# PR 485 Physics Questions

#### WingMD

##### Junior Member
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Can someone explain to how the answer for number 75 on the physics portion is B.) the needle will spin counterclockwise? I looked at the answers and I still don't get it. Is it because the current carries electrons so according to the right hand rule, or left hand rule since I am dealing with electrons, will cause the Force to move out of the board on the left side and Force is moving into the board on the right?

Also for number 96 and 98, I thought that since there was friction, conservation of energy does not occur, yet the answers says that since coefficient of kinetic friction is really small, it can be ignored. So the answers applied PE=KE. But I always thought that even with some amount of friction energy is not conserved?

#### freakazoid

##### Guy Friend Extraordinaire
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Is there any way you could post the whole question so we can try to solve it even if we don't have PR485?

OP
W

#### WingMD

##### Junior Member
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Its kind of hard to post the whole question for 75 since it has to refer to the passage. So let me modify my question. I know that for a certain current in a wire I has to be perpendicular to B to feel a force. So is the current in the wire I always flowing of electrons so I cannot use the right hand rule to figure out the direction of the force? I have to use the left hand rule then?

Number 96 and 98 are really simple calculations if conservation of Energy holds, but I am confused because the question also states friction, which means that I cannot use conservation of energy right? But the answers says I have to use conservation of Energy because friction is really small. What do you think?

96) If a block with mass of 5kg slides down the inclined plane shown in Fig. 1 in 1.1 seconds and the coefficient of kinetic friction between the surface is 0.2, which one of the following best approximates the work done by gravity on the block as it slides down the plane?

A) 75J B) 100J C) 125J D) 150J

98) If the coefficient of kinetic friction between a 10kg block and the inclined plane is 0.01, what will be the speed of the block at the moment when it reaches the bottom?

A) 0.1m/s B) 0.5m/s C) 1m/s D) 5m/s

There is a picture of a block sliding down an inclined ramp at 37 degreeds at a height of 1.5m at a distance of 2.5m.

#### Focus

##### crazy!!
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Originally posted by WingMD
Its kind of hard to post the whole question for 75 since it has to refer to the passage. So let me modify my question. I know that for a certain current in a wire I has to be perpendicular to B to feel a force. So is the current in the wire I always flowing of electrons so I cannot use the right hand rule to figure out the direction of the force? I have to use the left hand rule then?

You can apply the right hand rule. Current flow is always opposite to electron flow. (think of current flow as "proton" flow, although protons don't actually move). However,the right hand rule applies to positive charge, if you are using it for a negative charge, just swich the signs of the opposite forces.

Number 96 and 98 are really simple calculations if conservation of Energy holds, but I am confused because the question also states friction, which means that I cannot use conservation of energy right? But the answers says I have to use conservation of Energy because friction is really small. What do you think?

96) If a block with mass of 5kg slides down the inclined plane shown in Fig. 1 in 1.1 seconds and the coefficient of kinetic friction between the surface is 0.2, which one of the following best approximates the work done by gravity on the block as it slides down the plane?00

A) 75J B) 100J C) 125J D) 150J

Work done by gravity is the work done in relations to the normal force, which in turn is related to the kinetic friction. Find the force done by friction, which is [Normal Force * KE friction] then multiply that by the ramp's length.

98) If the coefficient of kinetic friction between a 10kg block and the inclined plane is 0.01, what will be the speed of the block at the moment when it reaches the bottom?

A) 0.1m/s B) 0.5m/s C) 1m/s D) 5m/s

There is a picture of a block sliding down an inclined ramp at 37 degreeds at a height of 1.5m at a distance of 2.5m.
First you have to find the starting energy, which is the potential energy of MGH.

If there was no friction, then the speed at the bottom of the ramp would just be Potential energy = kinetic energy = MGH = (1/2)mv^2

however, since there's friction, you have to take that into account. So basically you have:

PE = KE + Friction Work

The friction work would be calculated the same way as the first problem. Solve for v, the speed of the block.

- Focus

#### freakazoid

##### Guy Friend Extraordinaire
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electrons always move in the opposite direction of the current, so you can either use the left hand rule or point the other way.

On 96, friction doesn't matter because it wants the work *done by gravity*.

On 98, friction does matter but because of the coefficient of friction of .01, it will be very very small in comparison to the potential energy.

mgh=(10)(10)(1.5)=150

(mu(k))*mgcos(theta) = (.01)*(10)(10)(some number less than one) = x<1