# Physics Notes

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• Published : November 25, 2012

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I noticed that I have not described the rule of F=ma in either the last email or this one. Where would you suggest it be described? Somehow the details of adding forces and balanced forces were missed in the last email and also it did not make perfect sense for me to note. As far as I am concerned the khan academy does not lecture it so I am not too sure in what to do about this. I am assuming finding velocity is the sole purpose of applying the law of conservation of momentum. Is this true? I also would like to note that a graph could not be drawn in some situations again due to me lacking the technology to send photos of handwritten notes. Hence there is sadly no examples of a problem for translational equilibrium and for the force-time graph in which impulse can be identified. I also have referred to explosions as divisions. Is this appropriate?

Newton's First Law of Motion:

A body will remain at rest or moving with constant velocity unless acted on by an unbalanced force. Example:
• Q: while traveling in train if one throws a ball up it lands on his palm though the train is moving. my doubt is that though the ball is detached from motion how does it manage to land on his palm though he is moving along with the train?

• A: he ball lands on your hand because the ball is, in reality, traveling at the same velocity as the train, you, and everything else on, or part of the train. The ball is not at rest, because assume while the train was accelerating, you were holding the ball. Since you were moving with the train, then the ball is moving at the same velocity you are, and therefore, the same speed the train is moving. Translational Equilibrium: The condition for translational equilibrium is for all the forces acting on a body to be balanced

Newtons Second Law of Motion:

Momentum is the product of mass and velocity (p = mv). It is measured in kg m /s and is a vector quantity. Impulse is the change in momentum when an object reacts to clashing with an external force (momentum after – momentum before) The rate of change of momentum of a body is directly proportional to the unbalanced force acting on that body and takes place in the same direction. Example:

• Q: There is a car with 500 KG mass and constant velocity 50 mph. As the car hits a wall what force will be applied on the wall? as the velocity is constant the acceleration would be zero and substituting in the second law F = 500 x 0 =0

• A: In the first question, the acceleration is not zero. It is zero before the car hits the wall, but when it hits the wall, the car will go from a speed of 50 mph to 0 mph in a very short space of time, which is a big deceleration (acceleration in the other direction), until it's speed is zero. The wall will experience an acceleration away from the car. Hence there is a substantial force.

Newtons Third Law of Motion:

If body A exerts a force on body B, Body B will exert an equal and opposite force on Body A. Example:
• Q: I have a pen and I push it with an arbitrary amount of force. The pen will exert the same amount of force on me. So wouldn't the forces cancel? And wouldn't the pen not move at all? • A: The forces are equal, but that does not mean this is no reaction. F=ma says that the reaction on each object (you and the pen) due to equal forces will be based on yours and the pens masses. If you and the pen are of equal mass, you and the pen will receive equal acceleration, just in the opposite directions. In space (no friction), the pen will start to move in one direction and you will start to move in the opposite direction, the speed of each based on the individuals or objects mass.

The Law of Conservation of Momentum:

Basically, this is just a combination of Newton's 3 laws but is useful when solving problems. For a system of isolated bodies, the total momentum is always the same. When solving problems for impulse...