Mass, acceleration and Newton's laws

Newton's first law: an object stays still or keeps moving at a steady velocity unless a resultant force acts on it. Newton's second law links force, mass and acceleration: force = mass × acceleration, or F = m × a. A bigger force gives a bigger acceleration, and a bigger mass needs a bigger force to accelerate the same amount.

Newton's third law: when two objects interact, they exert equal and opposite forces on each other. When you push on a wall, the wall pushes back on you just as hard. When a rocket pushes gas downwards, the gas pushes the rocket upwards. In a collision both objects feel the same size of force, but the one with less mass has the bigger change in motion.