Physics quiz question: impulse on Kenny Bräck

Physics 205A Quiz 4, fall semester 2018
Cuesta College, San Luis Obispo, CA

IndyCar race driver Kenny Bräck (mass 70 kg) blogged the details of his notorious accident at the Texas Motor Speedway in 2003:[*],[**]

My car caught air at [100 m/s], got airborne and smashed straight into a massive steel pole in the catch fence... It recorded a record [150,000 N force] impact [on me] and left me seriously injured.

Ignore friction and drag, and assume the car was completely stopped by crashing into the steel pole. (Kenny Bräck was later able to make a full recovery and return to racing.)

The magnitude of the stopping impulse was:
(A) 1.5×10³ N·s.
(B) 7.0×10³ N·s.
(C) 1.5×10⁵ N·s.
(D) 1.1×10⁷ N·s.

[*] kennybrack.com/pages/personal-info/2003.html.
[**] motorsportmagazine.com/archive/article/november-2014/102/lunch-kenny-br-ck.

Correct answer (highlight to unhide): (B)

The impulse J can be calculated as the initial-to-final change in momentum:

J = ∆p = m·∆v,

where ∆v = v_f – v₀.

The initial velocity vector is v₀ = +100 m/s (traveling to the right, in the +x direction), and the final velocity vector is v_f = 0 ("completely stopped"). Then with a mass m = 70 kg:

J = (70 kg)·((0 m/s) – (+100 m/s)) = –7,000 N·s,

or to two significant figures, the magnitude of the impulse is 7.0×10³ N·s (and the "–" sign indicates that it is directed to the left, opposite the direction of the initial velocity).

(Response (A) is the impact force divided by the initial velocity; response (C) is the impact force; response (D) is impact force multiplied by the mass.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 3 students
(B) : 34 students
(C) : 13 students
(D) : 1 student

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.74