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Do Heavier Boards Roll Faster? Testing Skate Park Banks

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Do Heavier Boards Roll Faster? Testing Skate Park Banks

Use bank runs to untangle how net force and mass change a skateboard's motion.

Planning the bank runs

Read the club notes and Tables 1 and 2, then answer the questions.

Physics Club Notes: Boards on the Banks

1Cottonwood Skate Park has three low practice banks, smooth concrete slopes that run down onto a flat floor. Bank 1 is the gentlest and Bank 3 the steepest. Many skaters at the park believe that heavier riders speed up faster on a bank. Eighth graders Leilani Fairclough and Mateus Abiodun, members of the school physics club, decided to test how the net force on a skateboard and its total mass affect how quickly its speed changes. Their adviser, Ms. Ngozi Albright, met them at the park early on a Saturday, before it opened.

2Nobody rode a board. The pair strapped sandbags onto one skateboard so the board and its load had a total mass of 4 kg, 8 kg or 12 kg. A thick crash mat lay across the flat floor below each bank, and nobody stood in front of a rolling board.

3To find the net force, Leilani clipped a force gauge to the nose of the loaded board and lowered it down the bank at a slow, steady crawl, keeping the gauge pulled straight up the slope. The wheels rub a little as they roll, and the pair assumed this friction stays about the same at higher speeds. They used the gauge reading during the crawl as the net force on the board once it rolled freely (Table 1). For example, on Bank 2 the 8 kg board crawled down at about 0.1 m/s while the gauge read 8 N.

4For each run in Table 2, Mateus let go of the board from rest at a chalk line near the top of the bank. A phone filmed the run next to tape marks every 0.5 m, and the pair used the video to find the board's speed 2.0 s after letting go, while it was still on the bank. Each run was repeated three times, and Table 2 shows the averages. In these runs, the boards stayed slower than a fast jog and then came to rest on the crash mat.

5Last, the pair let the 8 kg board go from rest 2 m up the slope of Bank 2, measured along the bank, and kept filming as it crossed the flat floor and rolled onto a strip of artificial turf beside the floor (Graph 1). For this run, they moved the crash mat to the far end of the turf.

Table 1. Gauge reading during the steady crawl (N)
Bank	4 kg board	8 kg board	12 kg board
Bank 1	2	4	6
Bank 2	4	8	12
Bank 3	6	12	18

Table 2. Runs from rest (averages of three trials)
Run	Bank	Total mass (kg)	Net force (N)	Speed after 2.0 s (m/s)
1	Bank 1	8	4	1
2	Bank 2	8	8	2
3	Bank 3	8	12	3
4	Bank 2	4	4	2
5	Bank 2	12	12	2

1.

Use paragraph 3. Why can the pair use the gauge reading during the steady crawl as the net force on the board once it rolls freely?

A. At a steady speed the forces are balanced, so the gauge holds back just the push left after friction.B. Friction stops acting once the board is let go, so the gauge reads the whole downhill pull.C. A moving board needs a forward force to keep moving, and the gauge measures that force.D. The gauge measures the board's mass, and a board's mass is the same as its net force.

2.

Use the pattern in Table 1. On Bank 3, the net force on a 10 kg board would be about N.

3.

Runs 4, 2 and 5 all used Bank 2, with masses of 4 kg, 8 kg and 12 kg. Why can't these three runs show how mass alone affects the change in speed?

A. The speeds were measured on the bank, so the boards had not finished speeding up.B. The net force grew along with the mass, so two things changed at once.C. Each run was repeated three times, so the averages hide the effect of mass.D. The boards were let go from different chalk lines, so they rolled different distances.

4.

The 4 kg, 8 kg and 12 kg boards in runs 4, 2 and 5 each reached 2.0 m/s after 2.0 s. Which explanation fits Tables 1 and 2 best?

A. Friction grew with the load and canceled the extra pull, so the net force stayed the same.B. The heavier boards pressed harder on the bank, so the bank pushed them along faster.C. More mass came with a matching larger net force, so each board's speed changed at the same rate.D. The sandbags slowed the lighter boards down, so they matched the heavier boards.

5.

Circle the two pairs of runs that each compare different masses with the same net force.

6.

Runs 3 and 5 both had a net force of 12 N. The 12 kg board has 1.5 times the mass of the 8 kg board.

The 12 kg board's speed after 2.0 s was of the 8 kg board's speed. Write your answer as a fraction.

7.

The pair plans one more run: a 6 kg board let go from rest on Bank 1.

Use both tables. Predict its speed 2.0 s after letting go: m/s

Down the bank and across the floor

Graph 1 shows the 8 kg board's speed during the run in paragraph 5. Use it with the tables.

The board rolls down Bank 2 from 0 s to 2 s, crosses the flat floor from 2 s to 4 s and rolls onto the turf at 4 s.

8.

From 2 s to 4 s, the board kept rolling at about 2 m/s across the flat floor with nothing pushing it forward. Which explanation fits the graph best?

A. The board's mass pushed it forward, so a heavier board would roll faster.B. The floor pushed the board forward just enough to keep its speed at 2 m/s.C. The push from the bank stayed with the board and kept driving it forward.D. The forces along the floor were about balanced, so its speed barely changed.

9.

On the bank, a net force of 8 N changed this 8 kg board's speed by 1 m/s each second (run 2). On the turf, from 4 s to 6 s, its speed dropped by about 1 m/s each second.

The net force from the turf on the board was about N, pointing opposite to its motion.

10.

Suppose the 4 kg board crosses the flat floor at 2 m/s and rolls onto the same turf, which pushes back on it with half the force it put on the 8 kg board. How should its speed change on the turf?

A. Its speed should drop by about 1 m/s each second, like the 8 kg board's.B. Its speed should drop by about 2 m/s each second, twice as fast as the 8 kg board's.C. Its speed should stay at 2 m/s, since a lighter board feels less of the turf.D. Its speed should drop by about 0.5 m/s each second, since the push is smaller.

11.

On flat ground, a 40 kg skater and a 60 kg skater each push off from rest with the same net force of 120 N for 1.0 s. Based on the runs, what should happen?

A. The 60 kg skater reaches about 3 m/s and the 40 kg skater about 2 m/s.B. The 40 kg skater reaches about 3 m/s and the 60 kg skater about 2 m/s.C. Both reach about 3 m/s, because a push off flat ground ignores mass.D. Both reach about 2 m/s, because the net force on each skater is the same.

12.

A skater at the park says, "I'll wear my heavy backpack down the bank, because heavier boards speed up faster." Is the skater right? Make a claim, use evidence from at least three runs, and explain your reasoning about net force and mass.