I need some serious help with my homework problems

[Kadaj]

I'm on static/kinetic friction, tension force, and Equilibrium applications of Newton's Laws of motion.

I have many homework problems that baffles my mind and need serious help understanding them. So, any help will be appreciated.

1] 'A large cube (mass= 25 kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (mass= 4.0 kg) is in contact with the front surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.17. What is the smallest magnitude that P can have in order to keep the small cube from sliding downward?'

2] 'A supertanker (mass= 1.70e8 kg) is moving with a constant velocity. Its engines generate a forward thrust of 7.40e5 N. Determine (a) the magnitude of the resistive force exerted on the tanker by the water and (b) the magnitude of the upward buoyant force exerted on the tanker by the water'
The problem here is that i don't understand what it wants =(

3] 'A circus clown who weighs 890 N is at rest. The coeffienct of static friction between the clown's feet and the ground is .53. He pulls vertically downwards on a rope that passes around three pulleys and is tied around his feet. What is the minimum force that the clown must exert to yank his feet out from under himself'
I thought the answer would just be (.53)(890), but that gves me 471.7, but my teacher has his answer as 308 N, Any ideas how he got this?

4] 'A skier is pulled up a slope at a constant velocity by a tow bar. The slope is inclinced at 25.0 degrees with respect tot he horizontal. The force applied to the skier by the tow bar is parallel to the slope. The skier's mass is 55.0 kg, and the coefficient of kinetic friction between the skis and the snow is .120. Find the magnitude of the force that the tow bar exerts on the skier'

5] A mountain climber, in the process of crossing btween two cliffs by a rope,pauses to rest. She weighs 535 N. She is closer to the left cliff than to the right cliff, with reselt that the tensions in the left and right sides of the rope are not the same. Find the tension in the rope to the left and to the right of the mountain climber. (theres a picture in the book that shows the climber which i will try and draw on paint) The link to the picture is here: http://www.villagephotos.com/pubbrowse.asp?selected=1002062

These are the problems that are causing me trouble Please anyone help me I seriously need it

 

[arildno]

Let's start at 1:
We know that both cubes must have the same HORIZONTAL acceleration "a".
Set up the horizontal equations of motion for BOTH cubes, and solve for "a" and the normal force between large&small cubes in terms of P!

 

[bross7]

Question 2. Think of of Newton's laws. A force is an object's mass multiplied by its acceleration. You were given something in the question regarding the tanker's movement. The same principle applies for both (a) and (b).