EK 30 minute in-class exam for lecture 2 (Passage 1 #25)

[yingao88]

At the high speed achieved by vehicles on the highway, turns must be very gradual. As a safety precaution, highway turns are banked toward the inside. Federal guidelines specify highway curve speed limits based upon the angle of the bank, the avg coefficient of friction between a vehicle and the pavement, and the radius of curvature of the turn. The radius of curvature of a turn is the radius of a circle that would be circumscribed by the vehicle if the vehicle were to complete a full circle.



#25 If the vehicle in Figure 1 is moving very fast, but not slipping off the bank, the frictional force on the vehicle is most likely

a. static and in the direction of vector C.
b. static and in the direction of vector B.
c. kinetic and in the direction of vector C.
d. kinetic and in the opposite direction of vector C.

i am really confused with the direction of the friction force. why the friction force is not opposite of direction C?

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[slz1900]

Both B and C point in directions that the car isn't moving (assuming it isn't sliding). Because the car isn't sliding, there is no kinetic friction in the directions that either B or C point, therefore you know it can't be D. B doesn't make sense since the friction force is going to be parallel to the surface that the body is actually on. The B vector would be;the direction of the centripetal force vector.

If process of elimination doesn't do it for you, know that the friction force is the force responsible for centripetal acceleration. Imagine if the ramp was made of ice. The car would fly over the edge of it, not toward the middle, so obviously the friction force isn't pulling it off of the ramp.

 

[Postictal Raiden]

At the high speed achieved by vehicles on the highway, turns must be very gradual. As a safety precaution, highway turns are banked toward the inside. Federal guidelines specify highway curve speed limits based upon the angle of the bank, the avg coefficient of friction between a vehicle and the pavement, and the radius of curvature of the turn. The radius of curvature of a turn is the radius of a circle that would be circumscribed by the vehicle if the vehicle were to complete a full circle.

View attachment 22645

#25 If the vehicle in Figure 1 is moving very fast, but not slipping off the bank, the frictional force on the vehicle is most likely

a. static and in the direction of vector C.
b. static and in the direction of vector B.
c. kinetic and in the direction of vector C.
d. kinetic and in the opposite direction of vector C.

i am really confused with the direction of the friction force. why the friction force is not opposite of direction C?

This question is best solved by process of elimination. C and D can be eliminated right off the bat since we know that the car isn't sliding. Between options A and B, only option A makes sense because vector c is parallel to the surface.

 

[yingao88]

Both B and C point in directions that the car isn't moving (assuming it isn't sliding). Because the car isn't sliding, there is no kinetic friction in the directions that either B or C point, therefore you know it can't be D. B doesn't make sense since the friction force is going to be parallel to the surface that the body is actually on. The B vector would be;the direction of the centripetal force vector.

If process of elimination doesn't do it for you, know that the friction force is the force responsible for centripetal acceleration. Imagine if the ramp was made of ice. The car would fly over the edge of it, not toward the middle, so obviously the friction force isn't pulling it off of the ramp.

so only in the case where mass has kinetic friction (sliding movement) the friction force is opposite to the direction of movement, in case of static force is not necessary opposite? and also is there force of gravity (mgsin theta) acting on the car too? how does that play in the picture? is it true that mgsinO+ static friction=centripetal force ( since centripetal force is the net force)?