Newton’s Second Law F = m*a Force = mass x acceleration Newton = kg x m/s2 “Force” is a push or pull acting on an object Rearrange equation to a = F / m which shows that the acceleration of an object is proportional to force applied to it and inversely proportional to the mass of the object

B. Gravity Gravity The force of attraction between any two objects with mass in the universe Measured in Newtons (N) 1 N = 1 kg·m/s2 since F = m * a Force of Gravity will increase as... mass increases distance between objects decreases

Which is true? Heavier objects fall faster than than lighter objects? Heavy objects and light objects will fall at the same rate? Go to http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/efff.html for more falling questions…

All objects, regardless of mass, fall with same acceleration (10m/s2 downward) as long as air resistance is negated. More gravitational force is on the elephant, yet it is proportionally harder to accelerate the elephant’s larger mass, so acceleration is same as mouse.

Gravity Check Who experiences more gravity - the astronaut or the politician? Which exerts more gravity - the Earth or the moon? more mass less distance

C. Gravity Calculations MASS always the same (kg) WEIGHT depends on gravity (N) Weight measured in Newton’s is the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2)

a F m Weight Calculations F(W) = 557 N m = W ÷ g m = ? Jenny weighs 557 N on Earth. What is her mass in kg? GIVEN: F(W) = 557 N m = ? a(g) = 10 m/s2 WORK: W = m * g m = W ÷ g m = (557 N) ÷ (10 m/s2) m = 55.7 kg m F a

C. Gravity Calculations Accel. due to gravity (ag or g) On Earth: g = 9.8 m/s2 For Class, use g = 10 m/s2 elephant feather Animation from “Multimedia Physics Studios.”

Gravity Comparisons Would you weigh more on Earth or Jupiter? Jupiter because... greater mass 318 times more massive greater gravity g = 25.4 m/s2 greater weight W = m * g

Other Forces Normal force (FN) is the force applied by a surface to resist force of gravity (Fg) Frictional force (Ff) is the force that is opposite to the direction of movement Air resistance (Fair) is the force of air molecules against the direction of motion

Drawing Free-Body Diagrams Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. The size of the arrow in a free-body diagram is reflects the magnitude of the force. The direction of the arrow shows the direction which the force is acting. The free-body diagram to the right depicts four forces acting upon the object. Objects do not necessarily always have four forces acting upon them. There will be cases in which the number of forces depicted by a free-body diagram will

http://www. glenbrook. k12. il. us/gbssci/Phys/Class/newtlaws/u2l2c http://www.glenbrook.k12.il.us/gbssci/Phys/Class/newtlaws/u2l2c.html A book is at rest on a table top. A free-body diagram for this situation looks like this:

An egg is free-falling from a nest in a tree. Neglect air resistance An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:

A) A skydiver is descending with a constant velocity A) A skydiver is descending with a constant velocity. Consider air resistance. B) Now Draw a skydiver with increasing velocity (still accelerating)

A rightward force is applied to a book in order to move it across a desk at increasing velocity (so it has acceleration). Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

Air Resistance Reminders a.k.a. “fluid friction” or “drag” force that air exerts on a moving object to oppose its motion depends on: speed surface area shape density of fluid (medium)

D. Air Resistance in Real Life Terminal Velocity maximum velocity reached by a falling object reached when… Fgrav = Fair Fair no net force on object  no acceleration  constant velocity Fgrav

Animation from Multimedia Physics Studios. D. Air Resistance increasing speed  increasing air resistance until… Fair = Fgrav Terminal Velocity is reached! Got to http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/sd.html Animation from Multimedia Physics Studios.

Concept Test Is the following statement true or false? An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

Aristotle (a greek philospher ~350 BC) Believed that there are two kinds of motion, natural and unnatural. Natural Motion = Objects have their “natural” place and will move seeking out that natural place. ie: Something falling etc. Unnatural motion = When an object is pushed and the speed is relative to the force of the push.

Galileo (Italian scientist early 1600s) Galileo built his own telescope and used scientific methods, not philosophy, to explain motion. Legend states that he dropped both heavy and light balls from the top of the Tower of Pisa, noticing that they hit the ground at the same time. This demonstrated that, contrary to popular belief, heavy and light objects fell at the same rate. Galileo is also credited with creating our current understanding of acceleration, falling objects accelerate uniformly, an understanding of inertia (prior to Newton), and that motion of a free-falling projectile is constant horizontally and of uniform acceleration vertically.

Galileo Although Copernicus was the first to discuss that the Earth revolved around the sun, not the other way around…which was heresy to Aristotle and to the beliefs of the Catholic church… Galileo was first to experiment scientifically with the way objects fall to Earth and even how the Earth revolves around the sun (a heliocentric view), not the other way around…following Copernicus’ views. Placed under house arrest for the last years of his life for his heliocentric views of the galaxy which were contrary to the geocentric views which had been the dominant belief of the Roman Catholic church since the time of Aristotle.