Why Am I Struggling with Newtonian Physics Concepts?

[PhysicsNovice]

I need more help please. I am moving along slowly in my physics quest to understand. The readings are more difficult and my understanding comes in and out. Please review, see how I did (answers marked with *) and make comments to help me learn. Thanks.


1. Which of the following has the largest momentum?
a. a tightrope walker crossing Niagara Falls
b. a pickup truck speeding along a highway*
c. a Mack truck parked in a parking lot
d. the Science building on campus
e. a dog running down the street



2. Two objects, A and B, have the same size and shape, but A is twice as heavy as B. When they are dropped simultaneously from a tower, they reach the ground at the same time, but A has a greater:
a. speed
b. acceleration
c. momentum*
d. all of these
e. none of these

3. A bullet is fired from a gun. The speed of the bullet will be the same as the speed of the recoiling gun because:
a. momentum is conserved
b. velocity is conserved
c. both velocity and momentum are conserved*
d. the mass of the bullet equals the mass of the gun
e. none of these

4. Two billiard balls having the same mass roll toward each other, each moving at the same speed. What is the combined momentum of the two balls?
a. 0 kg m/s*
b. 10 kg m/s
c. More information need to determine

5. A 5-kg fish swimming at a velocity of 1 m/s swallows an absent-minded 1-kg fish at rest. The speed of the larger fish after lunch is:
a. 1/2 m/s
b. 2/5 m/s*
c. 5/6 m/s
d. 6/5 m/s
e. 1 m/s

6. According to the impulse-momentum equation, Ft=change in (mv), a person will suffer less injury falling on a wooden floor (which gives) than on a more rigid cement floor. The F in the above equation stands for the force exerted on the:
a. person*
b. floor
c. both of these
d. none of these

7. A 1-kg chunk of putty moving at 1 m/s collides and sticks to a 5-kg bowling ball that is initially at rest. The bowling ball and putty then move with a momentum of:
a. 0 kg m/s
b. 1 kg m/s
c. 2 kg m/s*
d. 5 kg m/s
e. more than 5 kg m/s

8. A karate expert executes a swift blow and severs a cement block with her bare hand. The magnitude is the same for the:
a. impulse on both the block and the expert’s hand
b. force on both the block and the expert’s hand*
c. time of impact on both the block and the expert’s hand
d. all of these
e. none of these

9. The force that accelerates a rocket in outer space is exerted on the rocket by the:
a. rocket’s engine
b. rocket’s wings
c. atmospheric pressure
d. exhaust gases*
e. none of these

10. Compared to the force that brings a small car to a stop, the force required to bring a heavy truck traveling at the same speed to a stop:
a. is less
b. is more*
c. is the same
d. may be less and may be more

 

[Doc Al]

1. Which of the following has the largest momentum?
a. a tightrope walker crossing Niagara Falls
b. a pickup truck speeding along a highway*
c. a Mack truck parked in a parking lot
d. the Science building on campus
e. a dog running down the street

Right.

2. Two objects, A and B, have the same size and shape, but A is twice as heavy as B. When they are dropped simultaneously from a tower, they reach the ground at the same time, but A has a greater:
a. speed
b. acceleration
c. momentum*
d. all of these
e. none of these

Right.

3. A bullet is fired from a gun. The speed of the bullet will be the same as the speed of the recoiling gun because:
a. momentum is conserved
b. velocity is conserved
c. both velocity and momentum are conserved*
d. the mass of the bullet equals the mass of the gun
e. none of these

This is a weird question. I assume they mean for you to pretend that no one is holding the gun. That said, will the speed of a recoiling gun always be the same as the speed of the bullet? Or is this a special case?

4. Two billiard balls having the same mass roll toward each other, each moving at the same speed. What is the combined momentum of the two balls?
a. 0 kg m/s*
b. 10 kg m/s
c. More information need to determine

"toward each other" is a bit vague. If they mean directly toward each other, then you are correct. But what if they mean just that they smack into each other, perhaps at an angle?

5. A 5-kg fish swimming at a velocity of 1 m/s swallows an absent-minded 1-kg fish at rest. The speed of the larger fish after lunch is:
a. 1/2 m/s
b. 2/5 m/s*
c. 5/6 m/s
d. 6/5 m/s
e. 1 m/s

What's conserved during this "collision" of big fish and little fish?

6. According to the impulse-momentum equation, Ft=change in (mv), a person will suffer less injury falling on a wooden floor (which gives) than on a more rigid cement floor. The F in the above equation stands for the force exerted on the:
a. person*
b. floor
c. both of these
d. none of these

Consider Newton's 3rd law.

7. A 1-kg chunk of putty moving at 1 m/s collides and sticks to a 5-kg bowling ball that is initially at rest. The bowling ball and putty then move with a momentum of:
a. 0 kg m/s
b. 1 kg m/s
c. 2 kg m/s*
d. 5 kg m/s
e. more than 5 kg m/s

What's the initial momentum?

8. A karate expert executes a swift blow and severs a cement block with her bare hand. The magnitude is the same for the:
a. impulse on both the block and the expert’s hand
b. force on both the block and the expert’s hand*
c. time of impact on both the block and the expert’s hand
d. all of these
e. none of these

Consider Newton's 3rd law. What does it tell you about the time of impact on the two objects?

9. The force that accelerates a rocket in outer space is exerted on the rocket by the:
a. rocket’s engine
b. rocket’s wings
c. atmospheric pressure
d. exhaust gases*
e. none of these

Right.

10. Compared to the force that brings a small car to a stop, the force required to bring a heavy truck traveling at the same speed to a stop:
a. is less
b. is more*
c. is the same
d. may be less and may be more

Consider the impulse-momentum equation: the amount of force depends on how quickly you wish to stop the truck.

(Please use the homework help forum for these kinds of questions.)

 

[PhysicsNovice]

Hello again Doc Al. Thanks for the assistance. Based upon your information here are my edits:

3. A bullet is fired from a gun. The speed of the bullet will be the same as the speed of the recoiling gun because:
a. momentum is conserved
b. velocity is conserved
c. both velocity and momentum are conserved
d. the mass of the bullet equals the mass of the gun*
e. none of these
This problem represents a special case. If momentum = mv the the speed of the bullet = the speed of the recoiling gun then the mass of the gun and bullet must be equal.

5. A 5-kg fish swimming at a velocity of 1 m/s swallows an absent-minded 1-kg fish at rest. The speed of the larger fish after lunch is:
a. 1/2 m/s
b. 2/5 m/s
c. 5/6 m/s*
d. 6/5 m/s
e. 1 m/s
Using the concept of the conservation of momentum then a 5 kg fish traveling at 1m/s eats a "standing still" 1 kg fish would result in a 6 kg fish traveling at 5/6 m/s.

6. According to the impulse-momentum equation, Ft=change in (mv), a person will suffer less injury falling on a wooden floor (which gives) than on a more rigid cement floor. The F in the above equation stands for the force exerted on the:
a. person
b. floor
c. both of these*
d. none of these
Not exactly sure the reasoning but both the floor and the person have a force exerted on them.

10. Compared to the force that brings a small car to a stop, the force required to bring a heavy truck traveling at the same speed to a stop:
a. is less
b. is more
c. is the same
d. may be less and may be more*
Any force, greater or smaller than the first force, could stop the moving car. The time to stop the car would vary.

Questions 7 & 8 I am not sure about. Also, how do I access the homework help forum? I did not see that at the physicsforums.com site. Thanks again.

 

[Doc Al]

3. A bullet is fired from a gun. The speed of the bullet will be the same as the speed of the recoiling gun because:
a. momentum is conserved
b. velocity is conserved
c. both velocity and momentum are conserved
d. the mass of the bullet equals the mass of the gun*
e. none of these
This problem represents a special case. If momentum = mv the the speed of the bullet = the speed of the recoiling gun then the mass of the gun and bullet must be equal.

Right.

5. A 5-kg fish swimming at a velocity of 1 m/s swallows an absent-minded 1-kg fish at rest. The speed of the larger fish after lunch is:
a. 1/2 m/s
b. 2/5 m/s
c. 5/6 m/s*
d. 6/5 m/s
e. 1 m/s
Using the concept of the conservation of momentum then a 5 kg fish traveling at 1m/s eats a "standing still" 1 kg fish would result in a 6 kg fish traveling at 5/6 m/s.

Right.

6. According to the impulse-momentum equation, Ft=change in (mv), a person will suffer less injury falling on a wooden floor (which gives) than on a more rigid cement floor. The F in the above equation stands for the force exerted on the:
a. person
b. floor
c. both of these*
d. none of these
Not exactly sure the reasoning but both the floor and the person have a force exerted on them.

I'd agree. Both the person and the floor exert the same amount of force on each other (but in opposite directions). So F would be the magnitude of the force on both.

10. Compared to the force that brings a small car to a stop, the force required to bring a heavy truck traveling at the same speed to a stop:
a. is less
b. is more
c. is the same
d. may be less and may be more*
Any force, greater or smaller than the first force, could stop the moving car. The time to stop the car would vary.

Right.

Questions 7 & 8 I am not sure about.

For #7: During the collision momentum does not change. So what's the moment before the collision? (What's the only thing moving?)
For #8: If A pushes on B for 5 seconds, how long must B push on A?

Also, how do I access the homework help forum? I did not see that at the physicsforums.com site.

If you can read this... you are in the Homework Help section (College Level Help).

 

[PhysicsNovice]

Thanks Al. I think that I have all of the questions and answers figured out. I am still weak on the formulas but I undestand and can explain in my own words the concepts. Would you agree from my responses? Tks. again!

 

[Doc Al]

You're getting there. Keep at it.