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

Q1The weight of the box is 70kg and the pulling

force is 500N, the normal force will be about

1) 200N pointing up;

2) 200N pointing down;

3) 1200N pointing down;

4) 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 fs S n

Kinetic:

K= fk /n fk = K n,

S > K

Q3See 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)