Calculate the force of Earth's gravity on a spacecraft

[cabrady92]

I am in a classical physics class and the class requires that we use mastering physics for our homework. The only problem is that I can't get what mastering physics claims is the right answer to these problems. Can someone try these calculations to see if I am right or wrong?


Problem 6.1 : Calculate the force of Earth's gravity on a spacecraft 2.50 Earth radii above the Earth's surface if its mass is 1400 kg.
My calculations: F= G((m1mE)/r^2) = (6.67*10^-11)*((1400kg)*(5.97*10^24kg))/(6378km*2.5*10^3)^2= 2192.7 N

Their answer: 1120 n?


Problem 6.28: Two satellites orbit Earth at altitudes of 5600 km and 1.7×104 km.Which satellite is faster? By what factor? = vclose/vfar.
My calculations: vclose = sqrt(G(mE/r)), sqrt((6.67*10^-11)*(5.98*10^24kg)/(5600km*10^3)) = 8439.55 m/s

vfar = sqrt(G(mE/r)), sqrt((6.67*10^-11)*(5.98*10^24kg)/(17400*10^3)) = 4843.83 m/s

vclose/vfar = 1.74

Their answer: 1.4?

Problem 6.51: How far above the Earth's surface will the acceleration of gravity be half what it is at the surface?

My calculations: m1(1/2g) = Gm1mE/r^2, r^2 = 2GmE/g, r = sqrt(2GmE/g), r = sqrt( 2*(6.67*10^-11)*(5.98*10^24)/9.8) = 9022262 m

Their answer: 2640000 m?


Can you try solving these answers and tell me what I am doing wrong or if the homework program is flawed?

 

[cabrady92]

I'm even more confused... the book tells me to use r as altitude but mastering phyics tells me that r is the altitude + r...