3
• Vector sum of all
the forces acting on the object is called
the net force.
i.e. both magnitude and direction needs
to be considered
What is the net force on the car ?
4
Inertial Reference Frame
Newton’s laws are valid in an inertial frame, i.e. the acceleration of the frame should be
zero
Constant Velocity .
Ideally we do not have such a frame, but earth is a very good approximation.
5
Newton’s Second Law
• First law: no net force no change in velocity
• Second law: What happens if there is an external force? net
Acceleration of the object Net Force
Acceleration inversely proportional to mass
m 1
6
Only two factors determine the acceleration---
Force and Mass
Force Units Newton N = kg. ms
2
What is 1 Newton ?
m F
a
ma
F
or
7
Newton’s Second Law Of Motion
When a net external force acts on an object of mass m, the acceleration a results is directly
proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of
the acceleration is the same as the direction of the net force.
F
ma F
or m
F a
SI Unit of Force: kg.ms
2
= newton N
8
Units for Mass, Acceleration Force
System Mass Acceleration
Force
SI
kilogramkg metersecond
2
ms
2
newtonN
CGS
gramg centimetersescond
2
cms
2
dynedyn
BE
slugsl footsecond
2
fts
2
poundlb
9
Frictional force = 560 N Push = 275 N+ 395 N = 670 N
Net force = 670 N – 560 N, N= 110 N F = ma , 110 N = 1850 kg
a a = 110 N1850 kg = 0.059 ms
2
what is the direction ?
10
Forces In 2-D
F
x
= ma
x
F
y
= ma
y
Two forces are applied onto an object. 1. 12N towards North, and 2. 5N towards East, what is the direction of
the motion of the object?
tan θ = 125 =2.4
θ = tan
-1
125 = 67.38
12N
5N
θ
Resultant force
=
13 5
12
2 2
11
1300 kg raft P= Force by the man
A= Force of wind In 65 seconds,
where will the boat be if v
0x
= 0.15 ms ?
a
a
x
= ?, a
y
= ?
d
12
Force X- component
Y- Component P
17 N 0 N
A 15cos67 N
15sin67 N
Resultant
, 23
67 cos
15 17
N
F
x
, 018
. 1300
23
2
s m
kg N
m F
a
x x
N
F
y
14 67
sin 15
2
011 .
1300 14
s m
kg N
m F
a
y y
13
Displacement Of The Boat In 65 sec
y
=
v
0y
t + ½ a
y
t
2
= 0 65 + ½
0.011 65
2
= 23 m For x: v
0x
= 0.15 ms, a
x
= 0.018 ms
2
t = 65 sec, x = ?
x = v
0x
t + ½a
x
t
2
= 0.15 65 +12
0.018 65
2
= 48 m
For y: v
0y
= 0, a
y
= 0.011 ms
2
, t = 65 sec
14
acceleration to be doubled ?
a All of the forces acting on the object
doubles
b The net force acting on the object doubles
c Both the net force and the mass doubles d The mass of the object is reduced by a
factor of two.
a, b, d
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