Title: Physics 108 lecture 4: Forces and Newtons Laws
1Physics 108 lecture 4 Forces and Newtons
Laws
- Forces make things happen. They move mountains,
cars, your pencil, and subatomic particles.
Below is a classification of common terms for
forces.
Types of forces
- gravitational  attraction between any objects
having mass - strong nuclear  holds together protons and
neutrons in the nucleus - weak nuclear  governs radioactive decay
- electric  repulsion of like charges or
attraction of opposite charges - magnetic  repulsion of like poles or attraction
of opposite poles - friction  resists sliding motion across surfaces
- tension  acts through ropes, pulleys, and
cables - centripetal  keeps an object in circular motion
from flying off into space - centrifugal  not a real force, but the outward
motion felt during circular motion - push  acts to move objects apart
- pull  acts to move objects together
- normal  force perpendicular to a surface that
prevents objects from falling through it
2Newton's First Law of Motion (the law of inertia)
- Objects will remain either at rest or in uniform
motion in a straight line unless acted on by an
external, unbalanced force. - This law means objects keep on doing what they
are doing if undisturbed. - Â Â Â Â Â Â Â An object at rest has a _____________
velocity of zero.       An object in uniform
motion has a ______________, non-zero velocity. - Â Â Â Â Â Â An ______________force is applied from
outside an object or system.          An
______________force on an object is one that is
not opposed            by an equal and opposite
force acting on that same object. -        Force, two equivalent descriptions    Â
          A force is a quantity that is capable
of producing a change in motion.              Â
A force is a quantity that is capable of
producing an acceleration. - Inertia - the ______________ of an object to
changes in its motion.            Mass is a
measure of an object's inertia.Note Momentum
is NOT the same as inertia. Momentum comes from
the combination of an object's mass and
velocity. Inertia comes only from mass.
3Newton's Second Law of Motion (the force law)
- The acceleration of an object is directly
proportional to the force applied toit and
inversely proportional to its mass. - This law means increasing forces on objects give
increasing accelerationswhile increasing masses
give decreasing accelerations. - Newton's second law of motion states two
facts   1) The acceleration produced by an
unbalanced force is directly proportional       Â
   to the magnitude of the that force (more
force --gt more acceleration).   2) The
acceleration produced by an unbalanced force is
inversely proportional           to the mass of
the object being accelerated (more mass --gt less
acceleration). - FORCEÂ Â Â Â Â Â Â Â Â Â Â Â force(mass)x(acceleration)Â Â Â
         F ma              Â
or                       a F/m       The
unit of force is the Newton N 1 N 1 kg
x 1 m/s2 1 kgm/s2 - Â Â Â Â Â Â Â The force applied to an object due to
gravity is expressed as its ____________. - WEIGHTÂ Â Â Â Â Â Â weight (mass) x (acceleration of
gravity)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â W mg - Mass and weight are related, but they are not the
same. Because the acceleration of gravity is
different on the earth and on the moon,an
object's weight will be different on the earth or
on the moon.
4Example (force)Â You are able to push a portable
bed with a patient lying on it with a force of
130N. After 5 seconds the velocity of the bed is
6.5m/s. What is the mass of the patient and bed
together?
Given                   Path                 Â
Want          Conversions/Equations
5Example (weight)Â What is the weight of the
patient and bed in the example above?
Given                   Path                 Â
Want          Conversions/Equations
6Example (force) Which is bigger, the force
needed to accelerate a 1550kg truck at 1.2m/s2
(would give you 0 to 60mph in 22s) or the force
needed to lift the 300.kg Africa Stone in the
Worlds Strongest Man competition?
Given                   Path                 Â
Want          Conversions/Equations
7Newton's Third Law of Motion (the force pair law)
- Whenever one object exerts a force on a second
object, the second objectexerts an equal and
opposite force on the first object. - This law means that forces always come in pairs.
- F1 -F2
- or
- m1a1 -m2a2
- The opposing forces in Newton's third law are
called _________________________ pairs.
8Example Two 85kg employees stock boxes on
rolling ladders of mass 70.kg. One throws a 10kg
box with acceleration 5.0m/s2 towards the other,
but did not lock the ladder wheels. a) How fast
will the employee/ladder accelerate backwards?
b) If the ladder moves 2m/s after the throw and
slows down due to friction at 1m/s2, how far will
it move before it comes to rest?
Given                   Path                 Â
Want          Conversions/Equations
9Force Components
Forces are sometimes applied at angles other than
the direction of motion. This leads to the need
for calculating force components using
trigonometry.