Worksheet

Smooth Starts for a Delivery Robot: Net Force, Load and Speed

Grade 8 forces and motion: a robotics team tests a hallway delivery robot to link net force and load to changes in speed, then plans smooth starts and stops.

Grade 8ScienceInteractive · Printable

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1 NGSS standard
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10 questions
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Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Short answer

About this worksheet

What students practice

Students find the net force on a robot from its wheel push and friction.
Students use run data to describe how net force and mass each change the robot's speed.
Students use the patterns to plan the forces for a smooth start and stop.

How to use

Work through the friction table together before students find net forces. The delivery log in Section 2 is a good place to stop and discuss what a zero net force looks like.

Curriculum

NGSS standard covered

MS-PS2-2

Plan investigations showing that an object's change in motion depends on the net force on it and its mass.

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Smooth Starts for a Delivery Robot: Net Force, Load and Speed

Plan and analyze robot runs to see how net force and mass change motion.

The test runs

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

Robotics Team Notes: Teaching Relay to Start Smoothly

1The robotics team at Larkfield Middle School built Relay, a robot that carries lunch trays and library books down the school's long hallways. On its first trial run, Relay lurched forward so quickly that a cup of soup slid off its tray. Team members Ottilie Nakashima and Bram Castellucci decided to find out how the net force on Relay and its total mass affect how quickly its speed changes.

2With no load, Relay has a mass of 4.0 kg. To add mass, the pair strapped sandbags into its tray bin. Relay's program sets how hard its wheels push. To measure each setting, the pair hooked Relay to a force gauge tied to a wall and let its wheels push while the gauge held it still. Relay's wheels and motors also rub as it rolls, so friction pushes back on it. To measure friction, the pair towed Relay slowly at a steady speed with the gauge (Table 1). Then they subtracted friction from the wheel push to find the net force.

3Every run started from rest on the same stretch of smooth tile floor, and a motion sensor measured Relay's speed 2.0 s after the start. The pair repeated each run three times and recorded the averages in Table 2. The hallway was cleared during each run, and Relay never went faster than a slow walk.

4Before collecting data, the pair checked that the floor was level. They set Relay down with its motors off and made sure it did not start rolling in either direction. They also marked a start line with tape, so that every run began at the same spot, at least 10 m from the end of the hallway, and a teammate stood at the far end to stop Relay if anything went wrong. After the tests, Relay's own motion sensor kept a log of its speed during real deliveries, so the team could check how it started and stopped with a load on board.

5The cafeteria staff gave the team a requirement for soup trays. Relay's speed may change by no more than 0.25 m/s each second, and it should reach 0.50 m/s within 2.0 s of starting. Relay's motors can push with at most 8.0 N.

Table 1. Friction check (towed at a slow, steady speed)
Total mass (kg)	Gauge reading (N)
4	0.8
8	1.6
16	3.2

Table 2. Runs from rest (averages of three trials)
Run	Total mass (kg)	Wheel push (N)	Speed after 2.0 s (m/s)
1	4	1.8	0.5
2	4	2.8	1
3	8	3.6	0.5
4	16	5.2	0.25
5	16	7.2	0.5
6	8	2.6	0.25

Use Tables 1 and 2. In run 4, the wheels push forward with 5.2 N.

The net force on Relay in run 4 is N forward.

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

A. Runs 2, 3 and 4, since the net force was 2.0 N in each of them
B. Runs 1, 2 and 6, since the wheels pushed with less than 3 N
C. Runs 4 and 5, since Relay carried 16 kg in both of them
D. Runs 1, 3 and 5, since Relay reached 0.5 m/s in each of them

What pattern do the runs you chose in question 2 show?

A. With the same net force, Relay gained about the same speed with any load.
B. With the same net force, doubling the mass doubled the speed gained in 2.0 s.
C. With the same net force, doubling the mass halved the speed gained in 2.0 s.
D. With the same net force, mass mattered for loads over 8 kg but not for lighter ones.

Compare runs 4 and 5. What do they show?

A. With the same mass, the larger wheel push made the friction disappear.
B. With the same mass, doubling the net force doubled the speed gained.
C. With the same mass, the larger wheel push cut the speed gained in half.
D. With the same mass, a 2.0 N rise in push made the speed rise by 2.0 m/s.

Bram's first plan for the mass test used a wheel push of 2.8 N with every load. Ottilie changed the plan so that the push was larger for heavier loads. Why?

A. Friction grows with the load, so a fixed push would change the net force along with the mass.
B. The motion sensor reads a light robot more easily, so heavy loads need a push to show up.
C. A heavier load needs a forward net force just to keep rolling at a steady speed.
D. A larger push lets the heavy loads reach 1.0 m/s in 2.0 s, so the runs are easier to compare.

A delivery run and the soup requirement

Use the tables and paragraph 5.

On a real delivery, Relay carried a load that made its total mass 8 kg, the same as in runs 3 and 6. Its motion sensor logged its speed.

Table 3. Delivery log (total mass 8 kg)
Time (s)	Speed (m/s)
0	0
1	0.25
2	0.5
4	0.5
6	0.5
7	0.25
8	0

From 2 s to 6 s, Relay rolled at a steady speed. Use Table 1.

During that time, the wheels were pushing forward with about N.

From 6 s to 8 s, Relay's speed dropped by 0.25 m/s each second. In run 3, a net force of 2.0 N changed this 8 kg robot's speed by 0.25 m/s each second. Friction pushes backward with 1.6 N, and the wheels can also push backward to brake.

The net force while Relay slows down is N backward, so the wheels must push backward with N.

To meet the soup requirement exactly, Relay's speed must rise by 0.25 m/s each second. The runs show this takes a net force of 0.25 N for each kilogram of total mass, and friction from Table 1 adds 0.2 N for each kilogram. Use paragraph 5. Circle every total mass Relay can start smoothly.

10 kg14 kg16 kg20 kg24 kg

Suppose Relay carries a load that makes its total mass 12 kg, and the pair sets the wheel push so the net force is 2.0 N, as in runs 2, 3 and 4. What speed should Relay reach 2.0 s after starting?

A. Less than 0.25 m/s, since a 12 kg load rubs more than an 8 kg load
B. More than 0.5 m/s, since a heavier robot builds up more speed
C. 0.5 m/s, the same as run 3, since the net force is the same
D. Between 0.25 m/s and 0.5 m/s, since 12 kg is between 8 kg and 16 kg

10.

Bram says, "Run 4 was slow because the heavy load has more friction. If there were no friction at all, the load would not change how fast Relay speeds up." Do you agree? Make a claim, use evidence from Tables 1 and 2, and explain your reasoning about net force and mass.