Friction Lab – Worksheet Name(s) ______
Hypotheses:
1. Kinetic friction (fk) is proportional to the normal force (n) between an object and the surface it moves over.
2. The maximum static friction force (max fs) is proportional to the normal force (n).
3. The static coefficient () of friction is greater than the kinetic coefficient ().
Materials:
3 blocks, a spring scale, string to pull the blocks, ¼ inch graph paper
Procedure:
A. Testing Hypothesis 1
1. Find the mass of each block and compute their weight or Fg (w = mg), using g = 9.8m/s2 (not 10m/s2) to obtain accurate lab results. The thinking here is that if the blocks are pulled with a horizontal force (not angled upward), the weight of the blocks will equal the normal force. (Equilibrium in the vertical direction would then imply w or Fg = n.)
2. Use the string and spring scale to pull the block(s). The spring scale will give the
pulling force (Fp) in the string. Here we need to pull the block(s) at a constant velocity so that we assume this pulling force equals the kinetic friction force. (Equilibrium in the horizontal direction implies Fp = fk.)
3. Complete the table here (leave out the last column for section ‘B’ below:
Mass (kg) / n ( = w = mg) / fk ( = Fp) max fs ( = Fp)1st block
1st and 2nd blocks
All 3 blocks
4. Now complete the graph of this data. Use normal force (n) as the independent variable (think ‘x’) and fk as the dependent variable (think ‘y’).
(a) Label each axis with the proper letter and indicate the units (N) for each variable.
(b) Put an appropriate title (with words) at the top of the graph.
(c) Make sure the scale on each axis is indicated.
(d) Put cute little circles or squares or around each data point.
(e) Draw the best-fit line (statistics?) with a straight edge. (Don’t play dot-to-dot!)
(f) Select 2 pts on this line to calculate slope. Show their ordered-pairs on the graph.
(g) Draw a right triangle connecting the two points graphically showing the slope.
(h) Calculate the slope of the best-fit line. Record this value here: = _____
B. Testing Hypothesis 2 – Do these data points form a line thru the origin? (y/n) _____
1. Start to pull one block and record the scale reading when it just slips. (Do a number of trials and take the average.)
2. Repeat for 2 and then 3 blocks.
3. Complete the last column in the table above using the weights of 1,2 & 3 blocks.
4. On the same graph paper as in procedure A:
(d) Put cute little… (Okay, you get the idea! Just use different cute little…)
Repeat steps (e) through (g) and record the value of this 2nd slope here: = _____
C. Testing Hypothesis 3 - Which slope or coefficient of friction is greater? ____
Conclusions – Do the pts form a line through the origin in A? (yes/no) ___ and B? ____
Some Data
I. The mass of the blocks.
The wood blocks are too light so we weigh block #1 with weight #1. The heavier bricks and ceramic chunks are just weighed by themselves.
Now most science scales actually give the mass (not the weight) in grams (not SI units of kg)! Depending on how much time you have for the lab, you can let the students ‘mass’ their own blocks, multiply by 1000 to get kg and then multiply by g = 9.80m/s2 to get weight in newtons (w=mg) OR find the mass for them and give it to them in kg.
Here are the masses in kg for our eight lab boxes.
A1 - .608kg B1 - 1.211kg C1 - .421kg D1 - 1.244kg
A2 - .635kg B2 - 1.186kg C2 - .421kg* D2 - 1.233kg
A3 - .629kg B3 - 1.202kg C3 - .427kg D3 - 1.229kg
E1 - .599kg F1 - 1.166kg G1 - .754kg H1 - 1.272kg
E2 - .619kg F2 - 1.170kg G2 - .765kg H2 - 1.257kg
E3 - .607kg F3 - 1.176kg G3 - .722kg H3 - 1.272kg
G4 - .745kg
*C2 - .416kg without the metal tab
II. We could try this lab pulling the block at different speeds with the same blocks and surfaces. Our hypothesis might be that kinetic friction (fk) is independent of speed (an assumption made in most all of the physics texts).
III. We could try to guess the coefficient of friction on other surfaces. With more time each student in a group could pull the same blocks along a different surface.
IV. A more difficult hypothesis to test would be that: Kinetic friction is independent of the surface area of the block that is in contact with the floor surface. This is another simplifying assumption made in most all of the physics texts.
V. For extra credit have the students make a hinged ramp which can be raised to any angle. In the background behind the ramp set up or draw in a large protractor so the angle of inclination can be measured. Also on the background, post a table of tangent values for different angle measures. These tangent values are the coefficients of static friction when an object (eraser, penny, …) starts to slip. So a force diagram showing the proof of this can be added to the background too. This becomes a rather large demonstration!