$3-$ Assume that an all$-$wheel$-$drive rubber$-$tired tractor has a total weight of $18000lb.$ on the two driving tires. The maximum rim pull is $9000lb.$ If the tractor is operating on a surface with a coefficient of traction of $0\mathrm{.}2.$

a$)$ What is the usable power?

b$)$ Is the full engine power used?

$4-$ A four$-$wheel$-$drive tractor weighs $42,000lb.$ and produces a rim pull of $40,000lb.$ at sea level. The tractor is being operated at an altitude of $9,000ft$ on wet earth. The tractor is moving on a downgrade with a rolling resistance of $12,000lb.$ and a grade resistance of $2,000lb.$ Determine the required power, corrected rim pull, traction force, and usable power.