Download Review: Newton`s second Law

Review: Newton’s second Law

a

F
m
or


 F  ma
Solving Problems:
(1)Isolate the object of interest
(2)Sketch forces acting on object – indentify all the
external forces acting on an object.
(3) Choose coordinate system (x &y)
(4) Resolve into components
Apply 2nd law to each components.
In components
Fy  ma y
Fx  ma x



F2  16 N
Ex.

F1  12 N
m=2kg

F2  16 N
m=2kg
Find the acceleration

F1  12 N
5-4 Newton’s third law of motion

F2

F1

F2

F1
force hand exerts on box
force box exerts on hand

F2


F1   F2

F1
Canoe Problem
2
1
m1=150kg
46N
m2=250kg
(a) Find acceleration of each canoe? (b) what
is the separation of canoes after 1.2 s?
Ex.
2 boxes 10kg & 5kg, both are pushed in +x direction
with a force of 20N, what is the force of contact?
20N
F21
10kg
5kg
F12
5-5 The Vector Nature of Forces:
Forces in Two Dimensions

F1
θ

F2
Review:
F1x=F1cosθ
F1y=F1sinθ
F
x
 F1x  F2 x  F1 cos  F2
F2x=F2
F2y=0
F
y
 F1y  F2 y  F1 sin 
Ex.
Say dog 1 wants to go 320 north of east 12N,
and dog 2 goes east 16N. Find magnitude and
direction of accelerate?
y
F1=12N
F2=16N
m=2kg
x
5-6 Weight
Weight: force exerted on object by gravitational
pull of earth.

W
W=mg
F
y
y
ay =g
Unit: N
 ma y
Ex.
81 kg person has a weight ?
Ex.
What is the acceleration of the mass?
F=200N
8kg
5-7 Normal forces
Normal

force: N ;
force exerted by table on object
y
 
W N
Problem:
(a)What is the normal force exerted on the mass?
(b)Find the acceleration of the mass?
8N
300
10kg
Ice Block Ex 5-8
F1=13N; F2=11N; m=6kg
(a)Find the acceleration of ice box?
(b)Find the normal force?
F1
F2
600
300
Apparent weight
m=80kg
what is the reading on the scale of elevator is
going up at 3.2 m/s2
Ex.
Sliding down an incline, mass m, (neglect
friction), find acceleration and normal force?
y
N
x
W
α