6.1 Forces and Interaction Interaction- mutual action of two
objects• Required for a force to occur
You can’t touch without being touched
6.2 Newton’s Third Law “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
6.3 Identifying Action and Reaction
As objects react to each other, action-reaction objects doesn’t really matter.• Action: Object A exerts a force on Object B• Reaction: Object B exerts a force on Object A
6.4 Action and Reaction on Different Masses
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=r
elated• http://www.youtube.com/watch?v=f9tCpLNBwxQ&NR=1• http://www.youtube.com/watch?v=xSkxQxPThi4&feature=re
lated
6.5 Do Action and Reaction Forces Cancel?
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
6.6 Horse-Cart Problem 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)