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Every action has an equal and
opposite reaction.
Action and Reaction Pairs
Feet impact boardBoard impacts feet
Bedouin pulls sheepSheep pulls bedouin
Fist hits neckNeck hits fist
Wheels push roadRoad pushes wheels
Action and Reaction on Different Masses
am
FBullet :am
FRifle :
Consider the recoil of a rifle…
Same Force
amF
JACOBS : am
FSafety :
Same force
Action and Reaction on Different Masses
Consider the launch of a space shuttle…
The shuttle continually recoils against the expelled gas from the rockets. No atmosphere is needed for this interaction.
Do Action and Reaction Forces Cancel?It depends on context…
If the car is the system, then the man is an external force acting against it. This action reaction does not cancel out. The external force of the road against the man accelerates the system.
BUT…
If the car and the man are the system, then the force pair is internal. This action reaction does cancel out.
Force and Friction
The Rickshaw Problem
Pull of man on rickshaw Pull of rickshaw on man
Push of man on ground
Push of ground on manFriction b/w cart wheels and ground
Acceleration is due to net force
The Rickshaw Problem
Net force on the rickshaw?
Push of ground on man - Pull of rickshaw on man
Push of man on ground – friction of ground on wheels
Pull of man – friction of wheels on ground
Net force on the man?
Net force on the ground?