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# Classical Mechanics HW Question watch

1. Please note that I immediately assumed rectilinear motion (straight line motion only) and obtained the dot product of the kinematics equations used. Please help me ascertain the correctness of my values.

An astronaut of mass 165 kg, equipped with a portable propulsion unit, is about to travel in a straight line from one spacecraft to another.
He first accelerates from rest for 15.0 seconds under the influence of a propulsion force of magnitude 142N. Upon reaching point A he shuts off the propulsion unit and continues to move with constant velocity until point B is reached. At point B, which is 425m from the second spacecraft, he turns on the propulsion unit to produce a constant reverse thrust, so that his velocity will be reduced to zero at the instant he reaches the second spacecraft.

i) Calculate the acceleration of the astronaut from the first spacecraft to point A.

My answer: I obtained the value 0.861 m/s^2 using F=ma although I don't know if I should solve simultaneously with respect to kinematics equations involving time?

ii) What is his speed from point A to point B?

My answer: I solved using the second kinematics equation, with respect to acceleration and substituted the variable for velocity over 15.0s into F=ma and got 12.9m/s Is this right, if not, how should I go about it methodically?

iii) Calculate the magnitude of the reverse thrust needed to reduce the astronaut's velocity to zero as he reaches the second aircraft.

My answer: Again, using the 2nd and 3rd kinematics equations I obtained acceleration to be 0.195 m/s^2 and Thrust to be 32.175 N all acting in the opposite direction to the astronaut's motion.

Any help would be much apreciated
2. This is how to go about it methodically.

3. (Original post by Stonebridge)
This is how to go about it methodically.

Thanks for taking the time to check and for the method.
4. Thank you

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