1. 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.

2. Newton’s second Law
Fnet = ma
[kg m/s2 = 1N]
To understand net forces, we need to know free-body diagrams.

3. Normal Force

4. Q1
The weight of the box is 70kg and the pulling force is 500N, the normal force will be about
200N pointing up;
200N pointing down;
1200N pointing down;
0N.

5. 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.

6. 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.

7. Weight of a person in an elevator

8. Friction forces

9. Coefficients of friction
Static:
S= max(fs)/n fs   S n
Kinetic:
K= fk /n fk =  K n,
 S >  K

10. Q3
See page 114, “Stop to think 4.3”

11. 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?
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.

12. 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.

13. Interacting objects (textbook, page 121 more on the 3rd law)