course Phy 241
36.An advertisement claims that a particular automobile can “stop on a dime.” What net force would actually be necessary to stop a 850 kg automobile traveling an initial velocity of 45 km/hr in a distance equal to the diameter of a dime, which is 1.8 cm.
To find F we need to know that F/m equals a and also that vf^2=v0^2 + 2 a `ds. We can substitute the F/m for a in the second equation and have 0=(45^2) +(2)(F/850)(1.8). F therefore is equal to -47812.5 N.
Your procedure is good but the units of 45 and the units of 1.8 are not compatible, and neither is compatible with force in Newtons.
Can you submit this calculation with every quantity expressed in appropriate units?
38.A parachutist relies on air resistance to decrease her downward velocity.
She and her parachute have a mass of 55.0 kg and air resistance exerts a total upward force of 620 N on her and her parachute. A) What is the weight of the parachutist? B) Calculate the net force on the parachutist. Is the net force upward or downward? C) What is the acceleration of the parachutist?
The weight of the parachutist is found by multiplying the mass times the acceleration of gravity. This equals to -539 N if the downward direction is negative.
If we have 620 N in the positive direction pushing upward, the net force is 81 N. This means that the net force is in the upward direction.
The acceleration can be found by using the equation Fnet=ma where Fnet is 81 N, and mass is 55.0 kg. a therefore equals 1.47 m/s^2.
Good solution.
42. A spacecraft descends vertically near the surface of Planet X. An upward thrust of 25.0 kN from its engine slows down at a rate of 1.20 m/s^2, but it speeds up at a rate of .80 m/s^2 with an upward thrust of 10.0 kN. A) In each case, what is the direction of the acceleration of the spacecraft? B) In each case what is the net force C) What is the spacecraft’s weight near the surface?
In reference to the 25 kN situation, the acceleration is in the negative direction, the other situation is in the positive direction. We can find the net force by taking the – 25 kN and adding the 10.0 kN to obtain -15 kN. I am not sure how you can find a weight from this information considering we don’t know the effects of gravity on this certain planet.
You have enough information to set up two equations with two unknowns. The acceleration of gravity could be regarded as one of the unknowns.
This specific problem is addressed in more detail in the query. If you still have questions at that point I will of course be glad to answer them.