Worksheet

Pip the Delivery Robot: Force Pairs in Hallway Bumps

Grade 8 physical science: a robotics team's bump tests show equal force pairs, reveal a cart's mass and guide a safer bumper and speed for its robot.

Grade 8ScienceInteractive · Printable

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Single worksheet
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1 NGSS standard
Printable answer key
11 questions
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Details

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 use Newton's third law and collision data to compare speed changes of objects with different masses.
Students infer an unknown mass from the speed changes in a collision.
Students choose a bumper and speed that meet design criteria and explain the choice with force pairs.

How to use

Read the team notes and Table 1 together, then let students try the select and mystery-cart questions in pairs. Section 2 works as a design discussion before the written recommendation.

Curriculum

NGSS standard covered

MS-PS2-1

Use Newton's third law to explain what happens when two objects collide and design a solution for a collision problem.

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Pip the Delivery Robot: Force Pairs in Hallway Bumps

Use bump data and Newton's third law to find a mystery mass and design a safer robot.

Bumps with rolling carts

Read the team notes and Table 1, then answer the questions.

Robotics Team Notes: Pip Bumps Into Things

1The robotics team at Linwood Middle School built Pip, a small robot that carries library books and lab supplies between classrooms. With its tray, Pip has a mass of 6.0 kg. It rolls on smooth wheels and steers itself down the halls with a camera. In its first week on the job, Pip bumped into a rolling supply cart, a recycling bin and, once, a student's leg. No one was hurt, but coach Mr. Delroy Hutchins asked eighth graders Astrid Mensah and Joaquim Fairbanks to study the forces in a bump and to design a safer robot.

2First, the pair tested bumps with rolling carts on the smooth hallway floor. They loaded each cart with books to a measured mass. With its standard rubber bumper, Pip drove straight into the resting cart at 0.8 m/s. A switch in the bumper cut Pip's motors the moment it touched anything, so during each bump the only strong sideways pushes on Pip and the cart came from each other. A thin force sensor behind Pip's bumper recorded the largest force on Pip during the bump, called the peak force. A phone filmed each bump from the side, next to a meter stick, so the pair could find how much each object's speed changed. Each bump was repeated three times, and Table 1 shows the averages. In every bump, Pip either kept rolling forward more slowly or stopped. The pair also bumped Pip into Cart X, a cart of library returns whose mass they did not know.

3Next, the pair tested hallway safety with a shin dummy, a padded post bolted to the floor with a force sensor behind its padding. Pip drove straight into the dummy with different bumpers and top speeds (Table 2). Mr. Hutchins gave three criteria. The peak force on the dummy must be 65 N or less, so a bump would not hurt a person. Pip's top speed must be 0.5 m/s or more, so deliveries finish between classes. With its bumper, Pip must be 60 cm wide or less to fit through the gap beside the lab benches. The bumper wraps around the front corners, so it adds its thickness to both sides of Pip's 50 cm body.

4The school is also building a second robot, Quill, which has a mass of 12 kg. In narrow halls, the two robots could meet head-on.

Table 1. Pip (6.0 kg, 0.8 m/s) bumps a resting cart
Cart	Cart mass (kg)	Peak force on Pip (N)	Pip's speed change (m/s)	Cart speed change (m/s)
A	2	60	0.3	0.9
B	3	70	0.4	0.8
C	6	85	0.6	0.6
D	12	100	0.8	0.4
X	?	95	0.72	0.48

Look at Carts A to D. Which pattern does Table 1 show?

A. The heavier the cart, the more Pip's speed changed and the less the cart's speed changed.
B. Pip's speed and the cart's speed changed by the same 0.6 m/s in bumps A to D.
C. The heavier the cart, the less Pip's speed changed and the more the cart's speed changed.
D. The cart's speed changed more than Pip's in bumps A to D, because Pip pushed first.

Circle every statement about the bump with Cart D that is correct.

Pip pushed on the cart with a peak force of about 100 N.The cart pushed back on Pip with about 200 N, since it has twice Pip's mass.Pip's speed changed twice as much as the cart's speed.Pip pushed on the cart first, and the cart pushed back after Pip stopped.Pip came to a stop, and the cart rolled away at 0.4 m/s.

In the bump with Cart A, the cart's speed changed 3 times as much as Pip's, even though they pushed on each other with forces of the same size. Which explanation fits the data best?

A. Pip's motors kept pushing on the cart after the bump, which sped the cart up more.
B. The cart has one third of Pip's mass, so the same force changes its motion more.
C. The cart's wheels added an extra push to the cart that Pip did not feel.
D. The light cart felt its push first, so it had more time to speed up than Pip.

In bump C, the masses were equal and the speed changes were equal. In bump D, the cart had twice Pip's mass, and its speed changed half as much as Pip's. In bump X, Pip's speed changed 1.5 times as much as the cart's.

Use the same pattern. The mass of Cart X is about kg.

Designing a safer Pip

Use paragraph 3, Table 2 and the three criteria.

Table 2. Pip drives into the shin dummy
Design	Bumper	Top speed (m/s)	Peak force on dummy (N)	Width with bumper (cm)
A	Standard rubber, 1 cm	0.8	110	52
B	Standard rubber, 1 cm	0.4	55	52
C	Foam, 3 cm	0.8	75	56
D	Foam, 3 cm	0.6	56	56
E	Foam, 7 cm	0.8	40	64

Compare Designs A and B. With the same bumper, driving at 0.8 m/s gave a peak force times as large as driving at 0.4 m/s.

Which design meets all three of Mr. Hutchins's criteria?

A. Design E
B. Design C
C. Design B
D. Design D

During a bump, a box of lab supplies on Pip's tray slides forward and hits the front wall of the tray. Which two forces form a third-law force pair?

A. The tray wall pushes back on the box, and the dummy pushes back on Pip.
B. Earth pulls the box down, and the tray pushes the box up.
C. The box pushes forward on the tray wall, and the wall pushes back on the box.
D. Pip pushes forward on the dummy, and the box pushes forward on the tray wall.

When Pip meets Quill

Use paragraph 4 and the pattern in Table 1.

Pip (6.0 kg) rolls east at 0.5 m/s, and Quill (12 kg) rolls west at 0.5 m/s. Both motors cut off as they bump head-on. During the bump, Quill's speed changes by 0.5 m/s, so it stops.

Pip has half of Quill's mass, so Pip's velocity changes by about m/s.

Pip started at 0.5 m/s east, and its velocity changes by more than that, toward the west. What does Pip do right after the bump?

A. It stops and stays where the bump happened, next to Quill.
B. It rolls back toward the west at about 0.5 m/s.
C. It keeps rolling east at 0.5 m/s, pushing Quill along with it.
D. It rolls back toward the west at about 1.0 m/s.

10.

Both robots still meet head-on at 0.5 m/s each. Which change would make Pip's velocity change less in the bump?

A. Load Pip with heavier supplies so it has more mass
B. Program Quill to drive toward Pip faster than 0.5 m/s
C. Load Quill with heavier supplies so it has more mass
D. Program Pip to push harder on Quill than Quill pushes on Pip

11.

Joaquim says, "The sensor behind Pip's bumper only tells us the force on Pip. For people's safety we need a second sensor on the dummy." Astrid says, "One sensor is enough." Make a claim about who is right and about which design the team should use. Use evidence from Tables 1 and 2, and explain your reasoning with Newton's third law.