Newton’s First Law of Motion Inertia

Aristotle on Motion Divided motion into 2 types: Natural Motion- Motion straight up or down. Objects would seek their natural resting places. For example: A boulder falling toward the ground or smoke rising into the clouds. Unnatural (Violent) Motion- Motion resulting from forces that pushed or pulled. Motion due to some external cause. For example: Cart pulled by a horse or ship pushed by the wind.

Aristotle on Motion Continued… Aristotle also believed that circular motion was natural for the heavens. He believed that the planets and stars moved in perfect circles around the earth. Since the motions were natural, they were not caused by forces.

Copernicus and the Moving Earth Nicolaus Copernicus (1473-1543) believed that the earth and the other planets moved around the sun. Copernicus worked on his ideas in secret in fear of persecution. He published his ideas, De Revolutionibus, before his death on May 24, 1543.

Galileo Galileo was a Italian scientist during the late-renaissance.

Galileo on Motion Galileo demolished the notion that a force was necessary to keep an object moving. Force-Any push or pull Friction-Force that acts between materials that touch as they move past each other. If friction were absent, a moving object would need no force whatever to remain in motion. Galileo argued that only when friction is present, is a force needed to keep an object moving.

Galileo Tests His Idea He tested his idea by rolling balls along plane surfaces tilted at different angles. Ball rolling down an inclined plane picks up speed and moves in the direction of gravity. Ball rolling up an inclined plane, opposite to gravity, loses speed. Ball rolling on a level surface does not roll with or against gravity and has almost constant velocity. Without friction, the ball would move forever with no push or pull required to keep it moving.

Galileo Supports His Idea With Another Line of Reasoning

Galileo Continued…

Galileo Comes to a Conclusion Galileo stated that the tendency of a moving body to keep moving is natural and that every material object resists change to its state of motion. The property of a body to resist change is called inertia.

Netwon’s First Law- Inertia Newton develops his famous laws at the age of 24. 1st Law – Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.

Inertia

For Example Dishes on a tablecloth are in a state of rest. They tend to remain at rest, even if you snap the tablecloth out from beneath them.

Inertia

For Example If you slide a hockey puck along the surface of a city street, the puck will soon come to rest. If you slide it on the ice, it slides a longer distance. (Due to less friction). In the absences of forces, a moving object tends to move in a straight line indefinitely!

More Examples of Inertia Blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. To dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions. While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

Mass-A Measure of Inertia Kick an empty can and it moves. Kick a can filled steel nails and it will not move as much and you will hurt your foot. The nail filled can has more inertia that the empty can. The amount inertia an object has depends on its mass. Mass- the amount of material present in an object. If an object has a large mass, it may or may not have a large volume. The more mass an object has, the greater its inertia and the more force it takes to change its state of motion. Mass is the measure of the inertia of an object.

More Mass = Greater Inertia

Quiz- Which Has More Inertia?

Mass is not Weight!!! Mass = the quantity of matter in an object. More specifically, mass is a measure of inertia, or “laziness”, that an object exhibits in response to any effort made to start it, stop it, or change its state of motion. The mass of an object is the same whether its on the moon, on earth or in space. Weight = the force of gravity on an object. An objects weight would be very different on the moon, on earth or in outer space. On the moon, the object’s weight would be one-sixth than that on earth. In space, the objects weight would be zero.

One Kilogram Weighs 9.8 Newtons Weight = mass × acceleration due to gravity. (W = mg) In the US the unit for weight is pounds. 1-kg bag of nails has a weight of 2.2 pounds or 9.8 Newtons. A Newton is the SI unit for force named after Sir Isaac Newton.

Net Force Net Force = all the forces acting on an object. When you pull with a force of 10 N, the net force acting on the object is 10 N. If a friend assists you and pulls in the same direction with a force of 5 N, then the net force is the sum of the forces or 15 N. If your friend pulls with a force of 5 N in the opposite direction, then the net force is the difference of the forces, or 5 N.

Equilibrium-When Net Force Equals Zero What forces act on your book while it is motionless on a table? Normal force- Force perpendicular to surface. Often, not always, equals weight of the object.

Equilibrium Continued… When you hang from a rope, the tension of the rope equals your weight. The rope pulls you up while gravity pulls you down. Since the forces cancel each other out, you are motionless.

Vector Addition of Forces The tension is greater in a pair of non- vertical spring scales and depends on their angle from the vertical.