Final answer:
An object's trajectory is influenced by gravity and follows the principles of Newton's laws of motion. Objects in space continue in straight paths unless acted upon by another force, and external forces are indeed needed to set objects in motion in the absence of gravitational pull.
Step-by-step explanation:
The trajectory of an object, such as a bullet, rocket, or ball, is influenced by gravity. When an object is launched, gravity pulls it towards the Earth, affecting the path it follows. For example, if a bullet is fired with a given velocity (va), it falls to the ground at point a due to gravitational force. Increasing the muzzle velocity (Vb) will allow it to travel farther before hitting the ground at point b. This is intimately related to Newton's laws of motion and the concept of Newton's law of universal gravitation, which explains the paths of planets around the sun, among others.
According to Newton's first law, if a rocket is launched into space and escapes Earth's gravitational pull, it will indeed continue to move in a straight line until another force acts upon it. Additionally, in outer space, away from gravitational influences, an external force is required to set a stationary object in motion. As for the position vs time graph for an object speeding up, it would not be a straight line; rather, it represents a curve due to constant acceleration. This is also the case for displacement vs time for an object with constant acceleration, which shows as a curved line, while displacement vs the square of time shows as a straight line.
Normal reaction is not a force that acts in the direction of gravity but rather it acts in opposition to it. Lastly, according to Newton's third law of motion, every action has an equal and opposite reaction; thus, the Earth and an object like the Moon or a launching rocket exert forces on each other that are equal in magnitude but opposite in direction, irrespective of their respective masses.