A.A. Newton’s Second LawNewton’s Second LawF = m*aF = 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. GravityB. 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?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 All objects, regardless of mass, fall with same acceleration (10m/sacceleration (10m/s22 downward) as long as air downward) as long as air

resistance is negated.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 CheckGravity Check

Who experiences more gravity - the astronaut or the politician?

less distance

more mass

Which exerts more gravity - the Earth or the moon?

C. Gravity CalculationsC. Gravity Calculations

Weight measured in Newton’s

is the force of gravity on an object

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

Weight CalculationsWeight CalculationsJenny 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 CalculationsC. Gravity Calculations

Accel. due to gravity (ag or g) On Earth: g = 9.8 m/s2

For Class, use g = 10 m/s2

mW

g

elephant

m

Wg

featherAnimation from “Multimedia Physics Studios.”

Gravity ComparisonsGravity ComparisonsWould you weigh more on Earth

or Jupiter?

greater gravity g = 25.4 m/s2

greater weight W = m * g

greater mass

318 times more massive

Jupiter because...

Other ForcesOther 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 DiagramsDrawing 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/newtlawshttp://www.glenbrook.k12.il.us/gbssci/Phys/Class/newtlaws/u2l2c.html/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. A free-body diagram for this situation looks like this:

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 RemindersAir Resistance Reminders

Air Resistance 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 LifeD. Air Resistance in Real Life

Terminal Velocity

maximum velocity reached by a falling object

reached when…

Fgrav = Fair

Fair

Fgrav

no net force on object no acceleration constant velocity

D. Air ResistanceD. Air Resistance

Terminal Velocity is reached!

increasing speed increasing air resistance until…

Fair = Fgrav

Got to http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/newtlaws/sd.html Animation from Multimedia Physics Studios.

Concept TestConcept 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 Aristotle (a greek philospher ~350 BC)(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 Galileo (Italian scientist early 1600s)(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 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.