Kinetic Energy of Pulley Question

[Godlock]

In the Figure the pulley has negligible mass, and both it and the inclined plane are frictionless. Block A has a mass of 1.7 kg, block B has a mass of 2.7 kg, and angle θ is 27 °. If the blocks are released from rest with the connecting cord taut, what is their total kinetic energy when block B has fallen 20 cm?

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c08/q75.jpg

Ke = Pe so is it just [mass x gravity x height] = (4.4) (9.8) (0.2) = 8.624 J?

 

[Ofey]

Indeed potential energy becomes kinetic energy. But notice that mass A gets potential energy aswell, it goes up after all :)

 

[Godlock]

How do I find how far it goes up?

 

[Ofey]

When talking about potential energy, the only thing that matters is how much the object has risen in the vertical direction. You know that the system moves 20 cm, in other words mass A moves 20 cm along the surface. Using a bit of trigonometry and the angle given, you can figure out how much mass A rises in the vertical direction. This change represents the change in the potential energy of mass A.

 

[Godlock]

I don't get what you mean.

 

[Ofey]

How much higher up is mass A after it has moved 20 cm along the surface?

Since the system moves 20 cm in total, mass A also moves 20 cm. But mass A moves 20 cm along the surface not 20 cm in vertical like mass B. Notice that when mass A moves 20 cm along the surface it moves less in the vertical direction.

When you know how much mass A has moved in the vertical direction (in other words straight upwards), you can calculate how much potential energy it has gained.

 

[Godlock]

I did that but it still turned out to be incorrect.

 

[lanedance]

hey godlock, showing what you tried to do may help :rofl: