TBR Translational Motion Practice Exam #15

[salsasunrise123]

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The question has ramp with object being pushed up it. In original case there is no friction. Then friction is added to ramp. What happens to magnitude of acceleration of object? The answer says it increases but I don't understand how that is the case. Can someone please explain it. Thx.

 

[BestDoctorEver]

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[brood910]

Acceleration is change in velocity over time... The friction will change the velocity in a more drastic fashion so to speak; therefore, will increase the acceleration... This is the problem I have with TBR physics. They don't not explain these minor concepts properly at all, which can make it difficult to see the big picture. If you are not strong in physics, I would suggest to use NOVA physics first before doing TBR.

? You are just not understanding the concepts at all. Friction doesnt always increase acceleration.
Dont blame it on the book. It is excellent.


Back to OP,

Before you add friction, ma = F - mgsin.
After you add friction, ma = F - mgsin - umgcos

So, acceleration actually should have decreased. Can you tell me the full question? Maybe they are ignoring the acceleration during the initial upward push. Then it makes sense.


Btw if you were going DOWN, not up, then it is the opposite as mgsin and umgcos are opposite directions.

 

[BestDoctorEver]

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[salsasunrise123]

The question reads as follows: If the apparatus is changed to increase the friction acting upon the carts, what might the students find regarding the upward motion in subsequent experiments compared to previous experiments? In the original scenario there was no friction.

Also, another question asks what will happen to the acceleration of object if multiple trials are conducted all using same angle theta for incline but different initial velocities. I don't get why the acceleration would remain the same for each trial if you are starting with different initial velocities since acceleration = deltav/deltat. also in each trial different times and distances up ramp were recorded which would correspond to different initial velocities. I am just really having trouble with acceleration. Please help.

 

[brood910]

The question reads as follows: If the apparatus is changed to increase the friction acting upon the carts, what might the students find regarding the upward motion in subsequent experiments compared to previous experiments? In the original scenario there was no friction.

Also, another question asks what will happen to the acceleration of object if multiple trials are conducted all using same angle theta for incline but different initial velocities. I don't get why the acceleration would remain the same for each trial if you are starting with different initial velocities since acceleration = deltav/deltat. also in each trial different times and distances up ramp were recorded which would correspond to different initial velocities. I am just really having trouble with acceleration. Please help.

The passage probably mentioned about ignoring the upward acceleration somewhere. Otherwise, the answer to the question is wrong. This question is exactly the same with one of the questions from the book. The question tells us to ignore the upward acceleration generated by the initial upward push.

Also, initial velocities do not affect the acceleration. Look at the equation.

ma = F - mgsin - umgcos
If the upward force is ignored,
ma = -mgsin - umgcos.

 

[salsasunrise123]

I don't understand how initial velocity doesnt affect acceleration. acceleration = final velocity-initial velocity/ time. I think i am assuming there is an upward force to get object moving but there isnt. the passage only mentions that object starts with initial velocity but doesnt say anything about an upward force. I am confused as to how there can be an initial velocity up a ramp but no associated upward force. Can you explain this?

 

[brood910]

I don't understand how initial velocity doesnt affect acceleration. acceleration = final velocity-initial velocity/ time. I think i am assuming there is an upward force to get object moving but there isnt. the passage only mentions that object starts with initial velocity but doesnt say anything about an upward force. I am confused as to how there can be an initial velocity up a ramp but no associated upward force. Can you explain this?

That makes sense.
They are ignoring the force that created the initial upward velocity like they did for the similar question from the book. There WAS an upward force, but they are talking about the situation AFTER the force is applied.

If you push a skier up the ramp with a certain force, then the skier will go up initially, but after a certain amnt of time, he will go down again. I am pretty sure this is the situation they are talking about.

Yes, there was an initial velocity, but they are not applying a constant upward force to keep the object moving upward.

 

[Odi]

? You are just not understanding the concepts at all. Friction doesnt always increase acceleration.
Dont blame it on the book. It is excellent.

Y U CONDESCENDING ?

The question reads as follows: If the apparatus is changed to increase the friction acting upon the carts, what might the students find regarding the upward motion in subsequent experiments compared to previous experiments? In the original scenario there was no friction.

There is no upward force. The carts are released with the same initial velocity. The only force acting on the cart is downward [ma = -mgsin] for the first experiment. For the next experiment they add friction. So both the mg force and friction are now acting on the cart [ma= -mgsin - umgcos]. What's happening to the magnitude of acceleration now? It has increased in negative direction.

You don't have to do any free body diagram for this. Just think of it conceptually. Acceleration is in the negative direction... apparent by the carts leaving with an initial velocity then coming to a stop at some point on the ramp before rolling back down. If you add friction, the cart reaches a lower distance before coming to rest and rolling back down. The acceleration in the negative direction must have been greater in this scenario to "eat up" that initial velocity.

Also, another question asks what will happen to the acceleration of object if multiple trials are conducted all using same angle theta for incline but different initial velocities. I don't get why the acceleration would remain the same for each trial if you are starting with different initial velocities since acceleration = deltav/deltat. also in each trial different times and distances up ramp were recorded which would correspond to different initial velocities. I am just really having trouble with acceleration. Please help.

Acceleration is caused by forces--not velocity. When the cart is released the only force acting on it is [ma = -mgsin] as it goes up and [ma = mgsin] as it comes down. If you want to reduce it further, mass cancels on each side so the only force causing acceleration is [a = gsin]. Since g never changes, and they are holding sin(theta) constant... [a] doesn't change either.

 

[E pur si muove]

PREMED501,

Just think of it as a ball being thrown up into the air. Yes it took force to achieve the initial velocity, but that is irrelevent. The question only wants to know what happens to the acceleration once it has left the hand. Now if you think about the force acting on the ball, the only force is gravity.

Now apply that to the ramp. The ramp acts to alleviate the downward force of gravity acting on the object (vs. free falling). The sin of the angle determines what fraction of the force of gravity the block will be feeling. This is because, sin(0)=0 is a flat plane, so the force acting upon you is essentially 0 (there is still a gravitational force, but the normal force cancels it out). So sin(90) = 1 is a vertical plane, and it offers no resistance to the force of gravity.

No for friction. FRICTION ALWAYS ACTS AGAINST THE DIRECTION THE OBJECT IS MOVING. So what that means is while the block going up the incline (from that initial velocity from some force we don't care about), the force of friction is opposing that movement. So since gravity is pulling you down, and friction is pulling you down while you travel in the upwards manner, that means the net force pulling you down is
the force of gravity (mgsin(theta)) - force of friction (Fn = (-)mgsin(theta)).

Also, just because the magnitude is increasing, that doesn't mean the it is accelerating in a positive manner. The main question the question is asking is, if the two blocks are pushed up an incline which one will stop first? That is all the question cares about. So logically, which one stops first? The one with friction of course!