LD Physics Physi cs Leaflets Leafl ets
Mechanics Statics
P1.2.6.1
Friction
S ta tic tic fri fric tion, tion, s lid ing fri fric tion tion a nd ro lling fri fric tio tio n
Objects of the experiment Investigating the static and sliding friction as a function of the area, the weight and the material Co mpa riso riso n of s ta tic tic a nd s liding fri friction a s a function function o f the we ight a nd d etermini etermining ng the co efficient efficient of fri friction Co mpa riso riso n of roll rolling a nd s liding friction friction a s a function function of the we ight a nd d etermini etermining ng the c oefficient oefficient of fri friction
Principles is mea sured a s the force force which ma intains the friction friction block in in a uniform uniform mo tion on t he rolling rolling rods . The The s liding iding fri frict ion force F G is measured once more for comparison, whereby this time the friction block is pulled over the stand rods arranged as a fixed fixed b a s e. The The expe riment riment c onfirms onfirms the relations hip
In discussing friction between solid bodies, we distinguish be tw ee n sta tic friction, friction, sliding sliding frict frict ion a nd rolling rolling fri frict ion. S ta tic friction force is the minimum force required to set a body at res t on a so lid ba se in motion. Analogo Analogo usly, usly, d yna mic mic (kinetic) kinetic),, or sliding friction force is the force required to maintain a uniform motion of the body. Rolling friction force is the force which maintains the uniform motion of a body which rolls on another bod y. To b eg in, this expe riment riment ve rifi rifies es tha t the s ta tic fri frict ion force indep ende nt of the size F H and the sliding friction force F G are indep of the contact surface and proportional to the resting force G on the ba se surfac surfac e of the fri friction ction block. Thus we ca n sa y: F H
=
H ⋅ G
(I)
G ⋅ G
(II).
Setup – P repare clean, d ry a nd s mooth experiment experiment surfaces surfaces (e. g. –
a nd F G
=
The c oe fficients fficients of frict frict ion H a n d G depe nd on the material material of the contact surfaces. As the static friction force is always greater than the sliding friction force, we can always say H > G
=
R ⋅ G
laboratory bench) for the friction experiments. If the resulting frictional forces are too slight, use different base surfaces.
Carrying out the experiment – Using the dynamometer, determine the weight (force of
(III).
gravity) block.
To d istinguish b etw ee n s liding a nd rolli olling friction, friction, the frict frict ion block is placed on top of multiple stand rods laid parallel to ea c h oth er. The rolling rolling fric fric tion force F R
(V).
G > R
G 1 of
the large large w oode n block block and
G 2 for
the the s ma ll
a) Static and sliding friction as a function of the area, the weight and the material
(IV)
– Place the small block on the experiment surface with the – –
l e S 9 0 1 0
– – – 1
plas plas tic tic s ide d own. Using the dyna momete r, mea sure the maximum maximum hori horizonta l pulling force at which the body remains stationary on the experiment experiment s urfac urfac e a s the s ta tic tic friction friction force force F H. Measure the horizontal pulling force which maintains a uniform motion on the experiment surface as the sliding friction friction force F G . P lac e the wood en block block on the bas e surfac surfac e with the the wide wooden side and then the narrow wooden side down and repeat your measurements for F H a n d F G . Repea t the me a surements w ith the large large block for fri friction ction experiments. Repeat the mea surement surement on other surfaces surfaces as des ired.
P1.2.6.1
LD Physics Leaflets b) Static and sliding friction force as a function of the force of gravity
Apparatus
– Place the large block on the experiment surface with the
1 Pa ir of woo den blocks for friction experiments . . . . . . . . . . . . . 1 S et 7 we ights , 0.1 – 2 kg, with hook . . . .
342 10 315 36
1 Dynam ometer, 10.0 N . . . . . . . . . . . .
314 47
6 Sta nd rods 10 cm . . . . . . . . . . . . . .
300 40
– –
plas tic side do wn a nd mea sure the s tatic and sliding friction force. Increase the weight of the block by adding in turn the weights 0.1 kg, 0.2 kg, 0.5 kg and 1.0 kg and repeat the measurements. Ca rry out the same meas urements for the woo den side of the block as well.
c) Rolling and sliding friction force as a function of the force of gravity
– Lay the s tand rods next to ea ch other and plac e the large block on the rods with the plastic side down.
– Measure the horizontal pulling force which maintains a – –
uniform motion on the rolling rods as the rolling friction force F R . Increase the weight of the block by adding in turn the weights 0.1 kg, 0.2 kg, 0.5 kg and 1.0 kg and repeat the measurements. Align the block parallel to the rod axes and measure the sliding friction force.
Measuring example Fig. 1
Measuring the static friction force friction force F G (bo tto m).
F H (top)
and the sliding
G 1 =
1.5 N
G 2 =
3.0 N
a) Static and sliding friction as a function of the area, the weight and the material Experiment s urface : plastic-coa ted benchtop Ta b. 1: S ta tic frict ion force F H and sliding friction force F G a s a function of the force of the a erea , the w eight a nd the ma terial G
Material
N
Fig. 2
Measuring the static friction force F H (top) and the sliding friction force F G (botto m) a s a function of the force o f gravity G .
A
F H
F G
c m2
N
N
1.5
P la s tic
12 × 6
0.8
0.6
1.5
Wo o d
12 × 6
0.3
0.3
1.5
Wo o d
12 × 3
0.3
0.3
3.0
P la s tic
12 × 6
1.6
1.1
3.0
Wo o d
12 × 6
0.5
0.5
b) Comparison of static and sliding friction force Experiment s urface : plastic-coa ted benchtop Ta ble 2: S ta tic frict ion force F H and sliding friction force a function of the force of g ravity G P la s tic s id e
Fig. 3
Measuring the rolling friction force F R (top ) a nd the sliding friction force F G (botto m) a s a function of the force o f gravity G .
2
F G a s
Wo o d en s id e
G
F H
F G
F H
F G
N
N
N
N
N
3
1.6
1.1
0.5
0.4
4
2.2
2.0
0.8
0.6
5
3.1
2.8
0.9
0.8
8
5.0
4.6
1.9
1.3
13
8.3
8.0
3.0
2.0
P1.2.6.1
LD Physics Leaflets c) Comparison of sliding and rolling friction Ta ble 3 : S liding friction force F G a nd rolling frict ion fo rce a function of the force o f gravity G
F R a s
Fig. 4 show s the results of the mea surements. The slope of the line throug h the o rigin is e q uiva lent to the c oe fficients of frict ion H a n d G c a lculat ed us ing (I) a nd (II) (s ee Ta ble 2). Ta ble 4: S ta tic frict ion c oe fficient cient G for friction on p la s tic
G
F G
F R
N
N
N
3
3.0
0.1
Material
4
4.3
0.2
5
5.2
0.2
8
9.0
0.3
11
0.4
13
0.5
18
0.6
23
0.7
H a nd
s liding frict ion c oe ffi-
H
G
P la s tic
0.63
0.59
Wo o d
0.22
0.15
c) Comparison of sliding and rolling friction
10 F N
5
0 0
Evaluation and results
10
20
G N
a) Static and sliding friction as a function of the area, the weight and the material As the me a suring res ults in Ta ble 1 s how, bo th the sta tic friction force and the sliding friction force depend on the material properties of the friction surfaces and on the weight (force o f gra vity) of the b loc ks. How ever, the friction force s a re indepe ndent o f the size of the friction a rea .
Fig. 5
Sliding friction force F G (circles ) and rolling friction force (triang les) as a function o f the force of g ravity G
F R
Fig. 5 show s the results of the mea surements. The slope of the line throug h the o rigin is e q uiva lent to the c oe fficients of frict ion H a n d G c a lculat ed us ing (I) a nd (II) (s ee Ta ble 3). Ta b le 5: Ro lling fric tion co efficient cient G
R a nd
s liding frict ion c oe ffi-
b) Comparison of static and sliding friction force Material: P la s tic
G
R
1.09
0.03
10 F N
5
0 0
5
10
G 15 N
Fig . 4
S t a t i c fric t io n f o rc e F H and sliding friction force function o f the force o f gravity G C lo s e d s q u a re s : s t a tic fric t io n fo r p la s t ic O pe n s q ua re s : s lid ing fric tio n fo r p la s tic C lo se d circ le s : s ta tic fric tio n fo r w o od Op en c irc le s : s lid ing fric tio n fo r w o od Experiment surface : plastic-coa ted benc htop
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