 # Selina Solutions Class 9 Physics Solutions for Exercise 3E in Chapter 3 - Chapter 3 Laws Of Motion

A body is dropped from the top of a tower. It acquires a velocity 20m/s on reaching the ground. Calculate the height of the tower. (Take g=10m/s^2)

Solution:

Given: u=0m/s, v=20 m/s, g=10m/s^2 ;

Assume ‘h’ to be the height of the tower

We know from the equation of motion;

\begin{array}{l} v^{2}-u^{2}=2 g s \\ (20)^{2}-0=2(10)(h) \\ h=400 / 20 \Rightarrow 20 m \end{array}

Video transcript
hello everybody today we are going to look at the question an atom is a basic unit of an element draw a diagram of an atom revisible as seen today so consider friends on the screen you can see this is a example of an atom so uh this is an example of an atom which is a basic chemical element let's look at this one by one so here atoms you can see are made up of three main parts three main parts now what are these these are neutrons protons and electrons let's locate each one of them in this atom now what is this proton and neutron this proton and neutron together makes up the nucleus protons and neutrons are contained inside this very center of an atom which is called as nucleus now what is this electron then electrons are orbit around the outside of the nucleus which you can see here now why does this protons and neutrons are considered inside the neutrals are contained inside the nucleus and electrons orbit around the nucleus this is because the protons and neutrons are heavier than electrons i repeat the protons and neutrons these are heavier than the electrons and that is why they are contained at the center of the atom electrons are light weight as compared to protons and neutrons they are extremely light weight and that is why they exist in the outermost orbit of an atom now let's take an example look at this atom this is an atom of chlorine whose atomic number is equal to 9 now class let's look at what is an atomic number an atomic number is actually defined as the total number of protons present inside the nucleus of the atom so in this case there are nine protons which are present inside the nucleus of an atom for fluorine atom and it is always represented as z is equal to nine what is this set it is denoted by said that this atomic number is denoted by z so atomic numbers said for a fluorine atom i can write z is equal to number of protons that is equal to number of electrons this is an example of a fluorine atom and this is the basic unit of an atom now let's look at the charges proton is actually positive charge which is represented by small p electron is negatively charged and neutrons do not have any charge so this is the basic unit of an element thank you
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