TBR Friction: Constant Force?

[justadream]

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TBR Physics page 152 #15

Says that the force of friction required to slow down a moving car is "relatively constant"

Can someone provide more insight into what "relatively constant" means?

Does that mean "slightly decreasing over time" or does it actually mean "constant"?

 

[user12]

TBR Physics page 152 #15

Says that the force of friction required to slow down a moving car is "relatively constant"

Can someone provide more insight into what "relatively constant" means?

Does that mean "slightly decreasing over time" or does it actually mean "constant"?

the force of friction depends pretty much solely on the normal force and the coefficient of friction which is dependent on the two surfaces in contact. now, what I might ASSUME they mean by "relatively constant" would be that the friction IS constant, but if it happens to move to a different surface where the friction is different- such as perhaps a slippery part of the road or something- then it might be subject to change. but I wouldn't read too far into this. on the mcat, I think it's safe to assume that kinetic friction such as that between a car and the road WILL be constant. now, something like air resistance or "drag" is NOT constant, as it is dependent on the speed of the falling object until it reaches terminal velocity. but for your question, no, I wouldn't say that the friction is slightly decreasing in time.

 

[Cawolf]

I agree with @user12.

It just means to assume that the frictional force is constant - where in reality it would be variable.