Gear Sleds on the Snow: Net Force, Mass and Motion

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Gear Sleds on the Snow: Net Force, Mass and Motion

Use sled-pulling runs from a winter camping trip to link net force and mass to changes in motion.

The sled runs

Read the passage and the table, then answer the questions.

Hauling Gear to Camp

1On a February camping trip, an eighth-grade outdoor club skied 3 km to a winter campsite. Instead of carrying heavy packs, each pair of skiers pulled a gear sled, a long plastic tub on runners that slides over the snow behind a tow rope. Trip leader Ms. Henrike Castellanos noticed that a fully loaded sled was hard to get moving and hard to stop, while an empty one jumped forward with a light tug.

2The next morning, campers Yusuf Lindgren and Marisol Achterberg turned that observation into an investigation: how do the net force on a sled and its total mass affect how quickly its speed changes? They worked on a flat, hard-packed snow trail next to camp. They loaded the sled with duffel bags and checked each total mass on a luggage scale. A spring scale clipped into the tow rope showed the pull in newtons, and Yusuf kept the reading steady during each run. Cones every meter and a phone video let them find the sled's speed 2.0 s after it started from rest.

3Snow rubs on the bottom of a sled, so the pull on the rope is not the net force. To find the friction, the pair first pulled each load at a slow, steady speed. On this trail, the friction came out to about 1 N for each kilogram of total mass. For every run, they subtracted the friction from the pull to get the net force. They ran each setup three times and used the average.

4In one trial of run 2, the speed after 2.0 s was only 0.6 m/s, while the other two trials both gave 1.0 m/s. Marisol then noticed that this trial had started a few meters farther along, where the trail tilts slightly uphill. For run 6, the pair first got the 40 kg sled sliding at 0.8 m/s and then held the pull steady at 40 N.

5Ms. Castellanos asked the pair to use their results to plan safer hauling for the trip back, when the sleds would be loaded with wet, heavier gear and the tired campers would want to pull no harder than they had to.

Sled runs on flat packed snow (averages of three flat-snow trials)
Run	Total mass (kg)	Pull (N)	Friction (N)	Net force (N)	Speed after 2.0 s (m/s)
1	20	25	20	5	0.5
2	20	30	20	10	1.0
3	20	40	20	20	2.0
4	10	20	10	10	2.0
5	40	50	40	10	0.5
6 (already moving)	40	40	40	0	0.8

1.

Which runs should the pair compare to find out how the net force alone affects the change in speed?

A. Runs 1, 2 and 3B. Runs 5 and 6C. Runs 2, 4 and 5D. Runs 3 and 4

2.

Scientists state a unit with every measurement. Match each measurement from the investigation to its SI unit.

3.

Runs 2 and 5 have the same net force, but the sled in run 5 has twice the mass.

The sled's speed after 2.0 s in run 5 is times its speed after 2.0 s in run 2.

4.

What pattern do runs 2, 4 and 5 show?

A. With the same net force, a sled with more mass speeds up more, because it pushes harder on the snow.B. The mass of the sled changed how fast it sped up only when the pull was larger than 40 N.C. With the same net force, a sled with more mass speeds up less in the same time.D. The sleds sped up by the same amount, because the net force was 10 N in each run.

5.

Use paragraph 4 and the table. In run 6 the sled kept sliding at a steady 0.8 m/s for the whole run. Which explanation fits the data best?

A. The pull and the friction were balanced, so the net force was zero and the sled's motion did not change.B. The sled was already moving, and a moving sled keeps its speed as long as the pull is larger than friction.C. The friction stopped acting once the sled was moving, so the pull was the only force on the sled.D. The 40 kg sled was too heavy for a 40 N pull to change its speed in 2.0 s.

Planning the trip back

Use the passage and the table to answer the questions.

6.

During run 3, Marisol skis beside the sled, keeping pace with it the whole time. She says, "The sled is not moving at all." Yusuf, standing still by the cones, says, "The sled sped up to 2.0 m/s." Which statement is correct?

A. Yusuf is correct and Marisol is wrong, because a pulled sled is moving even when a skier keeps pace with it.B. Both are correct; each describes the motion from a different frame of reference.C. Neither is correct, because the sled's speed was changing during the run, so it had no speed to describe.D. Marisol is correct and Yusuf is wrong, because she was closer to the sled than he was.

7.

The 20 kg sled is sliding at 2.0 m/s when Yusuf lets go of the rope. Now the only force along the trail is friction, 20 N backward. In run 3, a net force of 20 N changed the speed of the 20 kg sled by 2.0 m/s in 2.0 s.

The sled will slide to a stop after about s.

8.

Next, the loaded 40 kg sled is sliding at 2.0 m/s when the rope is dropped. Friction on it is 40 N backward. Compared with the 20 kg sled in question 7, what should happen?

A. The 40 kg sled slides for longer, because friction acts less on heavy sleds.B. The 40 kg sled slides for longer, because a sled with more mass keeps going.C. The 40 kg sled stops sooner, because twice the friction acts on it.D. Both sleds stop after about the same time, because twice the backward force acts on twice the mass.

9.

Use paragraph 4. What should the pair do with the slow trial from run 2?

A. Keep it and add a heavier load, so the slope has less effect on the sled.B. Redo it on flat snow, because the uphill slope added a backward force on the sled.C. Average all three trials anyway, because repeated trials cancel out a mistake like this one.D. Keep it in the average, because the pull on the scale was still 30 N in that trial.

10.

For the trip back, Ms. Castellanos wants the 40 kg sled to reach 2.0 m/s in 2.0 s from rest on the flat trail, just like the 20 kg sled in run 3. About how hard must the campers pull on the rope? Write a claim, give evidence from at least two runs, and explain your reasoning about net force, mass and friction.