## Mastering Physics Solutions: Exercise 6.30

C Exercise 6.30

Part A = 230Ns
Part B = 780N
Part C = The other car exerts this force on the driver.

Solutions Below:

A 46 kg driver sits in her car waiting for the traffic light to change. Another car hits her from behind in a head-on, rear-end collision and her car suddenly receives an acceleration of 17 m/s2.

Part A

If all of this takes place in 0.30 s, what is the impulse on the driver?

Impulse is just FΔt:

I = FΔt
F = ma
F = 46 * 17
F = 782
I = 782Δt
I = 782 * 0.30
I = 234.6

I = 230Ns (2 significant figures)

Part B

What is the average force exerted on the driver?

Be careful here. My first reaction was to find average force by dividing 782N (46kg * 17m/s2 by 0.3s (Δt). But this isn’t correct. What you have to consider is that the force is 782N, period – and that it’s applied on the driver for 0.3s. Since the force is constant, the average force is just the force. To make an analogy, consider acceleration due to gravity, which is -9.8m/s2 (the average would be -9.8m/s2 regardless of how long something accelerated for).

F = 780N (2 significant figures)

Part C

What exerts this force?

The other car exerts this force on the driver.

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### 2 Responses to Mastering Physics Solutions: Exercise 6.30

1. Brittany says:

Could I get some help with these problems please?

Problem 6.37
A 71kg trampoline artist jumps vertically upward from the top of a platform with a speed of 4.7m/s .
(a)What is his speed as he lands on the trampoline, 2.2m below his jump off point?
(b)If the trampoline behaves like a spring with spring stiffness constant 7.1×104N/m , how far does he depress it? Any depression of the trampoline from equilibrium is to be taken as a negative distance.

Problem 6.73
In a certain library the first shelf is 15.0cm off the ground, and the remaining four shelves are each spaced 31.0cm above the previous one.
(a)If the average book has a mass of 1.4kg with a height of 22cm , and an average shelf holds 29 books, how much work is required to fill all the shelves, assuming the books are all laying flat on the floor to start?
Thanks!

• Mastering Physics Solutions says:

We posted answers to these problems here and here. Good luck!