CBSE Class 9 Science — NCERT Solutions

A recent prediction such as the outcome of a cricket match may be based on either evidence and reasoning or on guesswork. If we look at facts like the teams' recent form, players' fitness, pitch condition, and weather, then the prediction is based on evidence. If we simply say one team will win because we like it, then it is mainly guesswork.

Scientific thinking improves predictions by making us:

• use observations instead of opinions,
• look for patterns in past results,
• consider measurable factors,
• and check whether the prediction is testable.

So, scientific thinking makes predictions more reliable and less random.

An approximate answer is good enough when a rough idea is enough, such as estimating how much rice a family needs for a month or estimating the volume of air breathed in a day.

A very exact value is needed when small errors can cause big problems, such as measuring medicine doses, calculating fuel for an aircraft, or doing precise scientific experiments.

So, approximate answers help in rough planning and checking reasonableness, while exact values are necessary when accuracy and safety matter.

According to Cell Theory, new cells are formed only from pre-existing cells. So a cell comes from another cell by cell division.

Technological interventions, especially the development of the microscope, helped scientists study cells and other tiny structures that are beyond the limit of resolution of the naked eye. Robert Hooke first observed cells using a microscope, and later electron microscopes revealed fine details of cell structure at the nanometre scale. These tools improved magnification, resolution, and contrast, creating new knowledge about the cell and the world beyond what we can see directly.

The study of cells and tissues helps us understand how living organisms grow, develop and perform life processes. Since cells are the basic unit of life and tissues are groups of cells working together, this study explains the structure–function relationship in plants and animals. It is also significant for human welfare because knowledge of tissues helps researchers understand growth, repair, movement, transport and control in the body, and it supports advances such as plant tissue culture, crop improvement, and medical use of stem cells and transplants.

Plant and animal tissues are different because plants and animals have different body structures, life styles and needs. Plants are usually fixed in one place, so they need tissues for support, protection, transport of water and food, and growth at certain regions. Animals move from place to place, so they have tissues for movement, coordination, sensory functions, and internal support. Also, plant cells have a cell wall, while animal cells do not, so plant tissues are generally more rigid and animal tissues are more flexible. These differences arise because the structure and function of tissues are adapted to the way plants and animals live.

A safe distance is the distance needed so that, if the truck ahead brakes suddenly, your vehicle can also stop in time. The chapter says this distance depends on the speed of the vehicle, because a vehicle moving faster needs a larger stopping distance.

In muddy water, the solid mud particles are comparatively larger and heavier, so they settle down on standing due to gravity. In milk, the particles are much smaller and are part of a colloid, so they remain uniformly dispersed and do not settle easily.

When the canoeist pushes the water backwards, the water exerts an equal and opposite force on the paddle and canoe, pushing it forwards. This is an example of Newton’s third law of motion. If the canoeist pushes harder, the force on the water is larger, so the forward force on the canoe is larger too, and the canoe moves faster.

By conservation of mechanical energy (neglecting friction), the child's potential energy at the top converts into kinetic energy at the bottom:

$$\frac{1}{2}mv^2 = mgh$$

Cancelling $m$ and solving for $v$:

$$v = \sqrt{2gh}$$

So the magnitude of velocity at the bottom of the slide is $\sqrt{2gh}$.

No. The chapter says atoms were once thought to be indivisible, but later experiments showed that atoms are made of smaller subatomic particles like electrons, protons, and neutrons. So atoms are not the smallest indivisible particles.

O represents a single oxygen atom, while O$_2$ represents an oxygen molecule made of two oxygen atoms joined together.

No. In outer space there is a near vacuum, so sound cannot propagate. Therefore astronauts cannot directly hear each other speak or hear metal clanking. They communicate using special devices fitted into their spacesuits.

Most bats locate prey by echolocation: they emit short bursts of ultrasonic waves, and by sensing the reflected echoes they determine the position of obstacles and prey.

A farmer prefers asexual reproduction when he wants many genetically identical plants quickly, such as through cutting, grafting, layering, vegetative propagation or tissue culture. It is useful for large-scale propagation of desirable crops and for keeping the same useful traits. He prefers sexual reproduction when he wants variation and the development of new varieties through seeds, especially in plant breeding.

This means the immense variety of living organisms found on Earth.

Warm Arabian Sea water causes more evaporation from the sea. This leads to fluctuations in the southwest monsoon, causing variability in rainfall in India. Some regions may get floods, while others may face drought.