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Net Forces, Friction, Coefficients of Friction. Newton’s 3rd Law. Sliding Book Example. Why do things not continue to move at constant velocity? If the sliding book slows down, what’s the force responsible? How could I keep it moving at a constant velocity?

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sliding book example
Sliding Book Example
  • Why do things not continue to move at constant velocity?
  • If the sliding book slows down, what’s the force responsible?
    • How could I keep it moving at a constant velocity?
    • Do I need to apply a force to keep it moving? Why?



A net force of zero

No, Inertia keeps the object moving!

forces are vectors so directions are important

Total Force

Force #1

Force #2

Force #1

Force #2

Forces are Vectors so Directions are Important

Total Force = 0

Forces Add

Net Forces = 0 Newtons

newton s 3 rd law
Newton’s 3rd Law

For every action , there is an equal and opposite reaction.

Force on Newton by Einstein = Force on Einstein by Newton

Newton tries to outsmart Einstein

by loading his own cart with lead bricks)

friction opposes motion

Forceon person

by box

Force on box

by floor

Force on box

by person

Force on floor by box

Friction opposes Motion


What kind of motion is created by Unbalanced Net Force?

It’s thesum of all the forces that determines the type of motion.

friction due to the surface
Friction due to the Surface

How does the corrugated surface change Friction?

Corrugations in the surfaces grind when things slide.

Lubricants fill in the gaps and let things slide more easily.

why doesn t gravity make the box fall

Force of Floor acting on Box

Force of Earth acting on Box (weight)

Why Doesn’t Gravity Make the Box Fall?

Force from floor on box

and gravity = Net Force of zero.

If the floor vanished, the

box would begin to fall.

what s forces are not shown

Forceon person

by box

Force on box

by floor

Force on box

by person

Force on floor by box

What’s Forces are not shown?

The Normal Force and the Force of Gravity

When we drew the box and floor, with the “normal” force and the force of gravity, these weren’t strictly force pairs
    • forces on the box that result in a Net zero acceleration of the box
  • The real pairs have to involve the earth:



  • Force Pairs:
    • earth-box (grav)
    • box-floor (contact)
    • earth-satellite (grav)





  • Does friction always exert a force that tends to bring things to a halt?
  • What does this say about the direction of the frictional force, relative to the velocity vector?
  • What do you think would happen if we loaded lead bricks into the box? Would it become harder to slide?
  • What are some ways to reduce frictional forces?

Friction vector is opposite in direction of motion


Lubrication, change surface,

reduce normal force

static and sliding dynamic friction
Static and Sliding (Dynamic) Friction
  • Static frictional force: when nothing is sliding
  • Sliding frictional force: when surfaces are sliding
  • Static frictional forces always greater than sliding ones
normal forces and frictional forces

Reaction Force

From Ramp





Decompose Vector

Weight of block

Weight of block

“Normal” Forces and Frictional Forces

“Normal” means


Friction Force = Normal Force  (coefficient of friction)

Ffriction = Fnormal

stay on the road
Stay on the road!
  • What does it take to stay on the road around a curve?
    • using s = 0.8 as average for tires on road, Ffriction = 0.8mg
      • (Normal force is just mg on level surface)

Fcurve = macurve = mv2/r

      • where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp)
  • Got enough friction if Fcurve < Ffriction
    • happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h.
air resistance
Air Resistance
  • We’re always “neglecting air resistance” in physics
    • Can be difficult to deal with
  • Affects projectile motion
    • Friction force opposes velocity through medium
    • Imposes horizontal force, additional vertical forces
    • Terminal velocity for falling objects
  • Dominant energy drain on cars, bicyclists, planes
  • Every force has an equal, opposing force
  • Friction opposes motion, requiring continued application of force to maintain constant velocity
  • Air resistance produces terminal velocity, alters trajectories of projectiles (for the worse).