Transcript Class11

Class 11 - Force and Motion I
Chapter 5 - Friday September 17th
•Some particular forces
•Newton's third law
•Sample problems
Reading: pages 87 thru 107 (chapter 5) in HRW
Read and understand the sample problems
Assigned problems from chapter 5:
24, 26, 34, 38, 44, 46, 52, 56 (due Sun. Sept. 26)
Announcement:
Physics tutoring center in NPB 1100
•Mon/Wed
•Tues/Thurs
•Fri
periods 4 to 8
periods 4,5,8
periods 4 to 7
Review
Newton's 1st law: If no force acts on a body, then
the body's velocity cannot change; that is, it cannot
accelerate.
1 Newton is that force required to accelerate our
standardized mass (1 Kg) at a rate of 1 m.s-2.
Mass is simply the characteristic of a body that relates
a force on the body to the resulting acceleration
Newton's 2nd law:
Fnet   Fi  ma
i
F1
Free-body
diagrams
F1
S
F3
F2
F2
F3
F  0  a
Gravity:
Some particular forces
•During free fall
a   g ĵ

F  ma  mg ĵ
•Even when a mass is stationary on the surface of a table,
gravity still acts downwards with a magnitude equal to mg.
•This leads to the concept of a normal or contact force:
for the mass on the table to remain stationary, the table
must exert an upward force on the mass so as to exactly
balance the force due to gravity.
N  Fg  ma
Normal force
N  Fg  ma y
N  ma y  Fg  m  a y  g 
N  Fg  mg if a y  0
Weight (a force!):
•In the above example, the internal forces within the table
supply the normal force, which is normal to the surface.
•If we hold the mass in a stationary state, we must supply
the force. This is the sensation of weight, i.e.
W  Fg  mg Newtons
Friction and tension
•We will deal with friction next week
(chapter 6). Friction is a force that
acts parallel to a surface in the
opposite direction to any motion.
•A taut cord is said to be in a state
of tension.
•If the body pulling on the cord does
so with a force of 50 N, then the
tension in the cord is 50 N.
•A taut cord pulls on objects at either end with equal and
opposite force equal to the tension.
•Cords are massless, pulleys are massless and frictionless
Newton's 3rd law
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
For every "action" force, there is always an equal
and opposite "reaction" force; we call these a
"third-law force pair."
Newton's 3rd law
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
FBEg  FBTn  aB  0
FBTn
FBTn   FBEg
FBEg
Newton's 3rd law
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
FTEn
FTEn 
FTEg
( FTEg  FBEg )
FTBn  FBEg
Newton's 3rd law
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
Why doesn't the earth accelerate?
FETn   FTEn  FBEg  FTEg
Newton's 3rd law
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
FEBg  FETg 

 FBEg  FTEg

F  0  a
FETn   FTEn  FBEg  FTEg