Acceleration of a body is defined as the rate of change of its velocity with time. 

Acceleration = change in velocity/time taken

If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration a = v–u/t. The acceleration is taken to be positive if it is in the direction of velocity and negative when it is opposite to the direction of velocity. The SI unit of acceleration is ms^–2.

If an object travels in a straight line and its velocity increases or decreases by equal amounts in equal intervals of time, then the acceleration of the object is said to be uniform. The motion of a freely falling body is an example of uniformly accelerated motion. On the other hand, an object can travel with non-uniform acceleration if its velocity changes at a non-uniform rate. For example, if a car travelling along a straight road increases its speed by unequal amounts in equal intervals of time, then the car is said to be moving with non-uniform acceleration.

 Initial Speed  (u)= 54 km /hr = 54 *1000 m / 3600 s = 54*5/18 = 15 m/s
         ( 1km = 1000 m and 1hr = 60 mins = 60*60 seconds = 3600 seconds)
        Time = 8 seconds
       Final Velocity (V) = 0
 we know that ,
         v = u + at
         0 = 15 + a*8
       0 = 15 + 8a
       a= -15/8 m/s²
   Therefore , acceleration is -15/8 m/s²
also ,
           S = ut+1/2at²
           S = 15*8² + 1/2 *(-15/8)*8²
              =15*64 - 15*4
             = 15(64 - 4)
             =15(60)
             S=900 m
hence Stopping Distance traveled before stopping is 900m .

A car travels along the straight road 100 m east, then 50 m west. The total distance covered by the car is equal to the total path covered by it. In this case, distance of the car is equal to 100 m + 50 m = 150 meters.

The shortest path covered by the car is called its displacement. It is given by :

{tex}d=\sqrt{100^2+50^2} \\\\d=111.8\ m{/tex}

Hence, the distance and displacement of the car is 150 m and 111.8 m respectively.

A balloon shrinks inside a fridge because of the way gases behave. If the pressures remain the same on a fixed mass of gas, a drop in temperature will reduce the volume of the gas. The gas molecules get closer together and the density increases.

Discovery of Cell:

While examining a thin slice of cork, Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Cork is a substance which comes from the bark of a tree. In the year 1665, Robert Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells. Cell is a Latin word for ‘a little room’.

Thus, cells were first discovered by Robert Hooke  in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time.  Robert Brown in 1831 discovered the nucleus in the cell.  Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell. The cell theory was presented by two biologists, Schleiden (1838) and Schwann (1839). This theory states that all the plants and animals are composed of cells and that the cell is the basic unit of life. The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells.

Momentum is defined as the quantity of motion of the body. It is measured by mass×velocity , as momentum depends upon velocity, and it depends on the direction of the motion of the body as well. Momentum is a vector quantity since velocity is vector while mass is scalar.

Momentum = Mass X Velocity

p = mv

A group of cells having common origin, similar structure and performing a definite function is called a tissue.  Tissues are found in plants and animals.  Plants and animals are made up of many different kinds of tissues. For example, groups of bone cells form bone tissues and muscle cells form muscle tissue. Different types of tissues have distinctive architecture best suited for what they do.  We will see that plant tissues are different from animal tissues in many ways.

A group of cells having common origin, similar structure and performing a definite function is called a tissue.  Tissues are found in plants and animals.  Plants and animals are made up of many different kinds of tissues. For example, groups of bone cells form bone tissues and muscle cells form muscle tissue. Different types of tissues have distinctive architecture best suited for what they do.  We will see that plant tissues are different from animal tissues in many ways.

There are the certain features or properties used for the classification of living organisms which are known as characteristics. Organisms with same characteristics are placed in same groups.

Some important characteristics used for hierarchical classification are

• Presence and absence of nucleus in the cell.
• Body composed of single cell or group of cells.
• Autotrophs (producing own food) and heterotrophs (getting food from outside).
• Development and organisation of different body parts.

The thin walls of those cells are saturated with a fatty waxy substance, called suberin, which makes cork almost impervious to water and air. Suberin is the chemical present in cell wall of cork that makes it impervious to gases and water. Lignin is a complex polymer which acts as a cement and hardens the cell wall. The walls of cork cells are heavily thickened with an organic substance, suberin. Suberin makes these cells impervious to water and gases.

Distance is the actual length of the path travelled by the object.
Whereas displacement is the shortest length or path between the initial and final points.

So
a)when the object is moving in a straight line both distance and displacement are equal.B because here the shortest path and the actual path are the same

b) when the object is moving in a plane distance may be equal to or greater than displacement.
because here the actual path and shortest path may or may not be same.

D1 = 7.5×2/60= 0.25km

D2= 7.5×2/60= 0.25km

Total Distance= 0.25+0.25=0.50km

Total time taken = 2+2+56=60min= 1 HR

Average speed =total distance/ total time
Average speed= 0.50/1= 0.50 km/hr

Heat is the transfer of kinetic energy from one medium or object to another, or from an energy source to a medium or object. This is the amount of heat required to raise the temperature of one pound of pure liquid water by one degree Fahrenheit. Most of us use the word 'heat' to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object. Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases.

Plasmolysis is the process in which cells lose water in a hypertonic solution. The reverse process, cytolysis, can occur if the cell is in a hypotonic solution resulting in a lower external osmotic pressure and a net flow of water into the cell.

• Factors Affecting Evaporation:

1. Temperature: The rate of evaporation increases on increasing the temperature of the liquid. When the temperature of a liquid is increased by heating it, more particles of the liquid get enough kinetic energy to go into the vapour state. This increases the rate of evaporation.
2. Surface area: The rate of evaporation increases on increasing the surface area of the liquid. For example, while putting clothes for drying up we spread them out.
3. Humidity of air: Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. When the humidity of air is low, then the rate of evaporation is high, and water evaporates more readily. When the humidity of air is high, then the rate of evaporation is low, and water evaporates very slowly.
4. Wind speed: The rate of evaporation of a liquid increases with increasing wind speed. For example, the clothes dry faster on a windy day.