Forces

¿ Qué es un Force?

A push or a pull

The Four Fundamental Forces

1. Gravitational

2. Electromagnetic

3. Strong Nuclear

4. Weak Nuclear

Galileo

• 1564 - 1642

• Was a Copernican

• Convicted of Heresy 1633

• Pardoned by Pope in 1992

Galileo

• Studied Falling Bodies

• Theorized what was later to be known as Newton’s first law of motion

http://galileo.rice.edu/

Sir Isaac Newton

• 1642 – 1727

• Developed three Laws of Motion

• Invented Calculus

Newton’s Laws of Motion

1. An object at rest will remain at rest, an object in motion remain in motion with a constant velocity unless acted on by a net external force.

Newton’s Laws of Motion

2. The acceleration of a body is proportional to the net force on the body AND proportional to the inverse of the body’s mass.

Newton’s Laws of Motion

3. For every force there is an equal but opposite force

Fhead on hand

Fhand on head

Everyday Forces

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

Straight Downw = mg

2. Normal Force, FN: The force a surface applies to an object. It is ALWAYS perpendicular to surface

Everyday Forces

3. Tension, FT: The force applied by a rope. Or the force in a rope. Ropes only pull. It is always along the rope.

4. Friction, fk or fs: The force a surface applies that resists motion or an applied force. Always parallel to the surface opposite motion or potential motion

Everyday Forces

5. Applied Force: A force applied by some object, like a person, that does not fall into any previous category. In the direction specified

Friction

• Remember friction always opposes motion or the applied force

Friction, types

• Kinetic Friction: this is moving friction

• Static friction: this is non-moving friction

Both friction types are defined by the coefficient of friction, μ (either kinetic, μk or static, μs)

Friction Equations

k k N

s s N

f F

f F

Drawing Free Body Diagrams

1. Draw forces acting on object acting through the center of mass (ask the 5 questions on the next slide)

2. Label “everyday” Forces

3. Label acceleration if present

4. Apply ΣF=ma in x and y directions

FN

mg

5 Questions to ask when drawing an FBD

1. Is it on Earth?• If yes, then there is weight and it is straight down

2. Is it on a surface?• If yes, then there is a normal force and it is perpendicular to

the surface

3. Is there a rope involved?• If yes, then there is tension and it acts along the rope

4. Is it on a rough surface?• If yes, then there is friction and it is parallel to the surface

opposite the motion or intended motion

5. Is someone pushing or pulling the object?• If yes, then there is an applied force and it is in the direction

indicated

mg

FN

mg

FT

mg

Fapplied

FN

fs no motion

fk motion

FN

m2g

FT

FT

m1g

FN

m2g

FT

FT

m1g

Ff

mg

FN

θ

a

mg

FN

θ

a

FN

θ

mg

FN

θ

mg

x

y y

x

mg cos θ

mg sin θ

a

a

mg

FN

θ

a

Ff

FN

θ

mg

FN

θ

mg

x

y y

x

mg cos θ

mg sin θ

a

aFf

Ff