How do I solve these problems (kinematics)

  • Thread starter carrollk9
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In summary: The correct answer would be 66.3 kphYes, except for the sig figs. The correct answer would be 66.3 kphIn summary, the conversation is about determining the average speed of a trip where the driver drove for 40 minutes at 80 km/h, 10 minutes at 75 km/h, and 30 minutes at 45 km/h. The student attempted to solve the problem and got a result of 66.25 km/h, but was unsure if it was correct. It was mentioned that the proper equation to use is distance = rate × time, and that the only potential issue with the answer is the number of significant figures. The expert confirmed that the answer is correct, but should be
  • #1
carrollk9
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Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip
The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So what's the proper way I am suppose to solve this?
 
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  • #2
carrollk9 said:
Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip
The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So what's the proper way I am suppose to solve this?
What fraction of an hour is 10 minutes?
 
  • #3
I appreciate the answer the question but I would enjoy more on understanding how to do them myself

Other questions on my online homework I don't get
Two boats start together and race across a 70-km-wide lake and back. Boat A goes across at 70 km/h and returns at 70 km/h. Boat B goes across at 35 km/h, and its crew, realizing how far behind it is getting, returns at 105 km/h. Turnaround times are negligible, and the boat that completes the round trip first wins.

Boat A wins but by how much? What is the average velocity of the winning boat?
since the boat starts moving at 70km/h and returns at 70km/h how come its says I am wrongs when I enter 70?
 
  • #4
SteamKing said:
What fraction of an hour is 10 minutes?
1/6th
 
  • #5
carrollk9 said:
1/6th
Why did you divide your calculation in Post #1 by 8 then?
 
  • #6
SteamKing said:
Why did you divide your calculation in Post #1 by 8 then?


because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.
 
  • #7
carrollk9 said:
because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.
The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
 
  • #8
SteamKing said:
The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
but my answer is correct right?
 
  • #9
carrollk9 said:
but my answer is correct right?
Yes, except for the sig figs.
 

Related to How do I solve these problems (kinematics)

1. What are the basic principles of kinematics?

The basic principles of kinematics involve the study of motion and the factors that affect it, such as velocity, acceleration, displacement, and time. These principles are governed by Newton's laws of motion and can be described using mathematical equations.

2. How do I approach solving kinematics problems?

The first step in solving kinematics problems is to understand the given information and identify what is being asked. Then, you can use the appropriate kinematic equations to solve for the unknown variable. It is important to label your variables and units correctly and to use the correct equation for the given scenario.

3. What is the difference between displacement and distance in kinematics?

Displacement refers to the change in position of an object from its starting point, while distance is the total length of the path traveled by the object. Displacement is a vector quantity, meaning it has both magnitude and direction, while distance is a scalar quantity and only has magnitude.

4. How does acceleration affect an object's motion in kinematics?

Acceleration is the rate of change of an object's velocity over time. It can either increase or decrease an object's velocity, depending on whether it is positive or negative. In kinematics, acceleration is affected by the net force acting on an object, as described by Newton's second law of motion.

5. Can kinematics equations be used for objects moving in curved paths?

Yes, kinematics equations can be used for objects moving in curved paths, as long as the motion is constant and the acceleration remains constant. In this case, the equations can be modified to incorporate the curved path and the motion can be analyzed using the same principles as linear motion.

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