Page 10 - GALIET PHYSICS BLOSSOMS II+

(ii) Explain Newton’s 2nd and 3rd Laws of motion. Then, using the possibility of experiments involving interacting masses, explain how the concepts of mass and force are then defined. To do this you should consider how they would be measured in these experiments.
Newton’s Second Law
Newton’s Second Law is about acceleration, an object’s change of velocity whether it is speeding up, slowing down, or changing direction. Newton’s Second Law posits that the acceleration of an object is directly proportional to the force applied, and inversely proportional to its mass:
When a force F acts on a body of mass m, it produces in it an acceleration a equal to the force divided by the mass. Thus, a = F/m, or F = ma.
Consequently, the greater the force that acts on the object, or the smaller the mass of the object, the greater is its acceleration. If we pull two objects with the same force, the larger one will accelerate less; if we pull two identical objects with different forces, the one moved by the greater force will accelerate more.
Acceleration is the rate of velocity’s change. Its units are velocity units per unit of time, such as meters per second per second (m/s2). We name the SI unit of force after Newton. 1 newton (N) is the force needed to cause a mass of 1 kilogram to accelerate at a rate of 1 meter per second every second (1 m/s2). One Newton is more or less 0.22 pound. The force of gravity, at the surface of the earth, produces a downward acceleration of approximately 9.8 m/s2 on all bodies, regardless of mass. According to Newton’s Second Law, this means that my weight (in newtons) is directly proportional to my mass (in kilograms).2
Newton’s Third Law Newton’s Third Law posits that forces do not occur in isolation:
For every force acting on a body, there is an equal and opposite reaction, acting somewhere.
If body A exerts a force on body B, then body B necessarily exerts a force on body A that is equal in magnitude, but of opposing direction. For example, when a baseball player hits a home run, Newton’s Third Law posits that the bat and the ball exert equal and opposite forces on one another during the instant they are in contact. According to Newton’s Second Law, the ball moves away much faster than the bat because the ball’s mass is lesser than the combined mass of the bat plus the batter (whose body absorbs much of the reaction force), so the ball’s acceleration is much greater.3
2 Chaisson, McMillan. Astronomy Today. The Solar System. 7th Edition. Vol. I. 48-49. 3 Chaisson, McMillan. Astronomy Today. The Solar System. 7th Edition. Vol. I. 48-49.
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