The Laws of Motion

1
Chapter 4

• The Laws of Motion

2
Classical Mechanics

• We will deal with objects larger than atoms
• (10 -10m) and those that travel slower than that
  of the speed of light (3 x 10 8 m/s)
• Quantum mechanics deals with the very small and
  very fast.

3
The Concept of Force

• Push or pull on an object
• Anything that has the ability to change motion.
• Vector quantity
• Force (F) units are Newtons (N)

4
Contact Forces

• Physical contact between two objects.
• Ex. Pull on a string, kick a ball, stretch a
  spring, etc.

5
Field Forces

• Do not involve physical contact between the
  objects.
• Ex. Gravitational force

6
Fundamental Forces in Nature

• Strong Force
• Responsible for the tight binding of quarks to
  form neutrons and protons. It is the strongest of
  the fundamental forces.

7

• Electromagnetic Force
• Responsible for the binding of atoms and
  molecules.

8

• Weak Force
• Responsible for beta decay. It is now believed
  that the electromagnetic and weak forces are
  manifestations of a single force called the
  electroweak force.

9

• Gravitational Force
• Responsible for holding the planets, stars and
  galaxies together, its effect on elementary
  particles is negligible. It is the weakest of the
  fundamental forces.

10
Newtons 1st law

• Before 1600, scientists felt the natural state of
  motion for an object was that of rest.
• Galileo was the first to state that the natural
  state of motion was to resist acceleration.
• Newton later formalized this as the 1st Law of
  Motion.

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1st Law

• An object at rest remains at rest, an object in
  motion remains in motion with constant velocity
  unless acted upon by a net external force.
• When the net external force on an object is zero,
  its acceleration is zero.

12
Mass and Inertia

• Inertia - the tendency of an object to resist a
  change in motion.
• Mass - a quantity of matter.
• Mass is a measurement of inertia.

13
Newtons 2nd law

• The acceleration of an object is directly
  proportional to the net force acting on it and
  inversely proportional to its mass.
• F ma
• w mg

14
Newtons 3rd Law

• For every action there is an equal and opposite
  reaction.
• Forces always occur in pairs.
• A single isolated force can not exist.

15
Normal Force

• Normal force - force perpendicular to the surface
  the object is on.
• Variable (n)
• Units (N)

16
Applications of Newtons Laws

• We must assume
• Constant external forces.
• Objects behave as particles.
• We are neglecting friction.
• We can neglect the mass of ropes and strings.
• We are interested only in external forces that
  act on the object.

17
Free Body Diagrams
18
Ex. Problems

• Equilibrium
• Net force 0.
• Forces in x and y direction are in equilibrium
  and equal to zero!

19
Friction

• Force of friction - force arising from the
  contact between two or more objects.
• Static friction - stationary friction ( fs )
• Kinetic friction - sliding friction ( fk )

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Friction

• fs lt msn
• ms is the coefficient of static friction.
• fk mkn
• mk is the coefficient of kinetic friction.
• The values of mk and ms depend on the nature of
  the surfaces in contact, in general mk lt ms.
• The coefficients of friction are nearly
  independent of the area of contact between the
  surfaces.

21
Measuring u

• us tan?c
• Quick lab