Physics Chapter 6Physics Chapter 6

ForcesForces

Newton’s Laws of MotionNewton’s Laws of Motion11stst Law (Law of inertia) Law (Law of inertia)– An object moving at constant velocity An object moving at constant velocity

keeps moving at that velocity unless a keeps moving at that velocity unless a net force acts on itnet force acts on it

22ndnd Law (F=ma) Law (F=ma)– The net force acting on an object causes The net force acting on an object causes

the object to accelerate in the direction the object to accelerate in the direction of the net forceof the net force

33rdrd Law Law– For every action there is an equal but For every action there is an equal but

opposite reactionopposite reaction

ForceForce–A push or a pullA push or a pull

SystemSystem–The object and what is The object and what is around itaround it

EnvironmentEnvironment–World around object that World around object that exerts forces on itexerts forces on it

Contact forceContact force– Acts on an object only by touching itActs on an object only by touching it

Long-range forceLong-range force– A force exerted without contactA force exerted without contact

Force of gravityForce of gravity– An attractive force that exists An attractive force that exists

between all objectsbetween all objects

AgentAgent– A specific, identifiable, immediate A specific, identifiable, immediate

cause of a forcecause of a force

Newton’s 1st Law of MotionNewton’s 1st Law of Motion

Inertia (deals with 1Inertia (deals with 1stst law) law)– The tendency of an object to resist The tendency of an object to resist

any change in motionany change in motion

EquilibriumEquilibrium– When an object is moving at constant When an object is moving at constant

velocityvelocity

An object at rest is moving at a An object at rest is moving at a constant velocity of 0 and is in constant velocity of 0 and is in equilibriumequilibrium

Free body diagramFree body diagram

Uses dots to represent objectsUses dots to represent objects

Uses lines to represent forcesUses lines to represent forces– Lines are always drawn in the Lines are always drawn in the

direction of the force which acts direction of the force which acts upon the objectupon the object

– The tail of the line representing the The tail of the line representing the force is always placed on the dot of force is always placed on the dot of the object that it is acting on.the object that it is acting on.

Net Force (FNet Force (Fnetnet))– The vector sum of two or more forces on an The vector sum of two or more forces on an

objectobject

Friction (FFriction (Fff))– The contact force that acts to oppose sliding The contact force that acts to oppose sliding

motion between surfacesmotion between surfaces– Parallel to the surface and opposite the Parallel to the surface and opposite the

direction of sliding frictiondirection of sliding friction

Normal Force (FNormal Force (FNN))– contact force exerted by surface on an objectcontact force exerted by surface on an object– Perpendicular to and away from the surfacePerpendicular to and away from the surface

FORCESFORCES

FORCES con’tFORCES con’tSpring Force (FSpring Force (Fspsp))– Restoring force, that is, the push or pull a Restoring force, that is, the push or pull a

spring exerts on an objectspring exerts on an object– Opposite the displacement of the object at Opposite the displacement of the object at

the end of the springthe end of the spring

Tension (FTension (FTT))– The pull exerted by a string, rope, or cable The pull exerted by a string, rope, or cable

when attached to a body and pulled tautwhen attached to a body and pulled taut– Away from the object and parallel to the Away from the object and parallel to the

string, rope, or cable at the point of string, rope, or cable at the point of attachmentattachment

FORCES con’t againFORCES con’t againThrust (FThrust (Fthrustthrust))– A general term for the forces that move A general term for the forces that move

objects such as rockets, planes, and carsobjects such as rockets, planes, and cars– In the same direction as the acceleration of In the same direction as the acceleration of

the object barring any resistive forcesthe object barring any resistive forces

Weight (FWeight (Fgg))– A long-range force due to gravitational A long-range force due to gravitational

attraction between two objects, generally attraction between two objects, generally Earth and an objectEarth and an object

– Straight down toward the center of the earthStraight down toward the center of the earth

W = mgW = mg

Common MisconceptionsCommon MisconceptionsWhen a ball has been thrown, When a ball has been thrown, the force of the hand that the force of the hand that threw it remains on itthrew it remains on it

A force is needed to keep an A force is needed to keep an object movingobject moving

Inertia is a forceInertia is a force

Air does not exert a forceAir does not exert a force

The quantity ma is a forceThe quantity ma is a force

Apparent weightApparent weight– The force exerted by the scaleThe force exerted by the scale

WeightlessnessWeightlessness– Doesn’t mean your weight is zeroDoesn’t mean your weight is zero– It means your APPARENT weight is zeroIt means your APPARENT weight is zero

Free fallFree fall– When an object is influenced only by When an object is influenced only by

the acceleration due to gravitythe acceleration due to gravity

Terminal velocityTerminal velocity– When the drag force is equal to the When the drag force is equal to the

acceleration due to gravityacceleration due to gravity

FRICTIONFRICTIONStatic friction forceStatic friction force– Force exerted on one surface by the Force exerted on one surface by the

other when there is no relative motion other when there is no relative motion between the two surfacesbetween the two surfaces

0 ≤ F0 ≤ Ff, staticf, static ≤ ≤ μμssFFNN

Kinetic friction forceKinetic friction force– Force exerted on one surface by the Force exerted on one surface by the

other when the surfaces are in relative other when the surfaces are in relative motionmotion

FFf, kineticf, kinetic = = μμkkFFNN

Periodic MotionPeriodic MotionSimple harmonic motionSimple harmonic motion– When the force that restores an object When the force that restores an object

to its equilibrium position is directly to its equilibrium position is directly proportional to the displacement of the proportional to the displacement of the objectobject

Period (T)Period (T)– Amount of time needed to repeat one Amount of time needed to repeat one

complete cycle of motioncomplete cycle of motion

AmplitudeAmplitude– The maximum distance the object The maximum distance the object

moves from equilibriummoves from equilibrium

Mechanical resonanceMechanical resonance– The increase vibration of the amplitude The increase vibration of the amplitude

due to small forces being applied at due to small forces being applied at regular intervals to a vibrating or regular intervals to a vibrating or oscillating objectoscillating object

Period of a PendulumPeriod of a Pendulum

T = 2T = 2 ll gg

If an object is on a flat surface, then If an object is on a flat surface, then the Force due to Weight will equal the Force due to Weight will equal the Normal Forcethe Normal Force

FFWW = F = FNN

The force of friction (FThe force of friction (Fff) is always ) is always opposing the applied force (Fopposing the applied force (Fappapp))

If the net force is zero (means no If the net force is zero (means no acceleration or constant velocity) the acceleration or constant velocity) the force due to friction will equal the force due to friction will equal the applied force.applied force.

FFff = F = Fappapp