Science & Mathematics » Physics » Jim is driving a 2268-kg pickup truck at 30.0 m/s...?

# Jim is driving a 2268-kg pickup truck at 30.0 m/s...?

Jim is driving a 2268-kg pickup truck at 30.0 m/s and releases his foot from the accelerator pedal. The car eventually stops due to an effective friction force that the road, air, and other things exert on the car. The friction force has an average magnitude of 900 N .

I got the first two parts of the question which was

Determine the initial kinetic energy of the truck?

1.02x10^6 J

Determine the stopping distance of the truck.
1130 m

It's this part that im stuck in, i'm not exactly sure what to use to plug into the equation.

Determine the coefficient of kinetic friction between tire and surface:

I know the equation is Fk=uFk*N

But i don't have "N"

• They already told you what the average frictional force is. So set the Work done by friction (Work = force x distance) equal to the kinetic energy. The Work is done to absorb all of the kinetic energy. (900 N)(distance) = 1.02 x 10^6 Joules. I get 1133.333 meters
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• Since the friction force causes the truck to decelerate from 30 m/s to 0 m/s, the work that is done by the friction force is equal to the truck’s initial kinetic energy.

KE = ½ * 2268 * 30^2 = 1,020,600 J
This rounds to 1.02 * 10^6 J.

900 * d = 1,020,600
d = 1,020,600 ÷ 900 = 1134 meters.
This rounds to 1130 m
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• N is the normal force of the road against the truck (via its tires). This is, by Newton's 3rd Law, equal to the weight of the truck = mg = (2268)(9.81) = 22,249 N.

Now here's where it get's confusing. The description in this question mentions "effective friction force" which is made up of MORE than the kinetic friction force because air friction and "other things" are included.
So the way it is worded => it's really not possible to compute the coefficient of kinetic friction between the truck's tires and road..because the effective friction force, given, is greater than the kinetic friction force.