__FRICTION NOTES__

WHAT IS FRICTION?

What are the types of friction?

*Starting (static), Sliding (kinetic), rolling, air,
etc*

What causes it?

*Friction is the large scale representation of the
electromagnetic force caused by the interaction between molecules of two
surfaces. It is a force that always acts opposite the direction of motion.
(opposite velocity, not necessarily A or F).*

Always depends on *the normal force (weight) or the
push between the two surfaces, and also on all the molecular stuff: density, molecular
size, stickiness, attractiveness, texture, etc..*

Which type of friction is strongest?

*Static friction is always higher than kinetic
friction.*

How do we measure friction?

F_{f}=
m F_{N}

**Force of friction can be measured by pushing/pulling
something at a constant velocity so Fnet = 0, then Ffriction = - Force push or
pull**

What is the coefficicnt of friction?

F_{f}=
m F_{N}

*m** = F _{f} /
F_{N} *

What is the normal force?

*The normal force is the push, according to Newton’s
Third Law, between tow surfaces. If an object is on a horizontal surface, then
the normal force is equal and opposite the weight. If the object is on a
diagonal or there is another force also pushing down or pulling up, the normal
force is the force perpendicular to the surface. On the ground then, usually F _{N}=W.
On a ramp F_{N}=Wcosθ*

-------------------------------

TO ANSWER:

A skier slides down a slope. What force causes him to go down?

*A component of his weight ( F down = W sin θ )*

What force opposes him?

*Friction opposes him.*

What changes in environmental conditions might cause a change in the frictional force?

*Friction could depend on the weight, the angle of the
slope, or the snow conditions.*

*F _{f}=
*

__FRICTION PROBLEMS__

1) D = (__Vi+Vf)__ * T 3)
Vf = Vi + A*T

2

2) Vavg = (__Vi+Vf)__ 4)
D = Vi*T + ½*A*T^{2}

2

5)
Vf^{2 }= Vi^{2} + 2*A*D

Newton’s Laws

Fnet = m * A F1
= F2

Weight = m * g F_{f}= m
F_{N}= m W = m mg ). m= F_{f}/
F_{N}

1) What net
external force is required to give a 25 kg suitcase an acceleration of 2.2 m/s^{2}
to the right? Draw a free body diagram. ** If it is wood on wood (**m = .2

*Fnet=Fpush-Ffric= m * A ,m=25 kg, A = 2.2 m/s ^{2}*

* so Fnet = 25*2.2 = 55 N*

*Fnormal = W = mg = 25 * 9.8 = 245 N*

*Ffriction = **m** F _{N} = .2 ( 245) = 49N*

*Fnet =
Fpush – Ffrict*

*55 = Fpush – 49*

*Fpush = 55
+ 49 = 104 N*

* *

2) What acceleration will you give to a 24.3 kg box if you
push it with a force of 85.5 N? ** If it is aluminum on steel so (**m = .47

*Fnormal = W = mg = 24.3 * 9.8 = 238.14 N*

*Ffriction = **m** F _{N} = .47 ( 238.14) =
111.93N*

*Fnet =
Fpush – Ffrict*

*Fnet = 85.5 – 111.93*

*Fnet =
-26.43 N = m*A*

*-26.43 =
24.3 * A , A = 26.43/24.3 = -1.088
m/s ^{s} *

* This object actually would not move at
all,*

* because friction can’t all of a sudden
make it go backwards!*

3) A 1850 kg car is moving to the right at a constant
velocity of 1.44 m/s. What is the net force on the cart? ** If it is rubber
on concrete so (**m
= .8

* *

*Fnormal = W = mg = 1850 * 9.8 = 18130 N*

*Ffriction = **m** F _{N} = .8 ( 18130) =
14504N*

*If velocity is constant, A=0, so Fnet=0, *

*Fnet=Fpush-Ffric so*

* Fpush=Ffriction*

*Fpush = 14504 N*

4) A man is pushing a 200 Newton box with a force of 50
Newtons along the floor. A dog is pushing against him with a force of 4 N .
What is the acceleration of the box? Draw a free body diagram for the box. ** If
it is glass on glass so (**m = .4

*Fnormal = W = mg = m * 9.8 = 200 N ,*

* m = 200/9.8 = 20.4 Kg*

*Ffriction = **m** F _{N} = .4 ( 200) = 80N*

*Fnet =
Fpush –Fdog-Ffric*

* = 50N – 4N – 80 N*

* = -34 N =Fnet= mA *

*=
20.4 * A, *

*A
= -1.67 m/s ^{2} Note that once again, this is impossible so the box
would NOT move!*

9) A 1200 kg boat moves through the water with two forces
acting on it. One is a 2100 N forward push by the motor, and the other is a
1800 N resistive **friction **force of the
water.

__What is the coefficient of friction?__

*Fnormal = W = mg = 1200 * 9.8 = 11760 N*

*Ffriction =1800 = **m** F _{N} = *

* **m= 1800 / 11760 = .153*

** Honors:

13) A block with a mass of 50 kg is sliding down a ramp, **that is wood on wood, ** that is
at an angle of 60 degrees. What is
the component of the force causing it to slide? **What is the force of
of friction?** What is its acceleration?
Draw a picture and force diagram.

W = m * g = 50 * 9.8 = 490 N , ** m**=
.2 for wood on wood

Fdown =Fparallel=Fpush=
W sin θ = 490 sin
60 = __424.35 N__

F_{N} = W cosθ =
490 cos 60 = 245 N

Ffric =** m**F

Fnet = Fpush – Ffric = 424.35 – 49 = 375.35 N =
Fnet

Fnet = m * A, 375.35
= 50 * A , __A =7.5 m/s ^{2}__