Confusion over 2nd law and conservation of momentum

AI Thread Summary
Adding weight to a moving cart affects its momentum, but the cart's speed can change depending on how the weight is added. If the weight is introduced at zero speed relative to the ground, the cart will experience a backward acceleration as the weight accelerates forward to match the cart's speed. The interaction between the cart and the weight creates a force that accounts for this change. When removing the weight by lifting it straight up, the method of removal can influence the cart's motion, suggesting that careful handling is crucial. Understanding these dynamics clarifies the relationship between momentum conservation and acceleration in both linear and rotational motion.
steven george
Messages
11
Reaction score
1
Hi everybody, I apologize if something like this has been asked before but I have been unable to find an answer through searching.

If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum. How is it that the cart has accelerated without a horizontal force?

This issue originally came up while discussing rotational motion. If a figure skater is spinning and brings his hands in he will speed up. How can he accelerate without any torque?

There must be something very basic that I am missing here, so thank you to anybody who can help with an answer.
 
Physics news on Phys.org
steven george said:
If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum.
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.
steven george said:
How is it that the cart has accelerated without a horizontal force?
There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..
 
  • Like
Likes steven george
mfb said:
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..

I knew there must be a simple answer. Thanks for that. Now, what if that weight is taken off by carefully lifting it straight up?
 
What do you mean by "carefully"? The answer may depend on this.
 
  • Like
Likes Nugatory

Thread '1:1 versus 2:1 Mechanical Advantage debate'

The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
View full post »

Thread 'Newton's first law?'

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?
View full post »
Thread 'Beam on an inclined plane'

Thread 'Beam on an inclined plane'

Hello! I have a question regarding a beam on an inclined plane. I was considering a beam resting on two supports attached to an inclined plane. I was almost sure that the lower support must be more loaded. My imagination about this problem is shown in the picture below. Here is how I wrote the condition of equilibrium forces: $$ \begin{cases} F_{g\parallel}=F_{t1}+F_{t2}, \\ F_{g\perp}=F_{r1}+F_{r2} \end{cases}. $$ On the other hand...
View full post »

Similar threads

I Conservation of Angular Momentum vs a gyro at the North Pole
Replies
16
Views
960
I Conservation of angular momentum during collisions
Replies
3
Views
3K
Conceptual questions about Angular Momentum Conservation and torque
Replies
2
Views
2K
Debate over the experimental procedure for Newton's 2nd Law
Replies
6
Views
1K
Jumping off a moving cart on a frictionless track
Replies
15
Views
5K
The conservation of angular momentum
Replies
9
Views
5K
Conservation of angular momentum and its counterpart for linear momentum
Replies
13
Views
3K
Symmetries and Conservation Laws
Replies
2
Views
2K
Confusion about conservation of energy vs. momentum
Replies
3
Views
2K
Orbital Slingshot and Conservation of Momentum Confusion
Replies
9
Views
7K

Hot Threads

Recent Insights

Back
Top