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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 11: Treating systems

# Masses on incline system problem

In this video David explains how to find the acceleration and tension for a system of masses involving an incline. Created by David SantoPietro.

## Want to join the conversation?

• Wait, what's an internal force? I presume gravity is an external force, as well as friction, as well the force of large dragons trying to impede your motion. What are forces that come from within? It almost sounds like some sort of chinese proverb.
• Internal forces result in conservation of momentum for the defined system, and external forces do not. It depends on what you have defined your system to be. Example, if you are in space floating with a ball and define that as the system. Then when you apply a force to the ball to throw it (and the ball applies a force to you), then the total momentum of the system remains unchanged since all those forces were internal. Now if something from outside your system pulls you (ex. gravity from planet), the system's momentum is no longer conserved because that additional force was external to the system, but if you expand the system to include the planet and take into account its momentum, then the total momentum of the larger system remains conserved. So it depends how you define what your system is, whether a force is internal or external to it.
• At , why is tension considered an internal force? What is the difference between internal and external forces?
• My teacher taught me to just draw a big circle around the whole system you're trying to deal with. Anything outside of that circle is external, and anything inside is internal. If you drew a circle around both of the boxes and the string attaching them, the tension force is inside of the circle and thus internal.
• Are the tensions in the system considered Third Law Force Pairs?
• To your surprise no!, in order there to be third law force pairs you need to have contact force. In other words there should be another object that will push that block. But our tension is not pushing it is pulling.
• I've watched all the videos on treating systems as a whole and one thing which I don't get is why don't we consider the coefficient of static friction along with the coefficient of kinetic friction? I mean, before kinetic friction starts acting on the box there's got to be static friction, so what am I missing here?
• CORRECT! But , We're looking at a problem(s) where the beginning of the problem(s) states that the objects have already been in motion before we looked/observed at it, Therefore,We consider Only The Kinetic Friction

Or if we you are still confused,
THE OBJECT IS SLIDING NOT ROLLING OR ANYTHING ELSE!
• Are the two tension forces equal? I know at he said that the internal forces cancel, but is that the same thing as saying they are equal in separate directions?
• A pulley is a rotating piece that is meant to convert horizontal tension force into vertical tension force. In these videos, we are assuming there's no resistance from the pulley, so the tension of one string is "converted" into the tension of the other string with no force being subtracted. In short, yes they are equal, but in different directions.
• How exactly do we determine which body is more massive? Do we compare the vertical components of the gravitational forces on the two bodies or something?
• In the video, the masses are given to us: The 9 kg mass is falling vertically, while the 4 kg mass is on the incline. The forces of gravity, or Weight, is directly proportional to mass, and both be positioned vertically.
• At , how come the force of friction is opposing the system? I thought that the force of friction is always upwards on an incline to prevent it from sliding down. In class for example, the professor said that friction acts upward on a truck driving up an incline. Doesn't friction only oppose motion when an object is sliding, but the 9 kg box is pulling the 4 kg uphill, so wouldn't the friction of the 4 kg box be positive then?
• Dynamic friction is always in the opposite direction of motion and static friction is opposite to the net forces (excluding friction) on an object.
• What if there's a friction in the pulley.. Does it affect the whole system