Ch. 4 Forces and Laws of Ch. 4 Forces and Laws of MotionMotion

4-1 Changes in Motion4-1 Changes in Motion

Key Terms pg. 150Key Terms pg. 150

Force-push or pullForce-push or pull Force causes change in velocity Force causes change in velocity

(acceleration)(acceleration)

SI unit of force is Newton (N)SI unit of force is Newton (N) Weight is a measure of gravitational Weight is a measure of gravitational

force on an objectforce on an object

Contact forces-physical contact of Contact forces-physical contact of two objectstwo objects

Ex: pull a stringEx: pull a string

pull a wagonpull a wagon

catch a footballcatch a football

Field Forces-force between two Field Forces-force between two objects without physical contactobjects without physical contact

Ex: gravityEx: gravity

electrical chargeselectrical charges

Force DiagramsForce Diagrams

Magnitude and direction of force Magnitude and direction of force affects an object’s motionaffects an object’s motion

Force is a vectorForce is a vector Force diagrams show forces acting in Force diagrams show forces acting in

a situationa situation Free-body diagrams show only forces Free-body diagrams show only forces

acting on an objectacting on an object

Force DiagramForce Diagram

Figure 4-4 pg. 127Figure 4-4 pg. 127

4-1 Section Review Questions

4-2 Newton’s First Law4-2 Newton’s First Law

An object at rest remains at rest, and An object at rest remains at rest, and an object in motion continues in an object in motion continues in motion with constant velocity(that is, motion with constant velocity(that is, constant speed in a straight line) constant speed in a straight line) unless the object experiences a net unless the object experiences a net external force.external force.

Began by Galileo in 1630s, then Began by Galileo in 1630s, then further developed by Newton in further developed by Newton in 1687.1687.

Inertia- the tendency of an object not Inertia- the tendency of an object not to accelerateto accelerate

When the net external force on an When the net external force on an object is zero, its acceleration (or object is zero, its acceleration (or change in its velocity) is zero.change in its velocity) is zero.

Acceleration is determined by net Acceleration is determined by net external forceexternal force

Net external force is the vector sum Net external force is the vector sum of all the forces acting on an object. of all the forces acting on an object.

An object’s acceleration is the same An object’s acceleration is the same as the net external force.as the net external force.

Mass is a measurement of inertiaMass is a measurement of inertia Ex: basketball vs golf ballEx: basketball vs golf ball

Golf ball will have larger acceleration Golf ball will have larger acceleration due to less inertiadue to less inertia

Objects in motion tend to stay in Objects in motion tend to stay in motionmotion

Ex: crash test dummy/seat beltEx: crash test dummy/seat belt Fig. 4-14Fig. 4-14

EquilibriumEquilibrium

Objects at rest or moving with Objects at rest or moving with constant velocity are at equilibriumconstant velocity are at equilibrium

The net external force acting on a The net external force acting on a body in equilibrium must be equal to body in equilibrium must be equal to zero.zero.

Resolve all vectors into their x and y Resolve all vectors into their x and y components.components.

When all the x vectors equal 0 and When all the x vectors equal 0 and all the y vectors equal 0, then the all the y vectors equal 0, then the vector sum is zero and the body is in vector sum is zero and the body is in equilibrium.equilibrium.

Sample Problem 4A in book pg. 132Sample Problem 4A in book pg. 132

Another Sample Problem 4A:Another Sample Problem 4A:Sample Problem 4A.docx

You have to find the net Force You have to find the net Force (hypotenuse force) and angle.(hypotenuse force) and angle.

4-3 Newton’s 24-3 Newton’s 2ndnd and 3 and 3rdrd Laws Laws

Acceleration of an object is Acceleration of an object is directly directly proportionalproportional to the net external force to the net external force acting on it.acting on it.

Less force is needed to accelerate a Less force is needed to accelerate a low mass object than a high mass low mass object than a high mass object at the same rate.object at the same rate.

In other words…In other words…

A low mass object accelerates faster A low mass object accelerates faster than a heavy object if the same force than a heavy object if the same force is applied.is applied.

Newton’s Second LawNewton’s Second Law

The acceleration of an object is The acceleration of an object is directly proportional to the net directly proportional to the net external force acting on the object external force acting on the object and inversely proportional to the and inversely proportional to the object’s mass.object’s mass.

∑∑F = maF = ma

Sample 4BSample 4B

Sample Problem 4B-Newton’s 2Sample Problem 4B-Newton’s 2ndnd Law Law

FFnet net = ΣF = ma= ΣF = ma

   Roberto and Laura are studying across from Roberto and Laura are studying across from

each other at a wide table. Laura slides a 2.2 kg each other at a wide table. Laura slides a 2.2 kg book toward Roberto. If the net external force book toward Roberto. If the net external force acting on the book is 2.6 N to the right, what is acting on the book is 2.6 N to the right, what is the book’s acceleration?the book’s acceleration?

4-3 continued…4-3 continued…

Forces always exist in pairsForces always exist in pairs

Ex: kicking a ball-the ball exerts a Ex: kicking a ball-the ball exerts a force on you and you exert a force on force on you and you exert a force on the ballthe ball

Newton’s 3Newton’s 3rdrd Law Law

If two objects interact, the magnitude If two objects interact, the magnitude of the force exerted on object 1 by of the force exerted on object 1 by object 2 is equal to the magnitude of object 2 is equal to the magnitude of the force simultaneously exerted on the force simultaneously exerted on object 2 by object 1, and these two object 2 by object 1, and these two forces are opposite in direction.forces are opposite in direction.

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

Action-reaction pair-describe the Action-reaction pair-describe the forces between 2 objects at the same forces between 2 objects at the same timetime

Action-reaction pairs do not result in Action-reaction pairs do not result in equilibrium fig. 4-18equilibrium fig. 4-18

Field forces also exist in action-Field forces also exist in action-reaction pairsreaction pairs

Ex: the force of Earth (gravity) on a Ex: the force of Earth (gravity) on a person and the force of a person on person and the force of a person on EarthEarth

4-4 Everyday Forces4-4 Everyday Forces

FFg g = Force of gravity is a vector = Force of gravity is a vector quantity toward the center of Earthquantity toward the center of Earth

Weight = a scalar quantity, the Weight = a scalar quantity, the magnitude of Force of gravitymagnitude of Force of gravity

FFg g = mg= mg Weight depends on location (g)Weight depends on location (g) Objects weigh less at higher altitudes Objects weigh less at higher altitudes

because g decreases because g decreases

Normal ForceNormal Force

Normal Force = FNormal Force = Fn n

The force perpendicular to the The force perpendicular to the surface of contactsurface of contact

Not always opposite of gravityNot always opposite of gravity In the absence of other forces, the In the absence of other forces, the

normal force is equal and opposite to normal force is equal and opposite to the force of gravity that is the force of gravity that is perpendicular to the contact surfaceperpendicular to the contact surface

FFn n = mg(cos= mg(cosθθ))

The angle is the angle between the The angle is the angle between the normal force and a vertical line and normal force and a vertical line and also the angle between the contact also the angle between the contact surface and a horizontal line. See surface and a horizontal line. See fig. 4-20fig. 4-20

Force of FrictionForce of Friction

Friction opposes the applied forceFriction opposes the applied force The resistive force that keeps an The resistive force that keeps an

object from moving is the static object from moving is the static friction (Ffriction (Fss))

FFs s = -F= -Fapplied applied as long as the object does as long as the object does not movenot move

FFs, max s, max = when the force is as great as = when the force is as great as it can be without moving the object.it can be without moving the object.

Kinetic FrictionKinetic Friction

FFapplied applied > F> Fs, max s, max =the object moves but =the object moves but there is still frictionthere is still friction

FFk k (kinetic friction) = the retarding (kinetic friction) = the retarding force on an object in motionforce on an object in motion

The net external force is equal to the The net external force is equal to the difference between applied force and difference between applied force and kinetic friction force kinetic friction force

(F(Fnet net = F= Fapplied applied – F– Fkk))

Friction arises from interactions at Friction arises from interactions at microscopic levelsmicroscopic levels

The force required to move a The force required to move a stationary object is greater than the stationary object is greater than the force to keep an object moving.force to keep an object moving.

FFs, max s, max > F> Fkk

Magnitude of the force of friction is Magnitude of the force of friction is proportional to the normal force proportional to the normal force exerted on an object by a surface exerted on an object by a surface (weight).(weight).

The force of friction depends on the The force of friction depends on the composition and quality of the composition and quality of the surfaces in contact. Ex: carpet vs. tilesurfaces in contact. Ex: carpet vs. tile

μμk k = = FFkk

FFnn

μμs s = = FFs, maxs, max

FFnn

FFf f = = μμFFnn

Coefficient of kinetic friction is always less than Coefficient of kinetic friction is always less than or equal to the coefficient of static frictionor equal to the coefficient of static friction

Air resistance is a form of frictionAir resistance is a form of friction Air resistance = FAir resistance = FRR

When air resistance equals FWhen air resistance equals Fgg (gravity), an object has terminal (gravity), an object has terminal speed.speed.

Sample Problem 4C-Coefficients of frictionSample Problem 4C-Coefficients of friction

A 24 kg crate initially at rest on a A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal horizontal floor requires a 75 N horizontal force to set it in motion. Find the force to set it in motion. Find the coefficient of static friction between the coefficient of static friction between the crate and the floor. crate and the floor.

μμs s = = FFs, maxs, max = = FFs, maxs, max

FFn n mgmg