DynamicsDynamicsKinematics: the study of HOW

objects moveDynamics: the study of WHY

objects move the way they do◦Force any push or pull on an object

Vector quantity Unit: Newtons (N) 1 N = 1 kgm/s2

◦2 types of Forces Contact Force: involves contact with the

object Field Force: force act on the object over a

distance…no contact

FORCESFORCESContact ForcesApplied Force: (Fa): the

force a person exerts on an object

Frictional Force: (Ff): the force that opposes motion. Always between two surfaces. Acts opposite the direction of an object’s motion

Normal Force: (FN): support force. Always perpendicular to the surface

Field ForceForce due to Gravity: (Fw):

the measure of attraction between an object and the center of the earth; the object’s WEIGHT. Always downwards.

FREE BODY DIAGRAMS: show all the forces acting on an object; help determine how an object will move.

Newton’s 1Newton’s 1stst Law of Law of MotionMotionThe Law of INERTIA

◦Inertia: the property of an object to resist any change in its motion

◦This means…an object at rest will stay at rest; an object in motion will stay in motion in a STRAIGHT LINE unless acted upon by an outside force.

◦Simply put…objects like to keep doing what they’re doing.

Mass and InertiaMass and InertiaMass and inertia

are DIRECTLY related.◦More mass means

more inertia.◦More inertia means

it’s HARDER to change the object’s motion.

Net Force Net Force

To find net force (Fnet) or resultant force, subtract forces in opposite direction.

Fnetx = (forces to right) – (forces to the left)

Fnety = (forces up) – (forces down)

EquilibriumEquilibriumEquilibrium: forces are

EQUAL and OPPOSITE◦ Fnet = 0 N◦ The object could be:

At rest (v = 0 & a = 0) Moving with CONSTANT

velocity

The object is not ACCELERATING when it is in equilibrium. (But, it can be moving.)

Weight vs. MassWeight vs. MassWeight (Fw) is a measure of the force due

to gravity pulling an object downwards; changes depending on how much gravity there is.◦Unit: Newtons (N)◦Weight = mass x gravity◦ Fw = mg

◦Gravity on earth (g) = 9.81 m/s2

Mass is how much matter an object is made up of; it stays the same no matter where in the universe you are◦Unit: kg

Newton’s 2Newton’s 2ndnd Law of Law of MotionMotionA net force causes an object to

accelerateAcceleration is directly proportional

to the net force on the object◦The greater the force, the faster the

object will accelerateAcceleration is inversely proportional

to the mass of the object◦The larger the mass, the slower it will

accelerate

Fnet = ma

Newton’s 3Newton’s 3rdrd Law of Law of MotionMotionFor every action, there is an equal but opposite reaction.◦Forces always occur in pairs.

Action Force Reaction Force

Force on object A

caused by object B

Force on object B

caused by object A

A B

FrictionFrictionAny force that resists the motion

of an object◦Acts parallel to the surfaces in

contact.◦Acts in the opposite direction of the

object’s motion.Amount of Ff depends on:

◦The type of materials in contact◦The amount of force pressing the

surfaces together

2 Types of Friction2 Types of FrictionStatic Friction

◦The friction keeping objects at restKinetic Friction

◦The friction slowing down moving objects

**Static friction is always greater than Kinetic friction because it is easier to keep an object moving than it is to start is motion. (Think INERTIA!)

**Coefficient of Friction (μ): number describing the type of material. The higher the number, the more friction produced. No UNITS!

Falling ObjectsFalling ObjectsObjects fall at the same rate if they

are falling in a vacuum (no air).Free Fall: Objects fall due to the

influence of gravity◦Forces Present:

Force due to gravity (weight) Force of air resistance (friction)

◦Terminal Velocity: the fastest an object can fall based on its surface area. Fnet = zero

Terminal VelocityTerminal VelocityAs an object falls, it

speeds up due to gravity. As the velocity increases, the amount of air hitting the object increases, so friction increases. When the friction is equal to the weight, the net force on the object is zero. No net force means no acceleration. CONSTANT VELOCITY!