Physics
Name: ______________________
Problem Solving Practice
1. Read: Giancoli Ch 2 sections 2-5 and 2-6, pp 26-30
2. Go back and study the box on page 28 called “Problem Solving”. I will be strict about your following
these guidelines.
3. Your understanding of these equations, which are used a lot in physics, will increase and then you will
know them by heart. For now, you should learn how to use them, as illustrated in examples 2-6,2-7, 28, and 2-9.
Constant Speed With No Conversions:
4. What must your average speed be to travel 5.0 m in 2.0 s?
a. Make a list of the known and unknown.
b. Using the symbols we have introduced, write down the equation you will use to find the average speed:
c. Rewrite the equation with numbers substituted for all the quantities whose values are known:
d. Calculate the car’s average speed, and put the complete (rounded, labeled) answer in a box.
5. During a sneeze, your eyes shut for 0.50 s. If you are driving a car at 24.0 m/s, how far does the car
move during your sneeze?
List of known and unknown:
Equation in symbol form solved for the unknown:
Substitution and solve:
Constant Speed With Conversions:
6. How long does it take a snail to move 3.5 m if his average speed is 1.00 cm/minute?
List with conversions:
Equation:
Substitution and solve:
7. What must your average speed be (in m/s) if you travel 30.25 km in 30.0 minutes?
List with conversions:
Equation:
Substitution and solve:
Shifting Frame of Reference:
8. Example: (Problem 10 pg. 39) Two locomotives approach each other on parallel tracks. Each has a
speed of 95 km/h with respect to the earth. If they are initially 8.5 km apart, how long will it be before
they pass each other.
The speeds of the trains are given with the respect to the earth. But if the earth is removed from the
picture, one train is approaching the other at a speed of 190 km/h. That is, move into the train's frame of
reference, and the speed of the first train compared to the second is 190 km/h. The question now
becomes, “ How long does it take a train traveling 190 km/h to cover a distance of 8.5 km?” This
problem is now plug and chug.
At an average speed of 190 km/h how long will it take a train to travel 8.5 km?
Δx = 8.5 km = 8,500 m
v = 190 km/h = 52.77778 m/s
Δt = Δx/v
Δt = 8,500 m / 52.777778 m/s
Δt = 161s
9. (Problem 12 pg. 39) A car traveling 88 km/h is 110 m behind a truck traveling at 75 km/h. How
long will it take the car to reach the truck?
Average Speed Problems:
If a journey is made up of legs, each with its own constant speed, the average speed over the whole trip is:
total distance distance of leg 1 + distance of leg 2 + ...
=
total time
time of leg 1 + time of leg 2 + ...
You may have to do a separate problem for each leg of the trip.
average speed =
10. Example: A car travels 200 km for 1.0 h, then it travels at a speed of 85 km/h for 1.5 h, and then it
travels a distance of 64 km at a speed of 120 km/h. What was its average speed?
Solution: You must find the total distance and total time by adding up the three distances and
times for the three legs of the journey. For leg 1, they are given. But you must solve for distance for leg
2 and time for leg 3 before you can complete the problem.
Leg 1:
∆x =
∆t =
v=
Leg 2:
∆x =
∆x = v∆t
∆t =
v=
Leg 3:
∆x =
∆t = ∆x/v
∆t =
v=
Final step:
average speed =
total distance
=
total time
11. Anne in C Building is walking to class. She walks 3 m/s for 10 s, then stops to talk to Dave for
5s. Realizing that she is late to class, she sprints at 6 m/s for 4 s to reach her class. What was her
average speed for the entire trip?
Average Velocity Problem:
12. A runner dashes from the starting line (x = 0) to a point 84 m away and then turns around and runs to a
point 18 m away from the starting point in 23 seconds. What is the average velocity?
List:
Equation in symbol form:
Substitution and solve (since velocity is a vector you need to include direction in your final answer):
Average Velocity Problem with too much info given (read carefully):
13. Kelly is standing at the open window of her physics room, 20m above the ground. She throws a ball
straight up at an initial speed of 14 m/s, it rises another 10m to a total height of 30m above the ground in
1.43s. It then falls to the ground reaching a speed 24.2 m/s before it hits in another 2.47s. Calculate the
average velocity of the ball.
List:
Equation in symbol form:
Substitution and solve (since velocity is a vector you need to include direction in your final answer):
Average Acceleration Problem:
14. A car that is traveling west at 15.0 m/s accelerates for 3 seconds until it reaches a velocity of 35 m/s.
List:
Equation in symbol form:
Substitution and solve (since acceleration is a vector you need to include direction in final answer):
15. Calculate the acceleration of Kelly’s ball in question 13 for the trip up to the peak and then calculate
the acceleration of the ball as it falls to the ground. If done correctly, you will get -9.8m/s2 for both. Be
very careful with the signs of your velocity! Remember that up is positive and down in negative.
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