# Newton's 3rd law

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#### armpitsOFmight

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Consider a situation when a shuttle is in space and fires its engines, creating a force accelerating the shuttle. Which of the following BEST describes the force accelerating the shuttle?

A- The force of the exhaust gases on the shuttle
B- The force of the shuttle on the exhaust gases.

Thanks!!

Consider a situation when a shuttle is in space and fires its engines, creating a force accelerating the shuttle. Which of the following BEST describes the force accelerating the shuttle?

A- The force of the exhaust gases on the shuttle
B- The force of the shuttle on the exhaust gases.

Thanks!!

In order to accelerate the shuttle, the force must be acting UPON the shuttle itself. Therefore choice B cannot be correct because it describes the force being performed BY the shuttle, not upon it.

Where is this question from?

Nova physics

I agree, the shuttle can not place a force on itself. It's the equal and opposite force from the gas that is accelerating it.

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i have a q:

lets say you have a pickup truck and in the back there are some pillars. The truck turns a corner and the pillars fall over since they are not secured properly.

Which of newtons laws causes the the pillars to fall?
its the 3rd law right?

i have a q:

lets say you have a pickup truck and in the back there are some pillars. The truck turns a corner and the pillars fall over since they are not secured properly.

Which of newtons laws causes the the pillars to fall?
its the 3rd law right?

Second, F=ma. You turned (changed direction)= accelerated them right off the truck. That make sense? But remember all three laws can be acting during the same event, don't get confused thinking it has to be one or the other.

Second, F=ma. You turned (changed direction)= accelerated them right off the truck. That make sense? But remember all three laws can be acting during the same event, don't get confused thinking it has to be one or the other.

Not quite. Its actually Newton's first law. The truck turns a corner and, thus, accelerates around that corner. However, since the pillars aren't secured, they aren't a part of the truck-system, so they don't experience the force that causes the truck to accelerate (Newton's second law). Since the pillars experience no net force, they continue to move in their initial direction, which is in the direction of the truck before turning, which causes the toppling of the pillars.

Furthermore, we can use this situation to show the relationship between the first two laws: the first law is actually a special case of the second law when Fnet=0 (i.e. a=0).

Not quite. Its actually Newton's first law. The truck turns a corner and, thus, accelerates around that corner. However, since the pillars aren't secured, they aren't a part of the truck-system, so they don't experience the force that causes the truck to accelerate (Newton's second law). Since the pillars experience no net force, they continue to move in their initial direction, which is in the direction of the truck before turning, which causes the toppling of the pillars.

Furthermore, we can use this situation to show the relationship between the first two laws: the first law is actually a special case of the second law when Fnet=0 (i.e. a=0).

Hmm, ok I could see that too. That's why I was warning about trying to make there only be 1 law in effect. The question wording needs to be really specific. But if the pillars aren't part of the system and you're saying no net force acted on them, how did they get accelerated with the truck in the first place?

thanks

i sort of reasoned my way through all three laws at one point and didn't have a clear answer...

i guess i dont know the laws as well as i think?

so the force law- otherwise known as the law of inertia?

i have it like this in my mind.....a object in motion stays in that motion unless compelled to change direction by another force...

The pillars in the pickup bed are traveling straight along the road just like the truck, the truck turns a corner, and now the pillars have the side of the pickup bed exerting a force on them, causing them to topple?

2nd F-ma! i didn't even think to consider that a acceleration was occurring with that example!
turning=change in direction=change in velocity=acceleration!

so the third law...i have the typical notion in mind for every action there is a equal and opposite reaction...but what does this mean really?

i know the typical...you stand on the ground, the earth pushes up but anyone have some more sort of complicated examples?

in the truck eg, i was thinking that....well since the pillars are being pushed aside by side of the truck bed during the turn, then they too must be exerting a force back on the truck? im not entirely sure!

thanks

i sort of reasoned my way through all three laws at one point and didn't have a clear answer...

i guess i dont know the laws as well as i think?

so the force law- otherwise known as the law of inertia?

i have it like this in my mind.....a object in motion stays in that motion unless compelled to change direction by another force...

The pillars in the pickup bed are traveling straight along the road just like the truck, the truck turns a corner, and now the pillars have the side of the pickup bed exerting a force on them, causing them to topple?

2nd F-ma! i didn't even think to consider that a acceleration was occurring with that example!
turning=change in direction=change in velocity=acceleration!

so the third law...i have the typical notion in mind for every action there is a equal and opposite reaction...but what does this mean really?

i know the typical...you stand on the ground, the earth pushes up but anyone have some more sort of complicated examples?

in the truck eg, i was thinking that....well since the pillars are being pushed aside by side of the truck bed during the turn, then they too must be exerting a force back on the truck? im not entirely sure!

We need more specifics to answer this question correctly, I don't think an actual MCAT question would be this vague, it would have to specify exactly which event in the sequence of events it wanted you to analyze, in my opinion.

Hmm, ok I could see that too. That's why I was warning about trying to make there only be 1 law in effect. The question wording needs to be really specific. But if the pillars aren't part of the system and you're saying no net force acted on them, how did they get accelerated with the truck in the first place?

The question stated/implied that the pillars were already moving along with the truck, so it before turning it was technically part of the system. But the frame of reference changed with the turn occurred. In that frame, the truck and the pillar were in separate systems. The truck-system was subject to a nonzero force, while the pillar-system moved forward as Fnet=0.

As to how the pillars would've been accelerated along with the truck initially, it depends on how the truck initially accelerated. Intuitively, if the truck accelerated from zero slowly and smoothly, then the pillars act as if they are a part of the system, even though they are loose. Now, if the truck accelerated from zero abruptly, then the pillars wouldn't have accelerated, and would have toppled over as the truck started moving.

What's happening here is when the initial jerk (change in acceleration) is very small, the truck and pillar are part of one system subject to acceleration (via force). But when the initial jerk is large, then the truck and pillars are different systems.

Also, I agree that all this extraneous info would be provided/assumed when taking the MCAT.

Don't overthink the question; only use data that is relevant.

Good luck with studies!

thanks

i sort of reasoned my way through all three laws at one point and didn't have a clear answer...

i guess i dont know the laws as well as i think?

so the force law- otherwise known as the law of inertia?

i have it like this in my mind.....a object in motion stays in that motion unless compelled to change direction by another force...

The pillars in the pickup bed are traveling straight along the road just like the truck, the truck turns a corner, and now the pillars have the side of the pickup bed exerting a force on them, causing them to topple?

2nd F-ma! i didn't even think to consider that a acceleration was occurring with that example!
turning=change in direction=change in velocity=acceleration!

so the third law...i have the typical notion in mind for every action there is a equal and opposite reaction...but what does this mean really?

i know the typical...you stand on the ground, the earth pushes up but anyone have some more sort of complicated examples?

in the truck eg, i was thinking that....well since the pillars are being pushed aside by side of the truck bed during the turn, then they too must be exerting a force back on the truck? im not entirely sure!

No, during the pickup truck example, there is absolutely NO force being exerted by the truck on the pillars (other than the normal force). The pillars topple because they are NOT acted upon by a net force (Newton's First Law).

As for more examples on the third law: think about someone rollerblading toward a wall. If the blader pushes on the wall, he exerts a nonzero force on the wall, but the wall doesn't move. Instead, the blader gets pushed back. What happens here is the blader exerts a force, lets say, F on the wall. By Newton's third law, the wall exerts a force -F on the blader that is equal in magnitude, but opposite in direction. Both of these forces are the same, but the outcomes are different: the blader accelerates, but the wall doesn't. This is because the respective masses are different. The blader has a relatively small mass and by Newton's second law (Fnet=ma), he accelerates in the direction of the force exerted upon him (by the wall). The wall, however, has a massive mass (no pun intended), so the acceleration is negligibly small and practically zero, which accounts for the lack of motion (actually its more complicated, but this is sufficient explanation for the MCAT).

Hope this helps!

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As to how the pillars would've been accelerated along with the truck initially, it depends on how the truck initially accelerated. Intuitively, if the truck accelerated from zero slowly and smoothly, then the pillars act as if they are a part of the system, even though they are loose.

I don't think that's physically possible unless friction is acting, no matter how small the initial acceleration, in theory? Thinking back to the "frictionless" cart and track in physics lab, if I pulled the track the cart would not stay in place on the track, the track would be pulled "out from under the cart" I believe.

I think this question is dumb, I give up.