notes - force

7/31/2019 Notes - Force

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Physics Notes

Unit 3

ForceChapter 7.2 and 4


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Causes of Motion Forces - any push or pull

Two Types: 1. Contact2. Field (long range)

Examples...

Measured in: Newtons, PoundsSI Unit:_____________One Newton accelerates 1 kg 1 m/s 2

Device used to measure force:_____________Force is a Vector


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Force of Gravity ( F g)

Commonly known as weight

2 equations based on different scenarios


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Force of GravityNewtons Universal Law of Gravitation

Every two objects in the universe have a mutualattraction between them

F g = Gm 1m 2

r 2 G = Constant of Universal GravitationG =6.67 x 10 -11 N x m 2 /kg 2

Acting from the center of mass

Example: Gravitational Force Between Earth & Sun


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Force of Gravity between the Earth and Sun

F g = Gm Sm E r S-E 2

Fg = 6.67 x 10 -11 N x m 2 /kg 2 x 5.98 x 10 24 kg x 1.99 x 10 30 kg1.49 x 10 11 m2

Fg = 3.58 x 10 22 N


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Weight of a teacher on Earth

A teacher has a mass of 90 kg (~200 lbs)

Fg = 6.67 x 10 -11 N x m 2 /kg 2 x 5.98 x 10 24 kg x 90 kg

(6.38 x 10 6 m) 2

F g = 882 N (attractive)


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Force of Gravity In a defined gravitational field

F g = mg

m = mass quantity of matterunits - _________

g = acceleration due to gravity9.81 m/s 2 (towards the ground) at sea levelchange with elevation (Mt Everest)


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Weight of a teacher on Earth

A teacher has a mass of 90 kg (~200 lbs)

Fg = mgFg = 90 kg x 9.81 m/s 2

Fg = 883 N


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Defining Acceleration Due to Gravity (g) (on Earth)(using numbers from Gravity Stats)

mog = F g = Gm omE rE2

g = Gm E rE2

g = 9.799 m/s 2

Sun: 274 m/s 2 Moon 1.63 m/s 2

Video - Moon Walk
http://www.csun.edu/science/geoscience/astronomy/movies/moon_walk1.movhttp://www.csun.edu/science/geoscience/astronomy/movies/moon_walk1.movhttp://www.csun.edu/science/geoscience/astronomy/movies/moon_walk1.movhttp://www.csun.edu/science/geoscience/astronomy/movies/moon_walk1.movhttp://www.csun.edu/science/geoscience/astronomy/movies/moon_walk1.mov


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Free Body Diagrams -all forces acting on a pointcenter of mass for the object


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Normal Force (F N) - a contact force actingperpendicularly to a common surface of contact

Static Equilibrium : Net Force = 0 (no acceleration)Forces are balanced

If there is a net force, then there is accelerationForces are not balanced

Often times (but not always)F Net y = F N + F g F Net x = F P + F F


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Friction - the force perpendicular to the normal forceat the surface of contact, which opposes the motionof an object

a.) Static frictionb.) Kinetic friction (Sliding)

c.) Rolling frictiond.) Fluid friction

Note: Friction is a resistive force

http://www.regentsprep.org/Regents/physics/phys01/friction/default.htm
http://www.regentsprep.org/Regents/physics/phys01/friction/default.htmhttp://www.regentsprep.org/Regents/physics/phys01/friction/default.htm


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Microscopic view

of two interactingsurfaces


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Coefficient of Friction (m, mu) - the characteristicability to slide between two interacting surfaces

Static Friction

F fs < sF N

Kinetic Friction

F fk = kF N


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Graph of Friction Force as an Applied Force Increases


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This rhyme is designed to help you remember theequation for the force of friction.

Hey diddle diddle the cat and the fiddle the cow jumped over the moon. Her leap into action took plenty of traction (FF) the product of force (FN)times mu (m).

FF

= m FN


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Newtons First Law of Motion Every body continues in its state of rest or constantvelocity unless acted on by a nonzero net force.(Law of Inertia)

Inertia - The tendency of a body to maintain its stateof rest or of uniform motion in a straight line(determined by mass not velocity)

(mass = inertia)Inertial Reference Frame - non accelerating frame of

reference or else 1st law does not hold.


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Ways in which we compensate

for inertia in the real world:


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N S d L f M i


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Newtons Second Law of Motion Acceleration of an object is proportional to the

Force acting on it Acceleration of an object inversely proportional to

the mass of the objectacceleration = Net Force

massF Net = ma

Units: N = kg m/s 2

Force due to gravity = mgConstant Velocity (a = 0) still can be in motion


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Newtons Third Law of Motion

Whenever one object exerts a force on a secondobject, the second exerts an equal and opposite forceon the first.

Examples:Push TableShot Gun

RocketNormal ForceGravity


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Application of the Net Force Often times (but not always)

(Big 3) ma = F Net x = F P + F F (FF = m FN)(Big 3) ma = F Net y = F N + F g (Fg = mg)


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A 5.0 kg box is pushed with a 12 N horizontal force.A frictional force of 8.0 N resists motion. The boxbegins at rest and is pushed for 10 s. Find the:

a. net force

b. acceleration

c. final velocity

F Net x = F P + F FF Net x = 12 N 8 N = 4.0 N

a = F Net / ma = 4.0 N / 5.0 kg = 0.80 m/s 2

vf = v i + at

vf = 0 + 0.8 m/s2

*10 s = 8.0 m/s


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A 2.0 kg box is moving at a speed of 15 m/s and slidesto a stop in 25 m. Find the:

a. acceleration

b. Net Force

c. Force of Friction

d. Coefficient of Friction

vf

2 = vi

2 + 2ada = v f 2 /2d = 225/50 = 4.5 m/s 2

FNet = maF

Net= 2 kg x 4.5 m/s 2 = 9.0 N

F Net x = F P + F F9.0 N = 0 FF = 9.0 N

FF = mFN = m x mgm = F F /mg = 9 N/19.6 N = .46


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A 3.0 kg box is pulled up with a 35 N Force. Find its:

a. Force of Gravity

b. Net Force

c. Acceleration

Fg

= mgFg = 3.0 kg x 9.81 m/s 2 = 29.4 N

FNet = Fp - FgFNet = 35 N 29.4 N = 5.6 N

a = F Net / m a = 5.6 N / 3.0 kg = 1.87 m/s 2

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