Every action has an equal & opposite reaction.
You cannot touch without being
touched back.
Newton’
Newton’s Third Law of Motion
PH 104 w/ dr. g
Lec 6
y Newton’s Third Law of Motion:
When an object exerts a force on a second object, the second
object exerts an equal and opposite force on the first.
Examples:
• Weight and support force
• Pushing against a wall
• Walking!
• Apple being pulled down by Earth
• Touching!
• Compare: wall versus curtain
Sir Isaac Newton (1642-1727)
Every action has an equal & opposite reaction. =
Forces come in pairs.
More Examples:
• Weight and support force
• Pushing against a wall
• Throwing a ball
• Walking!
• Apple being pulled down by Earth
• Touching!
• Compare: wall versus curtain
Every action has an equal & opposite reaction.
Force: h --> c
Force: c --> h
Force: g --> h
Force: h --> g
y Question: How can anything accelerate?
y Important: define your “system” of interest
y ANSWER: a pair’s forces are on different objects.
y Each object can still have a nonzero net force.
y More Examples:
y Shooting and recoil : projectile:
y Rocket motion : Can a rocket propel itself through a vacuum?
y Back to first spacewalk … and pushing a box w/o friction
Net force, when forces are not parallel:
Forces ADD, using the parallelogram rule.
y Parallel / antiparallel forces: add or subtract to get net.
y Vectors at angles (not 0o or 180o): parallelogram rule:
y Example: forces at right angles to each other
y Forces form corner of parallelogram
y Net force = diagonal of parallelogram.
y Two forces: add up using parallelogram rule.
y Velocities: two also add up like forces do:
y Recall: person in river: ground speed = person + river
(parallel velocities)
y Two forces: add up using parallelogram rule.
y Velocities: two also add up like forces do:
Newton’s Third Law:
You cannot touch without being touched back.
y Recall: person in river: ground speed = person + river
(parallel velocities)
y Forces come in pairs.
y Velocities not parallel: use parallelogram rule too!
y Every action has an equal & opposite reaction.
y Quantities that add up using plgram rule = VECTORS
y Vectors add using the parallelogram rule; and
y Force and velocity are vectors.
y Force and velocity are vector quantities.