Misconceptions — Laws of Motion & Friction — Notes

#	Misconception	Reality	Example to Clarify
1	"A moving body needs a continuous force to keep moving"	Newton's First Law: once in motion, no force needed to maintain constant velocity	A hockey puck sliding on ice (low friction) continues indefinitely
2	"Normal force and weight are an action-reaction pair"	Both act on the SAME body. A-R pairs act on different bodies	True pair: Earth pulls you down (weight), you pull Earth up (equal force)
3	"Heavier objects fall faster"	All objects fall with the same acceleration g in vacuum (First law of free fall)	Feather and hammer in vacuum fall together (Apollo 15 demonstration)
4	"Friction always opposes motion"	Static friction opposes the TENDENCY of motion, not motion itself	Static friction on a block at rest can act in any direction needed
5	"Static friction is always μ_s N"	$f_s$ is self-adjusting from 0 to μ_s N. Only equals μ_s N at the limiting condition	5 N push on a 60 N friction-capacity block → friction = 5 N, not 60 N
6	"Centripetal force is a separate force in the FBD"	Centripetal force is provided BY real forces (friction, tension, gravity). Never draw it as a separate arrow	On a banked road: N cos θ and N sin θ components together provide centripetal force
7	"In free fall, weight becomes zero"	Weight (gravitational force mg) still acts. It is APPARENT weight that becomes zero	An astronaut in orbit still has weight — they're in constant free fall
8	"Friction depends on the area of contact"	For macroscopic surfaces, friction force does NOT depend on contact area (Amontons' Law)	A wide brick and a narrow brick of same mass have the same friction force
9	"Kinetic friction is greater than static friction"	Always: μ_s > μ_k. It takes more force to start sliding than to sustain it	This is why you push hard initially, then less force to keep sliding
10	"The Atwood machine tension equals the weight of one mass"	T = 2 $m_{1}$$m_{2}$$\frac{g}{m_{1}+m_{2}}$ , which is less than $m_{1}$ g and greater than $m_{2}$ g	If $m_{1}$ = $m_{2}$ , T = mg (each mass is fully supported, a = 0) ✓