena related to the dynamics and not only to the kinematics of the problem. We are driven to say that an heavier object will fall before (or after?) because we think that it is more difficult to move with respect to a light one. But this is a different phenomenon and Physics is able to describe this: if we kick a ball, the velocity that it will get would depend on its mass, and this is a true fact. But it is, in the same time, a different story which has nothing to do with our problem above: yes, somebody gives a kick at the projectile, but we do not need details about that because we know the velocity just after... and that is all we need to describe the motion. And also yes, the same kick would have probably produced different trajectories for the two objects. But, as said before, this is an other story. I think that reversing the answer is a way to understand why we do not expect a certain phenomenon to be in some way. Personally, I do this very often in classical physics problems because in several cases the reverse question is posed better than the initial one, allowing to solve the doubt.
