Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 2.1, 2.2, 3.9
A Physics 205A student kicks a soccer ball horizontally with a certain speed off the edge of a cliff, and observes it landing on the ground below at a distance of 20 m from the edge. If the student kicks the soccer ball off the edge of this cliff with the same speed, but at angle of 45° below the horizontal, discuss why it will land on the ground below at a distance shorter than 20 m. Explain your reasoning using the properties of projectile motion.
Solution and grading rubric:
Correct. The ball kicked at a downwards angle will travel a shorter horizontal displacement Δx = vx⋅Δt because:
- it has a slower horizontal velocity component vx;
- and it has a shorter elapsed time Δt to travel before it hits the ground.
Nearly correct, but includes minor math errors. One of the two points (1)-(2) correct, other is problematic/incomplete.
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Only one of the two points (1)-(2) correct, other is missing, or both are problematic.
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some garbled attempt at finding velocity components, and applying kinematic equations of constant (zero) acceleration.
Implementation of ideas, but credit given for effort rather than merit.
Irrelevant discussion/effectively blank. This may include restating what must be proved.
Sections 70854, 70855, 73320
Exam code: finalm34T
p: 7 students
r: 2 students
t: 20 students
v: 11 students
x: 14 students
y: 8 students
z: 0 students
A sample "p" response (from student 0169):
A sample "t" response (from student 0123), neglecting the difference in horizontal velocities:
Another sample "t" response (from student 1107), neglecting the difference in elapsed times: