Newton's laws of motion
Isaac Newton was one of the most prominent, contentious and influential scientists of all time. He helped to invent calculus, explained gravity and identified the constituent colours of white light. His three laws of motion describe why B golf ball follows a curving path, why we are pressed against I lie side of a cornering car and why we feel the force through a baseball bat as it strikes the ball.
Although motorcycles had yet to be invented in Newton's time, his three laws of motion explain how a stunt rider can mount the vertical wall of death, and how Olympic cyclists race on inclined tracks. Newton, who lived in the 17th century, is considered one of the foremost intellects of science. It took his highly inquisitive character to understand some of the most seemingly simple yet profound aspects of our world, such as how a thrown ball curves through the air, why things fall down rather than up and how the planets move around the Sun. An average student at Cambridge in the 1660s, Newton began by reading the great works of mathematics. Through them he was drawn away from civic law into the laws of physics. Then, on sabbatical at home when the university was closed for an outbreak of plague, Newton took the first steps to developing his three laws of motion.
Borrowing Galileo's principle of inertia, Newton formulated his first law. It states that bodies do not move or change their speed unless a force acts. Bodies that are not moving will remain stationary unless a force is applied; bodies that are moving with some constant speed keep moving at that same speed unless acted upon by a force. A force (for instance a push) supplies an acceleration that changes the velocity of the object. Acceleration is a change in speed over some time.
This is hard to appreciate in our own experience. If we throw a hockey puck it skims along the ice but eventually slows due to friction with the ice. Friction causes a force that decelerates the puck. But Newton's first law may be seen in a special case where there is no friction. The nearest we might get to this is in space, but even here there are forces such as gravity at work. Nevertheless, this first law provides a basic touchstone from which to understand forces and motion.
Newton's second law of motion relates the size of the force to the acceleration it produces. The force needed to accelerate an object is proportional to the object's mass. Heavy objects - or rather ones with large inertia - need more force to accelerate them than lighter objects. So to accelerate a car from standing still to 100 kilometres an hour in one minute would take a force equal to the car's mass times its increase in speed per unit time.
Newton's second law is expressed algebraically as F = ma\ force (F) equals mass (m) times acceleration (a). Turning this definition around, the second law expressed in another way says that acceleration is equal to force per unit mass. For a constant acceleration, force per unit mass is also unchanged. So the same amount of force is needed to move a kilogram mass whether it is part of a small or large body. This explains Galileo's imaginary experiment that asks which would hit the ground first if dropped i (jgether: a cannonball or a feather? Visualizing it we may think that the cannonball would arrive ahead of the drifting feather. But this is simply due to the air resistance that wafts the feather. If there were no air, then both would fall at the same rate, hitting the ground together. They experience the same acceleration, gravity, so they fall side by side. Apollo 15 astronauts showed in 1971 that on the Moon, where there is no al mosphere to slow it down, the feather falls at the same rate as a geologist's heavy hammer.
Action equals reaction
Newton's third law states that any force applied to a body produces an equal and opposite reaction force in that body. In other words, for every action there is a reaction. The opposing fi iree is felt as recoil. If one roller-skater pushes another, then she will also loll backwards as she pushes against her partner's body. A marksman feels 11 uŽ kick of the rifle against his shoulder as he shoots. The recoil force is equal in size to that originally expressed in the shove or the bullet. In crime films the victim of a shooting often gets propelled backwards by the force of the bullet. This is misleading. If the force was really so great then 11 \(! shooter should also be hurled back by the recoil of his gun. Even if we jump up off the ground, we exert a small downward force on the Earth, but because the Earth is so much more massive than we are, it barely shows.