Transcript Lecture 23 and 24, Oct 29, Oct 31, 2008

Newton's First Law of Motion
An object will remain at rest or move in a
straight line at a constant speed if there
are no forces acting on it.
= Uniform motion with constant velocity is
The natural state of an object.
Newton’s second Law
Fnet = ma
[kg m/s2 = 1N]
To understand net forces, we need to
know free-body diagrams.
Normal Force
Q1
The weight of the box is 70kg and the pulling
force is 500N, the normal force will be
about
1)
2)
3)
4)
200N pointing up;
200N pointing down;
1200N pointing down;
0N.
Q2
Adam stands in an elevator that is
accelerating upward. The normal force acting
him is
A) larger than 500N;
B) smaller than 500N;
C) equal to 500N;
D) Not enough information.
Newton’s third law
Forces always come in action/reaction pairs
following the 3rd law:
1) Two members act on different objects;
2) They point in opposite directions;
and are equal in magnitude.
Weight of a person in an elevator
Friction forces
Coefficients of friction
Static:
 S= max(fs)/n
Kinetic:
fs   S n
 K= fk /n
fk =  K n,
S>K
Q3
See page 114, “Stop to think 4.3”
Q4
You are pressing a book (m=1kg) against a
wall. The static friction coefficient is 0.4.
Which of the following is true for the friction
on the book?
A) The friction force is pointing downward;
B) The friction force is pointing upward;
C) The friction force is pointing away from the
wall;
D) The friction force is pointing into the wall.
Q5
You see that a box is at rest on the top of
a car while the car is speeding up. What
is the direction of friction force acting on
the box?
A) forward;
B) backward;
C) zero;
D) depending on the velocity of the car.
Interacting objects
(textbook, page 121 more on the 3rd law)