Conservation of Linear Momentum. In the absence of a net external force, the linear momentum of a body, or of a system of bodies, is always conserved. We call the acceleration with which a body falls to Earth, the acceleration of gravity and use the symbol g to denote it. It is fairly constant over the surface of the Earth, but decreases with altitude as we shall see. The direction of g is towards the center of Earth.
Projectile motion is always a parabola, because there is only a vertical force.
The vertical speed changes due to gravity, but the horizontal speed is constant. There exists an attractive interaction between any two material bodies in the universe. Heat and Energy Temperature is different than heat and thermal energy. Temperature is what thermometers measure. It is an indication of the internal kinetic energy of the object.