Second Law: The force acting on a body is directly proportional to, and in the same direction as, its acceleration ] Newton’s first law of motion is, essentially, the definition of inertia — the tendency of an object to resist a change in motion. The more mass an object has, the greater its
inertia. An object with greater inertia is more difficult to start or stop. Newton’s second law tells us that the force (F) on an object is directly proportional to it’s acceleration, and that is dependent on its mass. So, the second law can be summed up as F=ma. Or, acceleration depends on the objects mass and on the net force acting on the object: Force Acceleration = ------------ Mass Orbital Motion: Why do Earth and the moon remain in their orbits? Newton concluded that two factors: inertia and gravity combine to keep the Earth in orbit around the sun and the moon in orbit around the earth. What exactly, is the force of gravity? 9.8 m/s 2 . Where have we seen that unit of measurement before, and what does it describe? [ it’s the acceleration ] The Earth’s gravity keeps pulling the moon toward it, preventing th e moon from
moving in a straight line. At the same time, the moon keeps moving forward because of inertia. If not for Earth’s gravity, the moon would shoot off into space in a straight line, and if not for the moon’s inertia, the moon would collide with Earth due to the pull of Earth’s gravity Image created by the author. License: Public Domain Actual Orbit Moon Earth Gravitational Force of Earth Moon’s motion without Gravity (due to inertia)
Newton’s Laws of Motion (cont’d) Newton proposed that whenever one object exerts a force on a second object, the second object exerts a force back on the first. The force exerted by the second object is equal in strength and opposite i n force to the first force. If we think of one force as the action and the other force as the reaction, then Newton’s third law states that for every action, there is an equal and opposite reaction. Can anyone think of an example of an action - reaction pa ir? [ jumping, rowing...] In those examples, there was always a motion as a result of the forces acting against each other, right? But, can we always detect a motion when paired forces are in action? The answer is no. For example, if I drop my pen, we see gravity pull the pen towards the ground. At the same time, we know from Newton’s third law, that the pen must be pulling Earth upward with an equal and opposite reaction force. But we don’t feel a giant jolt of Earth moving in that
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- Isaac Newton