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Chapter(5(
Part(1:((Forces(and((
Newton’s(Three(Laws(of(Mo:on(
Forces:(
(
•  Forces(are(the(pushes(and(pulls(that(act(upon(an(object.(
•  Forces(are(caused(by(the(ac:on(of(an(agent.(
•  Forces(are(vector(quan::es.(
•  Total(or(net(force(is(the(vector*sum*over(individual(forces.(
•  Forces(can(either(be(direct*contact(or(long*distance*(field*forces).(
Forces(types:(
Contact:*
D  Normal(
D  Spring(
D  Tension(
D  Drag(
D  Fric:on(
D  Thrust(
Long*Distance:*
(Field*forces)*
*
D  Gravity(
D  Electric/magne:c(
Fundamental:*
D  Gravity(
D  Electromagne:c(
D  Weak(nuclear(
D  Strong(nuclear(
iClicker(ques:ons(
What*is*a* net*force? *
A.  The weight excluding the container
B.  The vector sum of all forces in a problem
C.  The vector sum of all forces acting on an object
D.  The vector force applied by a net
E.  The vector sum of all forces that add up to zero
Forces*are*Vectors:*
Two*of*three*forces*exerted*on*an*object*are*shown.*The*net*force*
points*to*the*leE.*What*is*the*missing*third*force?*
Newton’s(Laws:(
(
1st(Law:((Law(of(Iner:a,(Galileo)(–(An(object(at(rest(will(remain(at(rest(OR(an(object(
that(is(moving(will(con:nue(to(move(in(a(straight(line(with(constant(velocity(IF(AND(
ONLY(IF,(the(net(force(on(the(object(is(zero(
ONLY(IF,(the(net(force(on(the(object(is(zero!(
Newton’s(Laws:(
(
1st(Law:((Law(of(Iner:a,(Galileo)(–(An(object(at(rest(will(remain(at(rest(OR(an(object(
that(is(moving(will(con:nue(to(move(in(a(straight(line(with(constant(velocity(IF(AND(
ONLY(IF,(the(net(force(on(the(object(is(zero(
ONLY(IF,(the(net(force(on(the(object(is(zero!(

v! = constant

   
!Fnet = ∑ F = F1 + F2 + F3 +... = 0
There(may(be(many(forces(ac:ng,(but(their(vector(sum(is(exactly(ZERO!(
Newton’s(Laws:(
(
2nd(Law:(The(accelera:on(of(an(object(is(directly(propor:onal(to(the(net(force:((
((((((((((((((((Fnet(~(a(((D>(constant(of(propor:onality(is(MASS:(((Fnet(~(ma((



F
=
F
=
m
a
! Net ∑
Example(FD1:(
(
Let(two(forces,(F1(=(10(N(@(135˚((from(+xDaxis)(and(F2(=(3(i(–(4(j((N)(act(on(a(50(kg(
box.((What(is(the(displacement(of(the(box(aeer(8(seconds?((Assume(that(the(box(
will(start(from(rest.(
F1*
F2*
Newton’s(Laws:(
(
3rd(Law:((If(an(object(exerts(a(force(on(a(second(object,(then(the(second(object(
exerts(a(force(equal(in(magnitude(and(in(the(opposite(direc:on(on(the(first(object.(
(
Forces(occur(as(part(of(an(ac:on(/(reac:on(pair.(
Ffloor(on(box(
Fbox(on(person(
Fperson(on(box(
Fbox(on(floor(
iClicker(ques:ons(
What*is*the*name*of*a*diagram*used*to*show*
all*forces*acHng*on*an*object?*
A. (Mo:on(diagram(
B. (Interac:on(diagram(
C. (FreeDbody(diagram(
D. (SecondDlaw(diagram(
iClicker(ques:ons:((Exploring(FreeDBody(Diagrams(
QuickCheck 5.8
An object on a rope is lowered at constant speed.
Which is true?
A.  The rope tension is greater than the object s weight.
B.  The rope tension equals the object s weight.
C.  The rope tension is less than the object s weight.
D.  The rope tension can t be compared to the object s
weight.
An(object(on(a(rope(is(being(lowered(with(a(speed(that(is(
decreasing.(Which(equa:on(is(true?(
((
a.  T(=(mg(
b.  T(>(mg(
c.  T(<(mg(
d.  T(=(0(
(
QuickCheck 5.11
An elevator, lifted by a cable, is moving upward and
slowing. Which is the correct free-body diagram?
A.
B.
C.
D.
E.
QuickCheck 5.12
A ball has been tossed straight up. Which is the correct
free-body diagram just after the ball has left the hand?
Ignore air resistance.
A.
B.
C.
D.
QuickCheck 5.13
A ball, hanging from the ceiling by a
string, is pulled back and released.
Which is the correct free-body
diagram just after its release?
A.
B.
C.
D.
E.
A(600(kg(sports(car(is(driving(along(the(
interstate.((In(one(empty(stretch,(the(
driver(is(traveling(at(a(constant(50(m/s.((
The(total(force(on(the(car(is:(
a) 
b) 
c) 
d) 
e) 
600(N(
5880(N(
0(N(
50(N(
Insufficient(informa:on(
10DyearDold(Sarah(stands(on(a(skateboard.(Her(older(brother(Jack(
starts(pushing(her(backward(and(she(starts(speeding(up.(The(force(
of(Jack(on(Sarah(is:(
(
A.  greater(than(the(force(of(Sarah(on(Jack.(
B.  equal(to(than(the(force(of(Sarah(on(Jack.(
C.  less(than(the(force(of(Sarah(on(Jack.(