# Homework Help: How to find acceleration

1. Dec 31, 2003

### Morgan

Alright there are 3 problems that I am just not getting. And I know that most of the time when I just cant get the answer, it is because it is staring me in the face. But anyway, please help me with the following if you could!

1) A 1.14x10^4 kg lunar landing craft is about to touch down on he surface of the moon, where the acceleration due to gravity is 1.6 m/s^2. At an altitude of 165m the craft's downward velocity is 18.0m/s. To slow down the craft, a retrorocket is firing to provide an upward thrust. Assuming the decent is verticalm find the magnitude of the thrust needed to reduce the velocity to zero at the instant when the craft touches the lunar surface.

2) A girl is sledding down a slope that is inclined 30degrees with respect to the horizontal. A moderate wind is aiding the motion by providing a steady force of 105 N that is parallel to the motion of the sled. The combined mass of the girl and the sled is 65kg, and the coefficient of kinetic friction between the runners of the sled and the snow is .150. How much time is required for the sled to travel down a 175-m slope, starting from rest?

3) A sports car is accelerating up a hill that rises 18 degrees above the horizontal. The coefficiant of static friction between the wheels and the road is .88 (the mew!). It is the static frictional force that propels the car forward. (a) What is the magnitiude of the maximum acceleration that the car can have? (b) What is the magnitude of the maximum acceleration if the car is being driven down the hill?

Please help!

no work is provided because all I did was draw my force diagrams and write down what i had and needed.. and then I didnt know exactly how to proceed! if you just help me with what equations i would need and why i need them.. etc.. i can do all the calculations! i just didnt know where to begin and what to do exactly.

Last edited: Dec 31, 2003
2. Dec 31, 2003

### himanshu121

1) Find the acceleration i.e $$\frac{u^2}{2s}=\frac{f}{m}-g$$
where f= thrust
g= acce due to gravity on moon
u= initial vel
s= distance

Last edited: Dec 31, 2003
3. Dec 31, 2003

### himanshu121

for the second part draw force diagram with your x-axis || the inclined. By resolving the Weight along and perpendicular to x-axis and taking others force under consideration u get
ma = 105 + mgsin30 - kmgcos30
u will get a from here and use it in the kinematic equation

4. Jan 1, 2004

### Morgan

Thanks so much!! Your information was just enough to give my brain a little push out of its mental block and i have successfully solved each problem! Thanks!

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