Dynamics …why an object moves the way it does Kinematics: how an object moves… what is its velocity, displacement, etc. Dynamics …why an object moves the way it does

If something is moving, then something MUST be pushing on it If something is moving, then something MUST be pushing on it. Things don’t move all by themselves! Aristotle

Oh, baloney! Once an object is moving in a straight line, it’s going to keep right on moving, unless it runs into something. That is, of course, as long as there is NO friction! Friction is what slows things down. Galileo

The Law of Falling Bodies Galileo… Born in Pisa 1564 Remember... Galileo found that, ignoring the effect of the air, all freely falling objects had the SAME acceleration, regardless of their mass or the height from which they were dropped. The Law of Falling Bodies

Galileo described HOW things moved, but not WHY they moved that way. Galileo wrote, “the present does not seem to be the proper time to investigate the cause of the acceleration of natural motion….” Galileo died in 1642. When Isaac Newton, born Christmas day, 1642, began his studies of motion in the second half of the seventeenth century, that statement was no longer appropriate.

Newton’s Three Laws of Motion Because Galileo had been so effective in describing motion, Newton could turn his attention to dynamics. Dynamics is the study of why an object moves the way it does- why it starts to move instead of remaining at rest, why it speeds up or moves on a curved path, and why it comes to a stop. Newton’s most famous book was “Principia”, published in 1687. This book first listed what came to be known as Newton’s Three Laws of Motion

Forces are vectors. ( a push or pull is always in some direction!) The unit for Force is the Newton, N If someone was pushing on you with one Newton of Force, it wouldn’t hurt much. One Newton of Force is about the same as the weight of a quarter pound hamburger patty.

There are four fundamental forces, often called “interactions”, between the elementary particles (protons, electrons, etc.): The strong force The weak force Gravitation Electromagnetism All of these interactions are non-contact. In this course, we will really only deal with the gravitational force.

The Strong Nuclear Force (but it acts over an infinite distance range) The strongest of these forces is…. The Strong Nuclear Force (it holds the nucleus of all atoms together, but only acts over a very limited distance range) The weakest of these forces is…. Gravity (but it acts over an infinite distance range)

Inertia Inertia: an object’s resistance to a change in its motion Mass: a way to measure inertia unit: kg

Newton’s First Law of Motion: “The Law of Inertia” An object at rest remains at rest unless a net external force acts on it. An object in motion continues that motion unless a net external force acts on it.

Newton’s First Law of Motion: “The Law of Inertia” An object at rest remains at rest unless a net external force acts on it. An object in motion continues that motion unless a net external force acts on it. Newton’s law of inertia confirmed what Galileo concluded: Once an object is moving, it requires no additional force to keep it moving. It will continue to move in a straight line unless a NET force acts upon it. I knew it first!!

Newton’s First Law An object in motion continues that motion unless a net external force acts on it. If an object is moving, but not accelerating, the NET force acting on it is ZERO. (constant velocity no acceleration zero net force)

Second Law If there is a net external force acting on an object, it will accelerate... Newton’s Second Law is expressed as an equation: (I call this “the granddaddy of all physics equations”!)

Same Force on different mass….

Action / Reaction forces Third Law For every force, there is an equal but opposite force. Action / Reaction forces

Action / Reaction forces For every force, there is an equal but opposite force. Action / Reaction forces

Action / Reaction forces For every force, there is an equal but opposite force. Action / Reaction forces

Action Force: Balloon pushes air downward Reaction Force: Air pushes balloon upward

The gun pushes on the bullet. The bullet pushes back on the gun!

I kick the wall (Action) The wall kicks me back (Reaction) The Action / Reaction forces do NOT act on the same object. I kick the wall (Action) The wall kicks me back (Reaction)

For every force, there is an EQUAL by opposite force! While driving down the road, an unfortunate bug strikes the windshield of a bus. The bug hit the bus and the bus hit the bug. Which of the two forces is greater: the force on the bug or the force on the bus? For every force, there is an EQUAL by opposite force!

Newton’s Three Laws An object at rest remains at rest and an object in motion continues that motion unless acted upon by a net external force. Fnet = mass x acceleration For every force, there is an equal but opposite force.

Fnet = ma A child pulls on a 5.0 kg wagon with a net force of 20.0 N. What is the wagon’s acceleration? m = 5.0 kg Fnet = 20.0 N a = Fnet / m a = 20.0 N / 5.0 kg = a = 4 m/s2

Fnet = ma A dog pulling a man on a sled over ice can accelerate the sled at 2 m/s2. If the sled and man have a combined mass of 150.0 kg, what was the net force on the sled? m = 150.0 kg a = 2 m/s2 Fnet = ma Fnet = 150.0 kg x 2 m/s2 = 300 N