physics 1710 chapter 5: laws of motion—iii

11′′ Lecture Lecture• Newton’s Laws of Motion are:Newton’s Laws of Motion are:

Acceleration (or deceleration) occurs if Acceleration (or deceleration) occurs if and and only if there is a net external only if there is a net external forceforce..

a a = = FF//mm [Note this is a vector eqn.][Note this is a vector eqn.]

The force exerted by a first object on The force exerted by a first object on a a second is second is always always equal and equal and opposite the opposite the the force exerted by the the force exerted by the second on the second on the first. first. FF1212 = = - F- F2121

Physics 1710Physics 1710 Chapter 5: Laws of Chapter 5: Laws of Motion—III Motion—III

11′′ Lecture – Lecture –continued (30continued (30″ ″ more)more)

• Weight is the force of gravity equal to Weight is the force of gravity equal to gg times the mass of the object.times the mass of the object.

• gg =9.80 =9.80 N/kgN/kg

• The force of friction is opposed to the The force of friction is opposed to the motion of a body and proportional to the motion of a body and proportional to the normal forcenormal force..

• Free body diagrams are sketches of all the Free body diagrams are sketches of all the forces acting on a body.forces acting on a body.


Laws of MotionLaws of Motion

11stst Law: Law:An acceleration is caused by a net An acceleration is caused by a net external force.external force.


REVIEWREVIEW

22ndnd Law of Motion: Law of Motion:

aa= = FF /m /mNote the vector nature of the Note the vector nature of the equation:equation:

aaxx= F= Fxx /m /m

aayy= F= Fyy /m /m


REVIEWREVIEW

Third Law of Motion:Third Law of Motion:

If two objects interact, If two objects interact, the force of the 1the force of the 1stst on the 2 on the 2ndnd is is equal equal and and oppositeopposite to the force of the 2to the force of the 2ndnd acting on the 1st body. acting on the 1st body.

FF1212 = - = - FF2121


FF1212

FF2121 ##11

##22

33rdrd Law in Action—Tug of War Law in Action—Tug of War


??

It’s the Law!It’s the Law!FF1212 = - = - FF2121

#1#1#2#2

FF1212FF2121

Third Law of Motion, Third Law of Motion, Implications Implications ::

Baron von MunchenhausenBaron von Munchenhausen

Draw “free body diagram”Draw “free body diagram”


Lou GeelsLou GeelsAs Baron von As Baron von

MunchenhausenMunchenhausen

Arm force Arm force ==

- Reaction - Reaction forceforce

Net force = 0Net force = 0FF1212 = - = - FF2121

1. The tree is not pushing on the elephant.

2. The tree is pushing back, but the elephant is pushing harder.

3. The tree is pushing harder on the elephant that she is pushing on the tree.

4. The tree and elephant push equally against each other.


1. The tree is not pushing on the elephant.

2. The tree is pushing back, but the elephant is pushing harder.

3. The tree is pushing harder on the elephant that she is pushing on the tree.

4. The tree and elephant push equally against each other.

But how can you have acceleration in this But how can you have acceleration in this case?case?

The acceleration of object #2 isThe acceleration of object #2 isaa22 = F = F12 12 /m/m22

Not Not 0 0 = ( = ( FF12 12 +F+F21 21 )/(m)/(m11 + m + m22))


Tug of warTug of war

Draw “free body diagram”Draw “free body diagram”



Tug of war :“free body diagram”Tug of war :“free body diagram”

mmMM

Rope tensionRope tensionFF1212 FF2121

--mgmg

-Mg-Mg

Floor reaction: Floor reaction: mg mg Mg Mg Friction:Friction:

- -μ (mg)μ (mg)

Friction:Friction: μ (Mg)μ (Mg)

WeightWeightResultant: Resultant: FFmm = F = F1212 – – μ(mg)μ(mg)

FFMM = 0 = 0



• Why does mud fly backward Why does mud fly backward from the tire when an automobile from the tire when an automobile accelerates in mud?accelerates in mud?• For each “action” and equal but For each “action” and equal but “opposite action” must happen.“opposite action” must happen.


22ndnd and 3 and 3rdrd Laws of Motion: Laws of Motion: ImplicationsImplications

Why does air have “resistance?”Why does air have “resistance?”


- gm- gm

FFdragdrag- gm- gm

FFdragdrag

No Talking!No Talking!

Think!Think!Confer!Confer!

Peer Instruction Peer Instruction TimeTime


22ndnd and 3 and 3rdrd Laws of Motion: Laws of Motion: ImplicationsImplications

Why does air have “resistance?”Why does air have “resistance?”


- gm- gm

FFdragdrag

ΔmΔmairair = ρAv Δt= ρAv Δt

aaairair = v/ = v/ ΔtΔt

FFair, maxair, max = = ΔmΔmairair aaairair = = (ρAv Δt)( (ρAv Δt)(v/ v/ Δt) = ρAv Δt) = ρAv

22 FFairair = ½ D = ½ D ρAv ρAv 22 = - F = - Fdragdrag

- gm- gm

FFdragdragInertia of the air in Inertia of the air in

the waythe way

• Why do objects stop moving?Why do objects stop moving?

• Friction is a force that resists Friction is a force that resists motion.motion.•Friction is proportional to the Friction is proportional to the “normal” force “normal” force (ie perpendicular force) and opposed (ie perpendicular force) and opposed to the to the direction of the applied force.direction of the applied force.


FFf f = - = - μμN, N, in opposite direction of applied force.in opposite direction of applied force.

μμ is the coefficient of friction, which is the coefficient of friction, whichdepends upon the nature of the two surfaces in depends upon the nature of the two surfaces in

contact and the state of motion. contact and the state of motion.


Empirical Observation:Empirical Observation:

Friction is caused by “welding” of Friction is caused by “welding” of the two surfaces.the two surfaces.

Lubrication changes the nature of Lubrication changes the nature of the contact and/or the surface, the contact and/or the surface,

atomically. atomically.


Friction is caused by “welding” of the Friction is caused by “welding” of the two surfaces.two surfaces.


WeldsWelds

Effect of Normal Force—more contact.Effect of Normal Force—more contact.


NNNN

More contact More contact pointspoints

FFf ,maxf ,max = = μNμN

Friction is caused by “welding” of the Friction is caused by “welding” of the two surfaces.two surfaces.


Lubrication changes the nature of Lubrication changes the nature of the contact and/or the surface, the contact and/or the surface,

atomically.atomically.

Rolling Friction:Rolling Friction:


FFff = = μNμN N N Normal Normal ForceForce

AdhesionAdhesion

Rolling Friction: LubricationRolling Friction: Lubrication


FFff = = μNμN N N Normal Normal ForceForce

No AdhesionNo Adhesion

No Talking!No Talking!Think!Think! Confer!Confer!

Peer Instruction Peer Instruction TimeTime


Friction, Friction, ApplicationApplication::

• For constant velocity F = -FFor constant velocity F = -Fff = -(- = -(- μ N)μ N)• N = m g = (5.2 kg)(9.8 N/kg) = N = m g = (5.2 kg)(9.8 N/kg) = 51. N51. N• F = (0.30) (51.F = (0.30) (51. N) = 15. N N) = 15. N



• A father (100 kg) and his A father (100 kg) and his daughter daughter (50 kg) are ice skating. The push (50 kg) are ice skating. The push off from each other. Who pushes off from each other. Who pushes harder?harder?

• What is there relative What is there relative acceleration? acceleration?


MM mmMM mm

FFDadDad FFkiki

dd


• A father (100 kg) and his daughter A father (100 kg) and his daughter (50 kg) are ice skating. The push off from each (50 kg) are ice skating. The push off from each other. Who pushes harder?other. Who pushes harder?


MM mmMM mm

-F-F FF


• A father (100 kg) and his daughter A father (100 kg) and his daughter (50 kg) are ice skating. The push off from each (50 kg) are ice skating. The push off from each other. Who pushes harder?other. Who pushes harder?


aaMM =-F/M =-F/M aamm =F/m =F/m

33rdrd Law of Motion says Law of Motion says they push the same and they push the same and

opposite.opposite.

Summary:Summary:• Newton’s Laws of Motion are:Newton’s Laws of Motion are:

(1) Acceleration (or deceleration) occurs if (1) Acceleration (or deceleration) occurs if and only if there is a net external and only if there is a net external forceforce..

(2)(2) a a = = FF//m m [Note this is a vector eqn.][Note this is a vector eqn.]

(3) The force exerted by a first object on a (3) The force exerted by a first object on a second is second is always always equal and opposite the the equal and opposite the the force exerted by the second on the first. force exerted by the second on the first. FF1212 = = - F- F2121


Summary (cont’d.) :Summary (cont’d.) :

• Weight is the force of gravity equal to Weight is the force of gravity equal to gg times times the mass of the object.the mass of the object.

• gg =9.80 =9.80 N/kgN/kg

• The force of friction is opposed to the motion The force of friction is opposed to the motion of a body and proportional to the of a body and proportional to the normal normal forceforce..

• Free body diagrams are sketches of all the Free body diagrams are sketches of all the forces acting on a bodyforces acting on a body..


physics 1710 chapter 5: laws of motion—iii

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