Transcript (field forces: magnetic force, gravitational force).

FORCE
A force is any influence that can change the velocity of
a body. Forces can act either through the physical
contact of two objects (contact forces: push or pull) or
at a distance (field forces: magnetic force,
gravitational force).
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Gravitational Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
What do we mean by balanced and unbalanced forces?
The forces on the book are unbalanced
Balanced forces are
EQUAL and
OPPOSITE in direction
A net force is the vector sum of the forces
acting on an object
If all the forces are balanced,
we same the object is in EQUILIBRIUM
What are the net forces?
These are free-body diagrams
FIRST LAW OF MOTION
According to Newton's
First Law of Motion:
Isaac Newton
(1642-1727)
" If no net force acts on it, a
body at rest remains at rest and
a body in motion remains in
motion at constant speed in a
straight line."
MASS and INERTIA
The property a body has of resisting any change in its
state of rest or of uniform motion is called inertia.
The inertia of a body is related to the amount of
matter it contains. A quantitative measure of inertia
is mass.
The SI unit of mass is the kilogram (kg).
Motion tends to continue
unchanged.
The elephant at rest tends
to remain at rest.
Tablecloth trick:
Too little force, too little time to
overcome "inertia" of tableware.
SECOND LAW OF MOTION
According to Newton's Second Law of Motion, the net
force acting on a body equals the product of the mass
and the acceleration of the body. The direction of the
force is the same as that of the acceleration. In
equation form:
F = ma
In the SI system, the unit for force is the newton
(N): A newton is that net force which, when applied to
a 1-kg mass, gives it an acceleration of 1 m/s2.
WEIGHT
The weight of a body is the gravitational force with which
the Earth attracts the body.
Weight (a vector quantity) is different from mass (a
scalar quantity). The weight of a body varies with its
location near the Earth (or other astronomical body),
whereas its mass is the same everywhere in the universe.
The weight of a body is the force that causes it to be
accelerated downward with the acceleration of gravity g.
Weight = mass x gravity
W = m x g
Newton = kg x m/s2
1 N = 1 kgm/s2
THIRD LAW OF MOTION
According to Newton's third law of motion, when one
body exerts a force on another body, the second body
exerts on the first an equal force in opposite
direction.
The Third Law of Motion
applies to two different forces
on two different objects: "The
action force one object exerts
on the other, and the equal but
opposite reaction force the
second object exerts on the
first."
Forces occur is pairs…
Net force is sometimes designated F.
The second law of motion is the key to understanding
the behavior of moving bodies since it links cause
(force) and effect (acceleration) in a definite way.
4.2 A force of 3000 N is applied to a 1500-kg car at rest.
a. What is its acceleration?
F = 3000 N
m = 1500 kg
vo = 0 m/s
F = ma
3000
F
= 2 m/s2

a
m 1500
b. What will its velocity be 5 s later?
vf = vo + at
= 2(5)
= 10 m/s
4.3 A 1000 kg car goes from 10 to 20 m/s in 5 s. What force is
acting on it?
m = 1000 kg
vo = 10 m/s
vf = 20 m/s
t=5s
v
 20  10  = 2000 N

1000


F = ma  m
 5 
t
4.4 A 60-g tennis ball approaches a racket at 15 m/s, is in contact
with the racket for 0.005 s, and then rebounds at 20 m/s. Find the
average force exerted by the racket.
m = 0.06 kg
vo = 15 m/s
t = 0.005 s
vf = - 20 m/s
v
F = ma m  0.06  20  15  = - 420 N
t
 0.005 
4.5 The brakes of a 1000-kg car exert 3000 N.
a. How long will it take the car to come to a stop from a velocity of
30 m/s?
m = 1000 kg
F  3000
F = -3000 N
2
a
=
3
m/s

vo = 30 m/s
1000
m
vf = 0 m/s
v f  vo 0  30
= 10 s

t
3
a
b. How far will the car travel during this time?
x = vot+½at2
= 30(10)+ ½ (-3)(10)2
= 150 m
4.7 A net horizontal force of 4000 N is applied to a car at rest whose
weight is 10,000 N. What will the car's speed be after 8 s?
Fa = 4000 N
Fg = 10,000 N
t = 8s
Fg
10000

m
9.8 = 1020.4 kg
g
F
4000
a
2

=
3.92
m/s
m 1020.4
vf = vo + at
= 0 +3.92(8)
= 3.14 m/s