Newton's second law of motion

carolene

2007-01-21 10:55 pm

The skid marks of a car show that the car has skidded for 80 , before coming to rest. Given that the friction on the is 10000N on the dry concrete, and the mass of the car is 1000kg. Find the speed of a car before coming to rest.

點計????????????

回答 (2)

琴生

2007-01-22 3:23 am

✔ 最佳答案

Taking the direction of the motion of the car be negative.
F = ma
10000 = (1000)a
a = 10 ms^(-2)

Therefore, the car is decelerating.

Taking the direction of the motion of the car be positive.
v^2 - u^2 = 2as
0^2 - u^2 = 2(-10)(80)
u^2 = 1600
u = 40 ms^(-1)

Why is the net force is 10000N?
It is easily found from the direction taking for the sign.
As the line of action of the force is parallel to the plan of road, and there is no other components lie on the same line of action, thus the net force is only the frictional force.

How about the weight of the car?
It is perpendicular to the road, so we cannot decomposite it into any components that parallel to the road. In other words, no other components are parallel to the road except the frictional force.

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manuel

2007-01-21 11:32 pm

F = ma
10000 = 1000 a
a =10 ms-2

v^2 = u^2 +2as
0 = u^2 +2(-10)(80) ... skidded for 80m
u = 40 ms^-1

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