Forces act on objects and can cause changes in motion or shape. Hooke's law states that the force needed to extend or compress a spring is proportional to the distance changed. Forces are vector quantities that combine according to geometric rules. Newton's three laws of motion describe how forces cause changes in motion: the first law states an object in motion stays in motion unless acted on by a net force, the second law relates net force, mass, and acceleration, and the third law states that every force induces an equal and opposite reaction force. Friction and tension are types of contact forces that influence motion. Dynamics problems apply Newton's laws to analyze forces and motion in physical situations.

1. FORCES
What are forces and how do they act

2.  Forces and Deformations: Hooke’s Law
 Forces as Vector Quantities
 First Newton’s Law of Motion (Inertia)
 Second Newton’s Law of Motion (F and Acceleration)
 Third Newton’s Law (Interaction)
 Weight and its Components (FWX and FWY)
 Friction and Tension
 Dynamics of Moving Objects
A Guide to Forces

3.  Forces acting on elastic objects produce changes in
their dimensions. The simpler case is when they only
modify an object’s length
 Consider a spring with a metallic object hanging from
it. Weight stretches the spring, so its length is longer
as the force is bigger. If we increase the amount of
objects hanging from the spring then it will become
longer and longer until a limit: elasticity ends when
the spring cannot hold it any more
 This fact is known as HOOKE’S LAW and is synthesized
in this equation: Fspring = −k·DL
Forces and Deformations

4. Hooke’s Law

5.  Forces are vector quantities. We will add them according
to geometric rules
 On the left vectors “a” and “b” add following the
parallelogram rule. Guess
 On the right the same vectors add according the polygon
rule. Guess
Forces as Vector Quantities

6.  According to Aristotle objects on the Earth surface
seek their natural resting places. Their motion can be
either a natural or a violent motion
 According to Galileo objects keep their motion until it
is compelled to change by a force. They can either
keep moving in a straight line or keep at rest
 Newton’s first law says:
EVERY OBJECT CONTINUES IN A STATE OF REST, OR OF
MOTION IN A STRAIGHT LINE AT CONSTANT SPEED,
UNLESS IT IS COMPELLED TO CHANGE THAT STATE BY
FORCES EXERCTED UPON IT
First Newton’s Law of Motion (Inertia)

8.  Mass is the quantity of matter of an object. More
specifically, mass is a measure of the inertia, or
“laziness”, that an object exhibits in response to any
effort made to start it, stop it, or otherwise change its
state of motion
 Weight is the force of gravity on an object
 All objects HAVE A MASS but only objects close to a
planet or star, for example, HAVE WEIGHT
Mass is not Weight

9. Massless? Weightless?

10. Second Newton’s Law of Motion
(Force and Acceleration)
 THE ACCELERATION PRODUCED BY A NET FORCE ON
AN OBJECT IS DIRECTLY PROPORTIONAL TO THE
MAGNITUDE OF THE NET FORCE, IS IN THE SAME
DIRECTION AS THE NET FORCE, AND IS INVERSELY
PROPORTIONAL TO THE MASS OF THE OBJECT
 In equation form:
a = F / m
IS Units: we use (N) newtons for force, (kg) kilograms for mass and
(m/s2) meters per second squared for acceleration

11. What force has the boy to push with?

12.  WHENEVER ONE OBJECT EXERTS A FORCE ON A
SECOND OBJECT, THE SECOND OBJECT EXERTS AN
EQUAL AND OPPOSITE FORCE ON THE FIRST OBJECT
FA = −FB
 Sometimes we say that object A exerts a force on
object B (action) while object B exerts a force on
object A (reaction) but, actually, both forces act at
the same time and we’d better say there is a
SIMULTANEOUS INTERACTION
Third Newton’s Law
(Interactions)

13. Interactions
Will the fish get it?
What will happen to their feet when they kick the ball?

14. Interactions
Can you kiss without being kissed?

15. Weight and its Components
(FWX and FWY)

16.  A force between surfaces in contact is called friction
 Fiction avoids objects moving against each other
 Friction acts in the opposite direction of moving objects
 Friction depends on normal force and on the roughness of
surfaces in contact
 Sometimes friction makes motion more difficult, but others it
helps: we couldn’t walk or run on the sidewalks without friction
Friction

17.  Forces inside chains or ropes and between objects
and chains are called tensions
 We don’t have a particular way of calculating
tensions, we just treat them as unknown and solve
the appropriate equation
Tension

18.  Solve some of these dynamic situations with the help
of Newton’s Laws
Dynamics of Moving Objects