Introduction to Motion

Who was Sir Isaac Newton?

• Born in 1642, 12 years after Johannes Kepler

• Studied at Cambridge University, but went home for 2 years during a break because of the plague

• English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian

Sir Isaac Newton and Physics

• Did not discover the 3 laws of motion, but explained them in his book Principia relating force to motion of objects

• Described universal gravitation

Newton’s First Law of Motion

DEFINITION: An object at rest stays at rest and an object in motion stays in motion at constant speed and in a straight line, unless acted on by an unbalanced force What is another way to say this? The law of inertia.

Newton’s First Law of Motion

• Inertia is the tendency of an object to stay at rest if its at rest, or to keep moving if it is moving. Objects resist change in motion.

• Inertia causes a mass to keep going unless it is acted upon by an outside force. Think of a seatbelt in a moving car. What happens if there is no air bag?

• More inertia means harder to stop and to change direction. Will something with more inertia be heavier or lighter than something with less inertia?

Newton’s First Law of Motion

• Demo: Book on table

• With no friction, what would happen?

• With no gravity, what would happen if you threw a ball up in the air?

Newton’s Second Law of Motion

DEFINITION: The acceleration of an object depends on the mass of the object and the amount of force applied

Force = mass * acceleration or F = ma

Newton’s Second Law of Motion

• VELOCITY is the speed of an object in a particular direction. Ex: 25 m/s South

• ACCELERATION is how fast an object’s velocity changes. An object accelerates if its speed changes, direction changes, or both. Ex: 5 m/s North to 10 m/s North

5 m/s North to 5 m/s East5 m/s North to 10 m/s East

Newton’s Second Law of Motion

• F = ma becomes a = F/m• Acceleration depends on mass: – an object’s acceleration as mass – acceleration as mass

• Acceleration depends on force:– acceleration as force – acceleration as force – acceleration is always in the same direction as the

force applied

Newton’s Second Law of Motion

• Acceleration due to gravity means how fast the speed of something changes because of gravity. It is the same, regardless of weight.

• If you dropped a bowling ball and a soccer ball from the roof, which one would have more force?

Newton’s Second Law of Motion

Feather vs. hammer videohttp://video.google.com/videoplay?docid=6926891572259784994

http://videos.howstuffworks.com/nasa/2047-a-hammer-and-a-feather-on-the-moon-video.htm

Newton’s Third Law of Motion

DEFINITION: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first

Newton’s Third Law of Motion

• Forces act in pairs– Action/reaction

• Force pairs do not act on the same object– Ex: sitting in a chair

• The effect of a reaction can be difficult to see– Ex: ball falling

Newton’s Third Law of MotionTerminal velocity: 120 MPHFall height: 14,000 feet

Newton’s Third Law of Motion

• How does skydiving demonstrate the 3rd law of motion?

• Is my acceleration constant as I fall downward from 14,000 feet?

Newton’s Third Law of Motion• At first there is no drag or air resistance and I

accelerate at 9.8 m/s/s downward. The NET force is downward. Does my weight change?

• My weight causes me to go faster, or to accelerate downward. The faster I fall, the more upward drag. These 2 forces are pushing in opposite directions. The NET force downward becomes less. Does my weight change?

Newton’s Third Law of Motion

• At some point the drag equals my weight and the NET force is zero, which means acceleration is zero (equilibrium). Since speed is constant, it is called terminal velocity. My terminal velocity is 120MPH. Does my weight change?

• Terminal velocity is determined by an object’s shape. Bigger, wider objects hit more air and experience more drag.

Newton’s Third Law of Motion

What do you feel when skydiving?• First few seconds feels like stomach is trying to

climb up into mouth like a roller coaster or elevator accelerating quickly downward

• Once terminal velocity is reached, it feels normal, just like when you’re on the ground, but it’s very windy

Newton’s Third Law of Motion

• When the parachute opens, the surface area suddenly increases dramatically and drag increases. I feel a jerk upward and my downward motion slows.

• As I slow down, the drag decreases until it once again equals my weight and terminal velocity is reached once again, but it is 20MPH instead of 120MPH for a safe landing.

Newton’s 3 Laws of Motion• 1st law (law of inertia): an object at rest

remains at rest and an object in motion remains in motion unless acted upon by an unbalanced force

• 2nd law: F = ma or a = F/m

• 3rd law: all forces act in pairs, action-reaction