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Forces

¿ Qué es un Force?

A push or a pull

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The Four Fundamental Forces

1. Gravitational

2. Electromagnetic

3. Strong Nuclear 

4. Weak Nuclear 

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Galileo

Studied Falling Bodies

Theorized what was later to be known as

 Newton¶s first law of motion

http://galileo.rice.edu/

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Sir Isaac Newton

1642 ± 1727

Developed three

Laws of Motion

Invented Calculus

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 Newton¶s Laws of Motion

1. An object at rest

will remain at rest,

an object in motionremain in motion

with a constant

velocity unless

acted on by a netexternal force.

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 Newton¶s Laws of Motion

2. The acceleration of a

 body is proportional

to the net force onthe body AND

 proportional to the

inverse of the body¶s

mass.

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 Newton¶s Laws of Motion

3. For every force

there is an equal but

opposite force

Fhead on hand

Fhand on head

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Everyday Forces

1. Weight : This is the force of gravity. Or gravity acting on a mass.

Straight Downw = mg

2.  Normal Force, F  N 

: The force a surfaceapplies to an object.  I t is ALWAYS  perpendicular to surface

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Everyday Forces

3  . Tensi on, F T : The force applied by a rope.

Or the force in a rope. Ropes only pull.  I t is

always along the rope.

4 . Fr i cti on, f k or f s: The force a surface applies

that resists motion or an applied force.

 Always parallel to the surface opposite

motion or potential motion

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Everyday Forces

5  . Appl ied Force: A force applied by some

object, like a person, that does not fall into

any previous category.  I n the direction

 specified 

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Friction

Remember friction always opposes motion or 

the applied force

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Friction Equations

k k N 

 s sN 

 f F 

 f F 

 Q

 Q

!

e

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Drawing Free Body Diagrams

1. Draw forces acting on

object acting through the

center of mass (ask the 5

questions on the nextslide)

2. Label ³everyday´ Forces

3. Label acceleration if 

 present4. Apply F=ma in x and y

directions

FN

mg

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mg

FN

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mg

FT

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mg

Fapplied

FN

f s no motion

f k motion

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FN

m2g

FT

FT

m1g

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FN

m2g

FT

FT

m1g

Ff 

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mg

FN

a

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mg

FN

a

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mg

FN

a

Ff 

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FN

mg

FN

y

x

mg cos

mg sin

a

aFf 

Ff 

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