Companion to the reading passage The Bug and the Windshield: What Newton's Third Law Really Says. Answer each question, then click Check. Multiple-choice and number questions grade themselves; for written answers, compare yours with the model answer and checklist, then score yourself honestly.
A car is traveling along a straight highway. A flying insect, initially moving very slowly compared with the car, is struck by the windshield. During the brief collision, the insect’s velocity changes until it is moving along with the car. Engineers studying windshield impacts used high-speed video to estimate the values in Table 1.
Table 1: Windshield Collision Data
| Quantity | Value |
|---|---|
| Mass of insect | 1.0 × 10−3 kg |
| Mass of car | 1500 kg |
| Speed of car | 27 m/s |
| Change in the insect’s speed during the collision | 27 m/s |
| Duration of the collision | 0.0020 s |
Data are representative values for practice.
1. Which row correctly compares the forces and accelerations of the insect and the car during the collision? (Magnitude of force of windshield on insect, compared with force of insect on windshield / Magnitude of acceleration of insect, compared with acceleration of car)
2. Substitute numerical values into a mathematical representation that can be used to determine the magnitude of the average force exerted by the windshield on the insect. Then calculate the force.
A group of students investigated whether a heavier or faster object exerts a larger force in a collision. They mounted force sensors on two carts on a level track: cart A (0.50 kg) and cart B (1.50 kg). Each sensor recorded the force exerted on its own cart during each collision. In trial 1 only cart A moved; in trial 2 only cart B moved; in trial 3 both carts moved toward each other. Table 2 shows the maximum force recorded by each sensor.
Table 2: Maximum Forces During Cart Collisions
| Trial | Cart(s) Moving Before Collision | Maximum Force on Cart A (N) | Maximum Force on Cart B (N) |
|---|---|---|---|
| 1 | A only (0.80 m/s) | 4.2 | 4.2 |
| 2 | B only (0.80 m/s) | 6.1 | 6.1 |
| 3 | A and B toward each other | 9.8 | 9.8 |
Data are representative values for practice.
3. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response with evidence from Table 2.
4. The sensors also recorded force versus time during trial 1, with forces toward the right counted as positive. Which graph best represents the force of cart A on cart B and the force of cart B on cart A during the collision?
When a student walks, each foot pushes on the ground and the ground pushes on the foot. A 60-kg student begins walking toward the right of the page. Her foot exerts a horizontal force of 300 N on the ground. The mass of Earth is 5.97 × 1024 kg.
5. Picture a horizontal line for the ground and a small box on it for the student’s foot (right is the direction she walks). Choose the direction of the horizontal force of the foot on the ground and determine its magnitude.
6. Now choose the direction of the horizontal force of the ground on the foot and determine its magnitude. (Your two arrows should be drawn with lengths that show their relative magnitudes.)
7. Using one or more mathematical relationships, support the claim that Earth’s acceleration caused by the student’s push is far too small to notice, even though the student’s own acceleration is easy to notice.