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## Class 11 Physics (India)

### Unit 9: Lesson 3

Newton's second law

# More on Newton's second law

David explains how to use Newton's second law when dealing with multiple forces, forces in two dimensions, and diagonal forces. Created by David SantoPietro.

## Want to join the conversation?

• Do all these forces happen at the same time any any given moment?

Also, do the same principles of acceleration apply here? For example, if the meteorite was going in the forward direction (positive) and the overall acceleration turned out to be negative, would this then become negative acceleration (i.e. slowing down)?

Thanks in advance for any help given! :) •  Suppose there is a meteor traveling with a constant velocity of 1000 km/h and there is no force acted on it. As it enters the gravity field of Earth, earth will start pulling the meteor with a constant force of 100 N resulting in 10 km/h^2 constant acceleration. Now there is only one force acted on meteor.

When meteor's velocity reaches 1500 km/h, we send a rocket to save the earth. The rocket starts pulling the meteor in the opposite direction with 50 N. Now there is two forces acting on meteor at the same time. The net force on meteor is 100-50=50 N towards the earth and it still accelerates but now its acceleration decreases to 5 km/h^2.

When meteor's velocity reaches 1750, we send a second rocket pulling the meteor with the same 50 N. Now net force on meteor is 100-50*2= 0 N. Since net force is 0, meteor stops accelerating but it continues with a constant velocity of 1750 towards earth.

Now we have to send a third rocket. Net force on the meteor becomes 100 - 3*50= - 50 in the opposite direction from earth, and the meteor; first starts to slow down from 1750, then stop completely, and eventually start moving in the opposite direction.
From first rocket onwards there are allways 2 or more forces acted on the meteor. Even when the meteor moves away from earth after third rocket, the force of earth's gravity continues to act on it.
• When a body is floating on water, what are the net forces acting on it? • While a body is in water or in any other fluid(regardless of whether it is floating or sinking), 2 forces act on the body, First is it's weight acting downwards, and second is the upward force exerted by water or fluid on the body in the upward direction, this force is known as upthrust. Now, for a body to float, the upthrust must be equal to the weight of the body. Hence, the net force would be zero.

• • When a body is floating on water, what are the net forces acting on it? • • In mathematics, the Pythagorean theorem, also known as Pythagoras's theorem, is a relation in Euclidean geometry among the three sides of a right triangle. It states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. The theorem can be written as an equation relating the lengths of the sides a, b and c, often called the "Pythagorean equation":

a^2 + b^2 = c^2 ,
where c represents the length of the hypotenuse and a and b the lengths of the triangle's other two sides.

Although it is often argued that knowledge of the theorem predates him, the theorem is named after the ancient Greek mathematician Pythagoras (c. 570 – c. 495 BC) as it is he who, by tradition, is credited with its first recorded proof. There is some evidence that Babylonian mathematicians understood the formula, although little of it indicates an application within a mathematical framework. Mesopotamian, Indian and Chinese mathematicians all discovered the theorem independently and, in some cases, provided proofs for special cases.

The theorem has been given numerous proofs – possibly the most for any mathematical theorem. They are very diverse, including both geometric proofs and algebraic proofs, with some dating back thousands of years. The theorem can be generalized in various ways, including higher-dimensional spaces, to spaces that are not Euclidean, to objects that are not right triangles, and indeed, to objects that are not triangles at all, but n-dimensional solids. The Pythagorean theorem has attracted interest outside mathematics as a symbol of mathematical abstruseness, mystique, or intellectual power; popular references in literature, plays, musicals, songs, stamps and cartoons abound.
• How we know the angle if we're only know the magnitude for horizontal and vertical direction? • My textbook states that Newton’s II law as “ The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.” Here it is different. So which one is correct? • When calculating the net force, why are things like normal force and weight included?   