Kaplan Physical Sciences #1 Questions

[Jay2910]

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Hey everyone,

Today I have questions on Kaplan Physical Sciences Test #1.
Its attached here:View attachment physical sciences 1.docx

Here are my questions on the problems:

12) I picked B and the answer was D. When looking through the archives, I noticed that people were saying that mgsin(theta) was the Frictional force? and that static force would increase if you put it on a deeper incline? if the static force increases, then why do objects fall on a near 90 degree angled incline? Can someone please explain these in more depth. I thought if you increase the coefficient of static friction then you increase that static force as well.

33) This was a part of the semiconductor passage.
I don't get how a semiconductor like silicon needs to have its covalent bonds broken nor do I get like how I was supposed to assume that.
If I missed this question, then what didn't I review?

22) So, this question asks about maximum sideways force a car can sustain without sliding. I thought that sideways force had to be the normal force and I linked it to a centripetal acceleration problem( a roller coaster cart going in a circle). I remember that the normal force is always jutting out from the surface( like how on a rollar coaster cart . . .the normal force keeps changing directions).

What I want to know is, A) Is the normal force the same althoughout a circle and only its direction change?
B) Is that normal force what's keeping the card on sideways? cause its pointing towards the center of a circle ( I'm assuming the car is on a similar track).

25) For this question I used V(f)=V(i)+at.
I knew my initial and final velocities but I didn't know my acceleration. I thought I could get the acceleration by using the fact "A 1000kg car can accelerate from rest to 28 m/s in 8 sec" but this was the wrong methodology.
Why is it wrong and what is the right way to do this?

Thanks in advance!

 

[Jay2910]

Guys . . . I'm really desperate for some help as I am on a timeline.
I know, that I'm not the only one that has kaplan resources
I have worked on these problems for over a day and I think, that I am overthinking things.
I would be greatful for your help!

 

[anon4895]

Hey everyone,

Today I have questions on Kaplan Physical Sciences Test #1.
Its attached here:View attachment 21999

Here are my questions on the problems:

12) I picked B and the answer was D. When looking through the archives, I noticed that people were saying that mgsin(theta) was the Frictional force? and that static force would increase if you put it on a deeper incline? if the static force increases, then why do objects fall on a near 90 degree angled incline? Can someone please explain these in more depth. I thought if you increase the coefficient of static friction then you increase that static force as well.

Friction matches an opposing force. If you put a block on the ground you can multiply the coefficient by the normal force but there's no friction actually acting. If you push the block with 5N friction will push back with 5N. This will happen until you exceed the maximum force friction can provide. mu times normal is the max force.

With the block on the incline friction matches the force pulling it down until it can't anymore. When you increase the slope the block wants to slide more so you're asking more of friction.

33) This was a part of the semiconductor passage.
I don't get how a semiconductor like silicon needs to have its covalent bonds broken nor do I get like how I was supposed to assume that.
If I missed this question, then what didn't I review?

This is just something you need to chalk up to memorization I guess. The resistance of materials depends on their temperatures. Normally the hotter the more resistance. This makes sense the molecules are more energetic and bounce around hence impeding the flow of electrons. Semi conductors have negative thermal coefficients when it comes to this, which is backwards. Hotter means less resistance. The explanation is what's stated in the question. I don't think I would have known that if I wasn't a physics major.

22) So, this question asks about maximum sideways force a car can sustain without sliding. I thought that sideways force had to be the normal force and I linked it to a centripetal acceleration problem( a roller coaster cart going in a circle). I remember that the normal force is always jutting out from the surface( like how on a rollar coaster cart . . .the normal force keeps changing directions).

What I want to know is, A) Is the normal force the same althoughout a circle and only its direction change?
B) Is that normal force what's keeping the card on sideways? cause its pointing towards the center of a circle ( I'm assuming the car is on a similar track).

The normal force is constant here. It opposes the weight of the car pointing up. Friction is what keeps the car from sliding. Essentially you just use the friction formula. The answer is a greater mass bc that means greater weight which means greater normal force which means greater max friction.

25) For this question I used V(f)=V(i)+at.
I knew my initial and final velocities but I didn't know my acceleration. I thought I could get the acceleration by using the fact "A 1000kg car can accelerate from rest to 28 m/s in 8 sec" but this was the wrong methodology.
Why is it wrong and what is the right way to do this?

Thanks in advance!

The question tells you that the force to stop the car should be the static friction force. F = ma where F is the static friction force gives you a.

I would think that max acceleration would be the same as max deceleration. I didn't work it out myself, but if they don't match, use what I said before. The acceleration isn't what they're talking about here or what you're concerned with.