Transcript Ch 4 Newton`s Laws

Ch 4 Newton’s Laws
This is a “big” chapter
Newton’s Laws
• #1: Law of Inertia or Laziness
– Objects want to keep doing what they are
already doing. Objects at rest stay at rest or
objects moving at constant speed in a straight
line continue doing this unless an outside
thing (force ) acts on them.
Newton’s Laws
• #2: F = ma (or better yet, a = F/m)
– An object changes its motion (accelerates)
when a net unbalanced force acts. The
amount of change (acceleration) also
depends on the mass of the object
Newton’s Laws
• #3: Forces exist in pairs
– For every force there is an equal and opposite
force acting on another object.
– Forces are interactions between 2 things. Golf
ball/golf club, tire/road, etc.
Mass vs. Weight
• Mass (kg) is a measure of inertia. It’s a
property of the material which doesn’t
change whether you are in outer space or
on mars.
• Weight is a force which depends on the
gravitational pull (or gravitational
acceleration) Fg = mg
Common Forces
• Fg = mg ( weight)
• Fnormal = contact force perpendicular to
plane of contact
• Ffriction acts parallel to the plane of surface
contact.
• Frictional force can be static or kinetic.
• Ffk = u Fn ( u = coefficient of friction, a property of the
materials in contact)
Free Body Diagrams
• FBD’s are essential to solving problems
involving multiple forces.
• Isolate the object and draw arrows to
represent each separate force acting.
• Force Arrows typically point away from the
object and should be drawn approximately
to scale.
• FBD’s show forces but not direction of
motion.