Can someone please help me with physics?

Question

I cannot understand this problem. Please help me :( I would appreciate it a lot. Thank you much.

Three objects with mases, m1 = 5.0 kg, m2 = 10.0 kg, and m3 = 15.0 kg, are attached by strings over frictionless pulleys as indicated in the figure linked below. The horizontal surface is frictionless and the system is released from rest. Using energy concepts, find the speed of m3 after it moves down 4.0 m.

http://www.webassign.net/sf/p5_32.gif

Answer 1

OK.
The force on m3 supplied by gravity is
F3 = m3*g = 15*9.8 = 147 Newtons
The force supplied by M1 is
F1 = m1*g = 5*9.8 = 49N
Now, the *total* mass of the system is
Mt = m1 + m2 + m3 = 30 kg and the total force acting on the system is
Ft = F2 - F1 = 147 - 49 = 98 N so all three masses will accelerate together at
a = Ft/Mt = 98/30 = 3.2667 m/s² Since velocity in terms of acceleration and distance is
v = √(2ax) then, when x = 4, the velocity will be
v = √(2*3.2667*4) = √26.11336 = 5.112 m/s

This is a general case of a thing called a 'Atwood Machine' and it's one of the more fun things to play with in physics.

HTH ☺


Doug

Answer 2

Alright, you're gonna have to help out here. I'll flesh out the concepts. The combined system m1+m2+m3=30 kg. There's a force on one side of 5kg x g, and on the other of 15kg x g. So there's a net force on the system of 10kg x g to the right. It's accelerating the entire system, and I'm glad everything is frictionless because that makes it easy. The entire system of 30kg has accelerated through 4m under the force of 10kg x g. This is really easy using energy concepts, which is why it's done. The work done is force through distance, 10kg x g x 4m (g = acceleration due to gravity). The equivalent energy is 1/2 mv^2. Equivalent as in, the work done is equal to the kinetic energy put into the system. Can you set up the equations and solve for v? I'm up kind of late. If you need more info, email. Don't be afraid to ask. Never be afraid to ask questions.