Conceptual Physics
Hewitt, 1999
 Interaction-
mutual action of two objects
• Required for a force to occur
 You
can’t touch without being touched
 “Whenever
one object exerts a force on a
second object, the second exerts an equal
force in the opposite direction on the first.”
• FA on B = -FB on A
 Action
force- causes the force
 Reaction force- reacts to the force in an
equal but opposite way to the action force
 As
objects react to each other, actionreaction objects doesn’t really matter.
• Action: Object A exerts a force on Object B
• Reaction: Object B exerts a force on Object A
Mass and acceleration are inversely proportional
 Force and acceleration are directly proportional


Example: Mallet & Post
• Mallet: large mass, small acceleration
• Post: small mass, large acceleration
• Both have an equal but opposite force on them

Example: Bullet being fired
• http://www.youtube.com/watch?v=h3PUXbo3tCo&feature=related
• http://www.youtube.com/watch?v=f9tCpLNBwxQ&NR=1
• http://www.youtube.com/watch?v=xSkxQxPThi4&feature=related
 If
they cancelled, nothing would ever move!
 We need to consider what is in the “system”
• System- the area around the force(s) that we are
concerned with
• If we only care about a football accelerating, we call
the football the system
• If we care about how the football and the foot
interact, we can include them both in the system

What is pulling what?
• FGW- force of the ground on the wagon (friction)
• FWG- force of the wagon on the ground
• FHW- force of the horse on the wagon
• FWH- force of the wagon on the horse (inertia)
• FHG- force of the horse on the ground (applied force)
• FGH- force of the ground on the horse (friction)
 Review
 Think
Questions 1-18
& Explain 1-15