Unit 2: Forces and Newton’s Laws

Isaac Newton, 1687: English scientist/mathematician who wrote about the 3 laws of motion in Philosophiae Naturalis Principia Mathematica, published in 1687.

Newton’s First Law (law of inertia) An object at rest stays at rest, or an object in motion continues to move at a constant velocity unless acted upon by a force.

Consequences of First Law Objects don’t move by themselves or stop moving by themselves (inertia =tendency to remain at rest or to not change velocity)

Consequences of First Law In a vacuum in zero gravity, an object will keep moving forever in one direction unless it collides with something else. Launched objects would not change velocity in the horizontal direction, except for the effects of the force of air resistance.

Spacewalkers must be tethered to the space station. Why then do objects rolling/sliding on a flat surface slow down?

Newton’s second law F = m·a The magnitude of the acceleration an object experiences (Δv/t) is equal to the force acting on it divided by the mass. The direction of acceleration an object experiences is in the same direction as the force. Everyday language: Forces accelerate things with mass F = m·a

Force is a vector! It has magnitude and direction. SI unit (derived): N (Newton). 1 N = 1 kg*m/s2 Pounds (lbs) are also units of force.

Consequences of 2nd Law F = ma F = ma F = ma A larger force will accelerate an object more than a smaller force An object that is more massive requires a larger force to accelerate it as much as a smaller object that is accelerated with a given force. A more massive object has more inertia. F = ma F = ma F = ma

Same mass, different force Same force, different mass

Which would be harder to push. A 150 lb motorcycle, or a 5-ton truck Which would be harder to push? A 150 lb motorcycle, or a 5-ton truck? Why?

Newton’s Third Law When one object exerts a force on a second object, the second object exerts an opposing force in the opposite direction with the same magnitude.

Consequences of Third Law If you push on a wall with a force of 20 N, the wall pushes back at you with the opposing force of the same magnitude.

An astronaut throws a wrench into space with all her might on the international space station propelling it quickly away. She accelerates slightly in the backwards direction. great acceleration little acceleration Same force

The force of gravity pushes a man down (200 lbs of force), and the opposing force (“normal force”) from the ground pushes back up at the man (200 lbs). Stress of gravitational force on joints

Other concepts Net Force: the resulting total force on an object when two or more forces act on it at the same time. Calculated by adding the force vectors. Free body diagrams: diagrams used to describe the various forces acting on an object. A box indicates the object. Arrows emanating from center/surface of box indicate Force vectors. Forces may be added and subtracted to find the net force on the object.

Balanced forces: add up to a net force of zero (they cancel out) Unbalanced forces: result in a non-zero net force. Results in acceleration