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Ana's car has a dead battery. She and 4 of her friends push the car closer to get help. The mass of the car with Ana inside is 1100 kg. Ana steers straight ahead while each of her four friends applies a 120 N force to the back of her car. If the car accelerates at 0.3 m/s2, what is the resistive force of friction that the friends are pushing against?

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Answer:

The resistive force is of magnitude 150 N

Explanation:

The net force Fnet applied on the car plus Ana is the additions of all forces acting which includes  that of four times 120 N (the total force applied by her friends = 480 N) minus the resistive force  Fr (minus because it is acting opposite the movement). Therefore, the net force is:

Fnet = 480 N - Fr

We can then apply Newton's second law to this:

Fnet = m * a

480 N - Fr = (1100 kg) * (0.3 m/s^2)

480 N - Fr = 330 N

480 N - 330 N = Fr

Fr = 150 N

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