Class 10 Science Lesson 1

Exercise

1. Choose the correct options for the following questions.

a. Which of the following is a fundamental unit?

Answer: iii. kilogram

b. Which of the following physical quantities has the unit ms⁻¹?

Answer: ii. velocity

c. Which of the following is a derived unit?

Answer: iii. joule

d. Which of the following units denote newton?

Answer: i. kg·ms⁻²

2. Give reason

a. Joule is the derived unit of work.

Reason:

Work is defined as force × displacement. SI unit of force = newton (kg·ms⁻²), displacement = meter (m), So, work = kg·ms⁻² × m = kg·m²·s⁻² = joule.

b. Some variables should be controlled while performing an experiment.

Reason:

Controlling variables ensures that the results of the experiment are valid and reliable. If other variables change, they can affect the outcome and make it difficult to determine the effect of the independent variable.

c. v² = ut is not a valid relation.

Reason:

The correct kinematic equation is:

v² = u² + 2as

The expression v² = ut is dimensionally incorrect and doesn’t relate the correct physical quantities.

3. Differentiate between:

a) Independent variable and dependent variable.

Independent Variable		Dependent Variable
i) The variable that is changed or controlled in an experiment.		i) The variable that is measured and affected in the experiment.
ii) It is the cause.		ii) It is the effect.
iii) Example: time in a speed experiment.		iii) Example: distance covered.

b) Fundamental unit and derived unit

Fundamental Unit	Derived Unit
i) A basic unit that is not derived from other units.	i) A unit that is derived from fundamental units.
ii) Example: meter, second, kilogram.	ii) Example: newton, joule, pascal.

4. Answer the following questions

a. What is a unit?

Answer: A unit is a standard quantity used to measure a physical quantity. It provides a reference by which measurements can be compared. For example, meter for length, kilogram for mass, and second for time.

b. Write the SI units of:

Mass: kilogram (kg)

Temperature: kelvin (K)

Energy: joule (J)

Density: kilogram per cubic meter (kg·m⁻³)

c. How is the validity of an equation checked? Write an example.

Answer: The validity of a physical equation is checked using dimensional analysis. This means verifying that the dimensions (units) on both sides of the equation are the same.

Example:

Let’s check the validity of the equation:

s = ut + ½at²

Left side: s (displacement) = m

Right side:

u·t = (m/s)·s = m

½·a·t² = (m/s²)·s² = m

So, both terms on the right have units of meter (m) → same as LHS.

Equation is valid.

d. Mention the fundamental units involved in the unit of pressure.

Answer:

Pressure = Force / Area

Force = mass × acceleration = kg·m·s⁻²

Area = m²

So, Pressure = kg·m·s⁻² ÷ m² = kg·m⁻¹·s⁻²

Fundamental units: kilogram (kg), meter (m), second (s)

e. Find out the fundamental units involved in the given derived units.

i. Newton (N) = kg·m·s⁻²

→ kg, m, s

ii. Watt (W) = joule / second = (kg·m²·s⁻²) / s = kg·m²·s⁻³

→ kg, m, s

iii. Joule (J) = newton × meter = (kg·m·s⁻²) × m = kg·m²·s⁻²

→ kg, m, s

iv. Pascal (Pa) = newton / m² = (kg·m·s⁻²) / m² = kg·m⁻¹·s⁻²

→ kg, m, s

f. Niva claimed that an alternative formula for power is P=mv2 and the formula of pressure; P=mv/A.

Check the validity of given formulae by the analysis of units.

i. P = mv² (claimed for power)

Unit of mass = kg

Unit of velocity² = (m/s)² = m²·s⁻²

→ Unit of mv² = kg·m²·s⁻² = joule (J)

This is the unit of energy, not power.

Correct unit of power = joule / second = kg·m²·s⁻³

Invalid formula for power

ii. P = mv / A (claimed for pressure)

m = kg, v = m/s, A = m²

→ mv / A = (kg· m/s) / m² = kg·m⁻¹·s⁻¹

Unit = kg·m⁻¹·s⁻¹, which is not the unit of pressure

Unit of pressure = kg·m⁻¹·s⁻²

Invalid formula for pressure

g. Describe the independent variable, dependent variable and controlled variable with a suitable example.

h. Karma connected a dry cell to a bulb using a few pieces of wire and lit the bulb. He was curious to know how the thickness of the used wire affects the life span of a dry cell. In this test or experiment, find out the independent variable, dependent variable and controlled variable.

Answer:

Independent Variable: Thickness of the wire

(Because Karma is changing the thickness to observe its effect)

Dependent Variable: Life span of the dry cell

(Because it’s what he is measuring)

Controlled Variables: Type of bulb, length of wire, type of cell, room temperature, material of wire

(These should be kept constant to ensure a fair test)

i. Chandani wanted to investigate the effects of substances mixed with the soil on plant growth. She decided to mix lime, urea fertilizer, common salt, and compost manure in the soil. Then she brought a bucket of soil from a nearby field and sieved it. She put that soil equally into 12 uniform sized pots. Then she mixed two spoons of salt in every three pots, two spoons of urea fertilizer in each of the next three pots, two spoons of table salt in each of the next three pots, and finally, two spoons of compost fertilizer in each of the remaining three pots. She then planted similar seeds of the same plant in every pot and placed them all in a sunny place in the house. She watered all the pots every day. After the plants grew, she measured the height of each plant daily and kept a record.

i. Identify the independent variable, dependent variable and controlled variable in Chandani

Experiment?

Answer: 

Independent Variable: Type of substance added to the soil (lime, urea, salt, compost)

Dependent Variable: Growth of the plant (measured by height)

Controlled Variables: Type and amount of soil, Size of pots, Type and number of seeds, Watering schedule, Amount of sunlight, Quantity of added substance (2 spoons each)

ii. Why did Chandani use 3 pots for each experiment?

Answer:

She used 3 pots for each substance to ensure reliability and accuracy of the results. If similar results are obtained from all 3 pots, it increases confidence that the results are due to the added substance and not a random variation. This is called replication in experiments.

j. Subodh wanted to find out how the colour of an object affects its ability to hold heat. For that, he took four conical flasks and coated the first flask with black coloured enamel, the second with white enamel, the third with green enamel and the fourth with red enamel. Then, he filled all the flasks with water, closed the mouth of each flask tightly with cork and then kept them in the sun. After some time he measured the temperature of water in each flask with the help of a thermometer and noted the readings. Identify the independent variable and dependent variable in Subodh’s experiment. Which variables should be controlled by Subodh?

Identify the variables:

Independent Variable: Colour of the flask (black, white, green, red)

Dependent Variable: Temperature of water in the flask (how much heat was absorbed)

Controlled Variables: Type and size of flasks, Amount of water in each flask, Duration and timing of sun exposure, Type of cork, Environmental conditions (temperature, wind, etc.)

k. Manisha wanted to test the eating habits of her dog. She decided to study how the amount of food and the time of giving food affect the speed at which the dog ate. What is wrong with the design of Manisha’s experiment and how can she correct it?

Problem in the design:

Manisha is changing two variables at the same time — both the amount of food and the time of feeding —

which makes it unclear which variable is affecting the speed of eating.

How to correct it:

Manisha should change only one variable at a time:

First, fix the feeding time and test the effect of different amounts of food on eating speed.

Then, fix the amount of food and vary the feeding time.

This way, she can isolate the effect of each factor on the dog’s eating behavior.

l. Prove that unit of electric resistance (Ohm) = kg·m²·s⁻³·A⁻²