ForceForce

A push or pull exerted on an object.

Forces…Forces… Forces are vectorsForces are vectors

– Have both magnitude and directionsHave both magnitude and directions Net ForceNet Force

– Vector sum of all forces acting on a Vector sum of all forces acting on a body body

When the net force on an object is When the net force on an object is zero, the object is in equilibriumzero, the object is in equilibrium– In otherwords, the acceleration is zero. In otherwords, the acceleration is zero.

Forces cause Forces cause accelerationsaccelerations– Change in velocityChange in velocity

Speed and/or directionSpeed and/or direction

Force & Force & MotionMotion

Consider the book on the table. In order for the book to move, an unbalanced force has to act on the book.

SystemsSystemsWhen considering how a force affects motion, it is important to identify the object of interest. This object is called the system.

Everything around the object that exerts forces on it is called the external world.

The identifiable cause is called an agent.

Contact Contact ForcesForces• A contact force exists when an object A contact force exists when an object

from the external world from the external world touchestouches a a system and thereby exerts a force on system and thereby exerts a force on it. it.

Friction

Normal

Tension

Applied

Field ForcesField Forces Field forces are exerted without Field forces are exerted without

contact.contact.– Also known as non contact forces or Also known as non contact forces or

action at distances forcesaction at distances forces

Gravitational

ElectricalMagneti

c

Free Body Free Body DiagramDiagram

Free Body Diagram:Free Body Diagram: a physical model a physical model that shows all the forces acting on that shows all the forces acting on the system. the system.

Each force is drawn as a vector leading away from the object.*Must be drawn for every problem

FBD ExampleFBD Example A car accelerating to the right A car accelerating to the right

experiencing friction.experiencing friction.

Weight

FrictionApplied

Normal

Newton’s First Newton’s First Law Law

An object in motion, stays in motion; An object in motion, stays in motion; an object at rest stays at rest unless an object at rest stays at rest unless acted on by a net force. acted on by a net force.

Also known as the law of inertia!!!

Inertia…Inertia…

InertiaInertia An object’s resistance to change its An object’s resistance to change its

state of motion. state of motion. An object’s mass is a measurement An object’s mass is a measurement

of the object’s inertia. of the object’s inertia. – The more mass an object has, the more The more mass an object has, the more

resistance it will have to change its state resistance it will have to change its state of motion. of motion.

                                                       

Mass vs. Mass vs. WeightWeight MassMass

– Amount of matter present in an objectAmount of matter present in an object– Constant – same everywhere in the universeConstant – same everywhere in the universe– A measurement of an object’s inertiaA measurement of an object’s inertia– Measured using a Measured using a BALANCEBALANCE

WeightWeight– Force due to gravityForce due to gravity– W = mg W = mg – Changes as the acceleration due to gravity Changes as the acceleration due to gravity

changeschanges– Measured using a Measured using a SCALESCALE

Newton’s 2Newton’s 2ndnd Law Law

Acceleration is directly proportional Acceleration is directly proportional to the net force acting on the object to the net force acting on the object and inversely proportional to the and inversely proportional to the object’s mass. object’s mass.

F = force F = force – Measured in NewtonsMeasured in Newtons

m = massm = mass– Measured in Measured in

kilogramskilograms a = accelerationa = acceleration

– Measured in m/sMeasured in m/s22

Newton’s 3Newton’s 3rdrd Law Law

For every action force there is an For every action force there is an equal and opposite reaction force.equal and opposite reaction force.

InteractionsInteractions All forces result from interactions All forces result from interactions

between objects.between objects. Action-Reaction force pairs act on Action-Reaction force pairs act on

different objects.different objects.– Action-Reaction forces do Action-Reaction forces do NOTNOT cancel cancel

out because they act on different out because they act on different objects.objects.