Question

phy 145
test #1
fall 2025
write your name with your non - dominant hand!
first principles to use on test:

newtons laws:
i. if an object has a constant velocity (including zero), there must be no unbalance of forces (i.e. \\(\sum f = 0\\)).
ii. if an objects velocity is changing, there is an unbalance of forces in the same direction as the change of velocity: \\(\sum f=ma = m\frac{\delta v}{\delta t}\\).
iii. forces in an interaction between objects are equal and opposite.

conversions
1.609 m = 1 mile
1 hr = 60 min = 3600 s
3280 ft = 1 mile
1.00 km = 1 m

tips & reminders

• include units on values you ultimately record while the rounding.
• ignore friction and air - resistance unless a problem states otherwise.
• draw diagrams.
• the majority of your physics work will be thinking & strategizing, not calculating.
• show all work! put your answers in the boxes provided.

here we go!

1. for the first hour of my 150 - mile trip, i averaged 65 mph. what must my speed be during the rest of my trip to average 70 mph for the total trip?

Explanation:

Step1: Calculate total time for the whole trip

The total distance of the trip is $d = 150$ miles. The desired average - speed is $v_{avg}=70$ mph. Using the formula $t=\frac{d}{v}$, the total time for the whole trip $t_{total}=\frac{150}{70}=\frac{15}{7}$ hours.

Step2: Calculate time for the first - hour part

The distance covered in the first hour is $d_1 = 65$ miles. Since the time for this part $t_1 = 1$ hour.

Step3: Calculate remaining distance and time

The remaining distance $d_2=d - d_1=150 - 65 = 85$ miles. The remaining time $t_2=t_{total}-t_1=\frac{15}{7}-1=\frac{15 - 7}{7}=\frac{8}{7}$ hours.

Step4: Calculate the required speed for the remaining part

Using the formula $v=\frac{d}{t}$, the speed for the remaining part $v_2=\frac{d_2}{t_2}=\frac{85}{\frac{8}{7}}=85\times\frac{7}{8}=\frac{595}{8}=74.375$ mph.

Answer:

$74.375$ mph