What is a force?

A force is an interaction that CAN cause an object to accelerate. A force won’t

always cause an acceleration. Only a net (non-zero) force will cause

an acceleration.

Inertia

First conceived by Galileo. Inertia is the tendency for an object to resist

changes in its motion.Inertia is proportional to the mass of an object

(and is often used synonymously with “mass” for translational motion).

Also a sweet idea for a roller derby name

1st Law = Law of InertiaAn 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.

The “disclaimer” at the end is very important!

2nd Law = The acceleration of a

body is proportional to the net force on the body AND inversely proportional to the body’s mass.

Mathematically m

Fa net

3rd Law = Action-Reaction Pairs

For every (action) force there is an equal but opposite (reaction) force

Fhead on hand

Fhand on head

Weight: is the force due to gravity on an object. It is always directed (straight) downward.

Fw = mg

Normal Force: is the force with which a surface pushes back on an object. This force is always perpendicular to the surface no matter the orientation of the surface.

Tension: The force applied by (or in) a rope. Ropes only pull.

Friction: The force a surface applies that resists motion or an applied force

Gravitational force: is the force on an object due to a second object. Any 2 objects will exert an equal and opposite gravitational force on each other. We’ll do more with this later.

1. Draw forces acting on object acting through the center of mass

2. Label “everyday” Forces** you do NOT label components as part of the free body diagram. The components are only to assist you in solving for a variable, they are not “stand alone forces”.

3. Label acceleration if present

4. Apply ΣF=ma in the necessary directions

FN

mg

mg

FN

θ

a

This is a free body diagram – notice the components of the weight are

NOT labeled!

The components are helpful in solving but are NOT

fundamental forces.

Friction is the resistive force between two bodies as they slide past one another.

Static friction must be overcome to initiate the motion of a body while kinetic friction must be overcome while an object is in motion.

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

k k N

s s N

f F

f F

What is UCM?It describes an object that moves in a circle with

a constant speed

Implications of UCM…An object accelerates without changing its speed.

The direction continuously changes as an object moves around a circle

Newton’s first law says that an object in motion continues in motion with a constant velocity unless acted upon by a net external force.

Velocity is a vector, speed and direction. A constant velocity means constant speed AND

direction. Direction changes, therefore a force was applied!

According to Newton’s first Law of Motion, an object’s natural path is a straight line, therefore to pull an object out of its natural path a net force must be applied.

Centripetal force is the force that is responsible for an object undergoing uniform circular motion (UCM). This force is always directed radially inward (tangent to the instantaneous velocity). It causes a change in direction without causing a change in speed.

The centripetal force is any “everyday force” that acts centripetally – that is acts so that it is directed toward the center of the circular path.

What is the equation for centripetal force? Remember that Fc is the net force toward the

center of the circle.cc maF

rr

vac

22

rmr

vmFc

22

OMG people….you better know this!The centrifugal force is a fictional force in an

inertial frame of reference. It is not real! It is usually used in place of the correct

Newton’s First Law when explaining certain behaviors exhibited in circular motion.

It is not real!Centrifugal translates from Latin to mean

“center fleeing.” The “centrifugal effect” was named such because it feels as if you are being pushed outward when moving in a circle. You know better than that….it is just inertia that you are feeling.

…that often give students trouble.

1. Define the direction of the motion/acceleration

2. Separate the system into discrete objects.

3. Write a F = ma equation for each object

4. Combine equations and solve

You have to pay attention to the direction of the acceleration AND the direction of the forces (and their components).

I hope that you are too…

…too bad if you’re not, we’re doing them anyway!