I posted the question below and got the answer from Kelvinmun. Thanks. I don't fully understand the answer. It assumes r is constant. I don't know...

I posted the question below and got the answer from Kelvinmun. Thanks. I don't fully understand the answer. It assumes r is constant. I don't know why we can assume r is constant through the tunnel. Can someone explain to me why r can be a constant? r is the distance between the mass and the center of the planet. x=r multiply by cos angle between horizontal and the tunnel. r is changing through the tunnel and cannot be assumed to be constant.

And, can someone also help me with 2nd part?

Find the maximum speed of the project.

An object of mass 67.2 kg moves in a smooth

straight tunnel of length 2440 km dug through

a chord of a planet of mass 4.04 × 1024 kg and

radius 5.76 × 106 m.

Find the effective force constant of the har-

monic motion. The value of gravitational

constant is 6.67259 × 10−11 Nm2/kg2.

Please help. x is a variable and r is not constant. What will be the force constant?