First of all, we need to convert the angular speed from rev/min into rev/s:

The angular acceleration is the variation of angular speed divided by the time:

And this is constant, so we can use the following equation to calculate the angle through which the engine has rotated:

so, 5 revolutions.
The vertical force exerted on the lawn is 68.8 N downward
Explanation:
The vertical force exerted by the lawnmower on the lawn is equal to the vertical component of the force applied, therefore:

where
F is the magnitude of the force applied
is the angle between the direction of the force and the horizontal
In this problem:
F = 120 N
Substituting,

where the negative sign means the direction of the force is downward.
Learn more about vector components and forces here:
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Answer:
About 66 miles per hour
Explanation:
Based on the information given we can assume the car traveled the same number of miles every hour meaning all we need to do is divide.
400/6 ≈ 66 miles per hour
More force needs to be applied