Lecture: Forces, Motion, Gravity and Friction

http://www.youtube.com/watch?v=YyJSlcIbd-s

What is the metric unit for mass?

What is the English unit for Mass (watch it, it’s slimy)?

What is the English unit for weight?

What is the metric unit for weight?

How does weight relate to a force?

Variable: English: Metric:

Mass slug Gram (g)

Weight pound Newton (N)

(Newton is a force)

(F= Mass x Accel)

(1N= 1kg x 1m/s2)

the Newton is a measure of two things: 1. weight2. forceTHEY ARE THE SAME THING: (YOUR NAMES?)

But, this also means weight is a different measure than mass

1. Force: can only occur as two types:A. pull orB. pushC. all forces transfer energy to an object,

whether it moves or not: wall?D. motion can only be caused by an applied force (w/ direction and magnitude),

thus force is a vectorE. the amount of force needed to move an object depends on:

1) the mass of the object (prius vs hummer?) and

2) the amount of friction applied to the object resisting motion(ie: smooth vs. rough floor?)

F. The metric unit for force is the Newton (N):the amount of force to cause 1kilogram

to accelerate at 1 meter per second squared:

-the same force a mouse applies to a table top due the gravity acting on its mass;

-also = cube of butter in your handG. force formula: 1N = 1kg x 1m (note: mass in kg)

s2 (measured with a Newton spring scale: ?)

H. forces always come in pairs: applied force (draw box w/ pairs?) andthe object’s resistant force

I. Mass is a measure of the amount of matter (atoms: # amu, see periodic table)

J. Weight (not mass!!)a measure (in Newtons of force) of the mass of the object andthe amount of acceleration acting on the object:

1. so, what two variables affect weight (force)?weight (force in N) = mass(kg) x acceleration

F = M x A (Newton’s 2nd law of motion)2. so, how much does 1kg of mass weigh on Earth?

a. weight = 1kg x 9.8 m/s2

so it is 9.8N, because the acceleration of gravity on Earth is, for all objects, 9.8m/s2 (spring scale) and weight is always in units of Newtonsb. if we were on the moon, would we weigh

six times less or six times more?c. are people on the moon really

weightless?

K. Show your work: (k, ?k, F, cu, ca, sf):

A 50.0 kg mouse runs w/ an acceleration of 2.30 m/s2, what force is it running at?

A 1.2 g humming bird is caught in a birding net w/ an applied force of 0.07 N. What was the bird’s acceleration at the time of capture?

L. Forces video #2: http://www.youtube.com/watch?v=MztWyY9z1jY

2. There are four universal forces:

A. gravity force

1. always a pull force

2. the size of this force depends on two aspects:

a. the mass of the object:

thus everything with mass creates its own gravity and

the more massive the object, the greater its gravitational field:

moon vs Earth vs Sun:

this is what keeps planets in orbit, otherwise they fly off into space

you vs your neighbor?

b. the distance from another object:

the greater the distance = less effect of its gravitational force

The sun is a more massive object, but what object in our skies effects

our tides and why?

c. so: increase mass =

increase g force

(direct relationship)

increase distance =

decrease g force

(indirect relationship)

B. electromagnetic force

1. when electricity flows, it creates a magnetic field.

Thus, there are two types of forces created which are related:

a. electric

-between opposite charged particles: balloon

-holds atoms together:

protons/electrons

-stronger than gravity:

why the ceiling does not fall down

b. magnetic force field

-can attract or repel (push or pull): magnets?

-magnetic levitation trains?

C. strong nuclear force1. protons and neutrons are made of quarksthose quarks are each individually held together by a strong force2. occurs only over very short distances, w/ non contact3. break the strong force:produce a nuclear explosion with electricity

releaseD. weak nuclear force

1. holds particles (p & n) within the nucleus together w/non-contact: weak force

2. weak force decay causes neutrons to change (“flavor”) into protons3. loss of weak force causes the nucleus to

decay, releasing radioactivity (beta decay)

3. FrictionA. a force opposing motionB. when surfaces move past each other they create this force, of which there are three types:

1. sliding friction: greatest frictional force

a. objects slide past one another: feet sliding, tires screeching to a stop b. weight of object effects its amount:

greater = more frictionrubbing hands together?

c. surface types effect its amount: rough vs smooth?

2. rolling frictiona. object rolling over another

-bowling ball-tires rolling on street-ball bearings

3. fluid friction: least frictional forcea. fluid is any object which flowsb. ie: gases (wind), liquids (water)c. lubricants change sliding friction to fluid friction: reduces friction

C. friction is not always bad: need it to walk, drive: start/stop

D. terminal velocity: due to the friction acting on the object such that it is still moving butis slowed by the friction so that it is not accelerating= 0 m/s2 thus reached it maximum (terminal) velocity

4. Forces and equilibriumthings begin moving only if the forces acting on them are unequal.A. balanced forces are when all the forces acting on an object equal zero; thus,

there is no speeding up or down in that direction: it is a constant motion (including stopped)for the airplane, which forces are balanced?

is it moving in that direction?which forces are unbalanced?

is it moving in that direction?

B. when the net force is zero the object is in equilibrium:

the object is either:not moving:

or moving at a constant speed

if the net force is not zero (unbalanced), the object is accelerating:

positive = going faster

negative = going slower

if the net force is not zero (unbalanced), the object is accelerating:

positive = going faster

negative = going slower

C. Normal force: an object resting on a surface is applying a

downward force on the surface it is resting upon.Because the object does not sink into the surface there is a force pushing it upward.This upward force is the Normal force.

It occurs opposite to the direction pushing into the surface.

D. free-body diagrama single object onlywith all forces acting are drawn on the objectup forces are positivedown forces are negative