Kinematics questions

[kingwinner]

1) What is the difference between free fall and projectile? The following are definitions from my textbook, but I don't understand the difference between these 2 terms...

free fall: motion of an object toward earth with no other force acting on it than gravity

projectile: an object that moves through the air, along a trajectory, without a propulsion system

 

True or False: (#2-4)

 

2) You toss a ball vertically and step aside. The ball rises and then falls down along the same path and hits the ground. The magnitude of the velocity of the ball just before landing is greater than its magnitude of initial velocity upon leaving your hand.[i put down False but the answer says True, why is it true?]

 

3) The time for the ball in question 1 to rise equals the time for it to fall.[i put down True but the answer says False, why? I don't understand...]

 

4) A model rocket launched in a vacuum chamber at an angle of 45 degrees above the horizontal, undergoes projectile motion.

[is this a projectile motion? Well, I would assume that a rocket has a propulsion system...reasonable, right? So the answer is False?]

 

5) A race car driver wants to attain a velocity of 54m/s[N] at the end of a curved stretch of track, experiencing an average acceleration of 0.15m/s^2 for 95s. What is the final velocity? (answer:54m/s[E])

[Does anyone know how to solve this problem? I really don't understand...isn't the final velocity 54m/s[N] (already given, right?), what is it asking for?]

 

Can someone help me out? I would appreciate! :steering:

[Jay-qu]

1) What is the difference between free fall and projectile? The following are definitions from my textbook, but I don't understand the difference between these 2 terms...

free fall: motion of an object toward earth with no other force acting on it than gravity

projectile: an object that moves through the air, along a trajectory, without a propulsion system

 

the definition of free fall is ok, except that it neglects drag and also aplies to projectile motion..

projectile - projected up normally at some angle, i would say that it is in freefall after it has reached a zero velocity in the up/down direction

 

its a bit of a grey area that i wouldnt worry to much about - take this example: would you say a sky diver is in free fall? I would, but dont forget they maintain the velocity of the plane they jumped out of..

 

2. true because the ground is at a lower point than your hand, hence more time to accelerate

 

3. false same reason as before

 

4. i think your spot on there - but i did have a question on my exam last year on which a model rocket was launched with a spring.. no propulsion after take off..

 

5. is a meaningless question without an initial velocity :steering:

[Qfwfq]

The rocket is in free fall as soon as its thrust ceases. It doesn't require the motion to be vertically downward. Even the moon is in free fall around Earth.

 

King, wouldn't it make sense if you started one thread asking for homework help, instead of a new thread for each little question? We've been considering having a forum especially for homework help but we're not sure yet. It would be better if you at least used one thread for all your questions. :steering:

[Jay-qu]

well it could be moved to the science projects and homework thread..

[kingwinner]

Hi,

 

4) But a rocket definitely has an engine (i.e. propulsion system), right? So the motion described in here isn't undergoing projectile motion...I think

 

6) The displacement from London to Rome is 1.4x10^3 km [s43E]. A wind is blowing with a velocity of 75km/h [E]. The pilot wants to fly directly from London to Rome in 3.5h. What velocity relative to the air must the pilot maintain?

 

[i actually worked this problem twice to see if I have made a careless mistake, but I still can't get the answer in my textbook! I got 3.5x10^2 km/h [E56S], but the answer in my textbook says 5.5x10^2 km/h [W15 S]. Since my answer is different, I suspect that I did something wrong. I hope someone can teach me how to solve this problem! Thank you!]

 

{Would it be better to put all questions in 1 post? I have tried doing this before in the Earth Science board, but someone told me to make new posts for new questions because it is easier to see...}

[Qfwfq]

But a rocket definitely has an engine (i.e. propulsion system), right? So the motion described in here isn't undergoing projectile motion...I think

While the engine is on, but not after the engine is stopped.

 

{Would it be better to put all questions in 1 post? I have tried doing this before in the Earth Science board, but someone told me to make new posts for new questions because it is easier to see...}

In the Science Projects and Homework thread you can do it however is best for those willing to help you, maybe you could start one for physics, one for Earth etc. and even ask what folks prefer... :confused:

[Jay-qu]

::moved thread to science projects and homework::

 

4. I would say that its not projectile motion as the question stands, but it could be different with more info..

 

6. you need a resultant velocity of 400km/h S43E

 

to get the resultant you need 273km/h E and 293km/h S

 

so the pilot must go 198km/h E and 293km/h S

 

which makes it 353km/h S34E

[Racoon]

::moved thread to science projects and homework::

 

4. I would say that its not projectile motion as the question stands, but it could be different with more info..

 

6. you need a resultant velocity of 400km/h S43E

 

to get the resultant you need 273km/h E and 293km/h S

 

so the pilot must go 198km/h E and 293km/h S

 

which makes it 353km/h S34E

 

Since when do you double-SPACE your type ? :singer: :confused: :hyper:

[Jay-qu]

I was just trying to make it clear and easy to understand - it looked all cluttered one after the other...:confused:

[kingwinner]

6) Thanks a lot! Now I know that my method is correct and the textbook messed up the answer! I hate textbooks with wrong answers :confused:

[TheBigDog]

6) Thanks a lot! Now I know that my method is correct and the textbook messed up the answer! I hate textbooks with wrong answers :hyper:

They are sometimes intentional, to check that people are understanding the process. Teachers can be tricky that way.:confused:

 

Bill (son of teachers)

[Jay-qu]

lol my lecturer made a mistake yesterday he said 'some teachers will try to tell you that they have done it on purpose to test if your paying attention, me, Im just getting old' :confused:

[kingwinner]

Whenever the textbook's answer is wrong, I have to spent hours checking all of my work again, for nothing......wrong answers just drive me crazy because I will be thinking that I did something wrong and this confuses my understanding of the topic even more...and more... :naughty: