7th std Science chapter 7 Motion Force and Work l Exercise

YADNYA e-Learning

Science Studies -  7th Standard  

7. Motion, Force and Work

Question & Answer

1. Fill in the blanks with the proper words from the brackets.

(stationary, zero, changing, constant, displacement, velocity, speed, acceleration, stationary but not zero, increases)

(a) If a body traverses a distance in direct proportion to the time, the speed of the body is constant.

(b) If a body is moving with a constant velocity its acceleration is zero.

(c) speed is a scalar quantity.

(d) velocity is the distance traversed by a body in a particular direction in unit time.

2. Observe the figure and answer the questions.

 

Sachin and Sameer started on a motorbike from place A, took the turn at B, did a task at C, travelled by the route CD to D and then went on to E. Altogether, they took one hour for this journey. Find out the actual distance  traversed by them and the displacement  from A to E. From this, deduce their speed. What was their velocity from A to E in the direction AE? Can this velocity be called average velocity?

Ans: Actual distance travelled by Sachin and Sameer from A to E = AB + BC + CD + DE = 3 + 4 + 5 + 3 = 15 km
Displacement from A to E = AB + BD + DE = 3 + 3 + 3 = 9 km

Yes, this can be called the average velocity of the motorbike from A to E.

3. From the groups B and C, choose the proper words, for each of the words in group A.

A	B	C
Work	Newton	erg
Force	Metre	cm
Displacement	Joule	dyne

 

Ans:

A	B	C
Work	Joule	erg
Force	Metre	dyne
Displacement	Newton	cm

4. A bird sitting on a wire, flies, circles around and comes back to its perch. Explain the total distance it traversed during its flight and its eventual displacement.

Ans: The total distance travelled by the bird during its flight = 2×(Distance between the point where the bird was sitting and the point from where it takes a turn) The eventual displacement of the bird is zero as it returns to its initial point i.e. where it was sitting.

5. Explain the following concepts in your own words with everyday examples : force, work, displacement, velocity, acceleration, distance.

Ans: Force: It is any kind of push or pull on a body due to another body when the bodies interact with each other. It is a vector quantity. For example, a person applies a force in the form of push or pull to open a door.

Work: It is defined as the work done by a force that causes a displacement in an object. It is a scalar quantity. For example, a child does work when he drags a toy car on the ground.

Displacement: It is the shortest distance between the initial and final position of an object. It is a vector quantity. For example, when we go to a mall for shopping from our house and then return to the house, the displacement would be 0 as our initial position (house) and final position (house) is same.

Velocity: Velocity is defined as the rate of change of displacement. It is a vector quantity. For example, a car running on a straight road has some velocity.

Acceleration: The rate of change of velocity is known as acceleration. It is a vector quantity. For example, if a car is moving on a straight with variable speed, it will posses some acceleration. In case the speed of the car remains same but the direction of car changes, then also the car would be accelerating.

Distance: It is the actual path length covered by an object during its motion. It is a scalar quantity.  For example, when we go to a mall for shopping from our house and then return to the house, the distance travelled by us would be twice the distance between our house and the mall.

 

6. A ball is rolling from A to D on a flat and smooth surface. Its speed is 2 cm/s. On reaching B, it was pushed continuously up to C. On reaching D from C, its speed had become 4cm/s. It took 2 seconds for it to go from B to C. What is the acceleration of the ball as it goes from B to C?

 

Ans: The acceleration of the ball between A to B is zero as the speed and direction of the ball is constant. After point B, a force is applied. Thus, the ball will get accelerated.
Acceleration of the ball from B to C=Change in velocity from B to C / Time taken for this change Acceleration of the ball from B to C=4−2 / 2=1 m/s²

7. Solve the following problems.

(a) A force of 1000N was applied to stop a car that was moving with a constant velocity. The car stopped after moving through 10m. How much is the work done ?

Ans:  Work done by the force to stop the car=F×S=1000×10

=10000 J

(b) A cart with mass 20 kg went 50m in a straight line on a plain and smooth road when a force of 2N was applied to it. How much work was done by the force ?

Ans: Work done by the force=F×S=2×50=100 J

 

7th std Science chapter 8 Static Electricity l Exercise

 YADNYA e-Learning

Science Studies -  7th Standard  

8. Static Electricity

Question & Answer

1. Choose the correct option and fill in the blanks.

(always repulsion, always attraction, displacement of negative charge, displacement of positive charge, atom, molecule, steel, copper, plastic, inflated balloon, charged object, gold)

(a) There is always repulsion between like charges.

(b) Displacement of negative charge is responsible for generation of electric charge in an object.

(c) A lightning conductor is made of a copper strip.

(d) Copper/Gold does not get electrically charged easily by rubbing.

(e) There is always attraction when opposite electric charges come near each other.

(f) A charged object can be detected with an electroscope.

 

2. Explain why it is not safe to go out with an umbrella when there is heavy rain, lightning or thunder.

Ans: Heavy rain and thundering is accompanied by lightning. Lightning is basically the discharge of huge amount of electric charges from clouds towards the ground. These charges may pass through the metallic rod of the umbrella to the person carrying it. Hence, the person might get severe electric shock and die. Thus, it is not advisable and safe to go out with an umbrella when there is heavy rain, lightning or thunder.

 

3. Answer in your own words.

(a) How will you protect yourself from lightning?

Ans:  ​Following are some safety measures to be followed to protect oneself from lightning:
If outdoor:
1. Avoid taking shelter under trees.
2. Avoid being near to any kind of metal objects which may include metal wires, fences, machinery and power appliances.
3. Take shelter inside a fully enclosed car with all the windows shut.
If indoor:
1. Turn off all the electrical appliances such as television and refrigerator.

 

(b) How are charges generated?

Ans:  Charges are generated in an object when the object loses or gains some electrons or when there is displacement between the positive and negative charges of an object. Methods of generating charges are:

· Rubbing: When two insulators are rubbed with each other, one of them loses electrons and the other gains electrons. The one which gains electrons gets negatively charged and the other one gets positively charged.

· Conduction: When a charged body is brought in contact with an uncharged conductor, the conductor gets charged.

· Induction: When a charged body is brought near to a neutral body say a conductor or an insulator, the electrons and protons in the neutral body gets separated. Thus, the body acquires temporary charge as long as the charged body is kept near it. 

 

(c) In the lightning conductor, what provision is made for spreading the electricity into the ground?

Ans: A pit is dug in the ground and is filled with coal and salt. A cast iron plate is placed upright in the pit and one end of the lightning conductor is connected to this plate. Also, a provision of pouring water into the pit is made. Thus, when the electric charges through the conductor is discharged into the pit, the water quickly spreads these charges into the ground and prevents damages.

 

(d) Why do farmers stick an iron staff into the ground while working in the field in rainy conditions ?

Ans:In rainy conditions, there is very high chance of lightning striking the Earth. So, the farmer by putting an iron staff into the ground while working in the field in rainy condition, makes sure that he and his crops remains safe in case lightning occurs. This is because the iron staff will be more prone to lightning than the crops and his body because of the iron staff being tall and metallic in nature. Thus, the electric charges from the clouds will flow to the ground through the staff without affecting the plants in case of lightning during rainy weather.

 

(e) Why is lightning not seen everyday during the rainy season ?

Ans: For lightning to happen, there must be some minimum amount of accumulation to happen in the clouds. So, it might be possible that this limit is not achieved everyday during rainy season because of which lightning does not occur everyday. Hence, we do not see lightning everyday during the rainy season.

 

4. What are the characteristics of a static electric charge?

Ans: The characteristics of a static electric charge are as follows:

· Charges exist in two types i.e. positive and negative. Positive charge is generally carried by a proton and negative charge by an electron.

· Like charges always attract each other and unlike charges always repel each other.

 

5. What is the damage caused by lightning? How will you create awareness to prevent it ?

Ans:  Damage caused by lightning are:

· A fire can start if the lightning strike makes contact with flammable material. These can include gas pipes, wood, paper, etc.

· If lightning follows electrical wiring, then the wires gets overheated which can cause a fire hazard.

· Lightning can cause damage to walls, concrete, plaster and glass of a building. It can damage the electrical appliances connected to damaged sockets.

· Lightning can damage tall trees and crops. It can even lead to forest fire.

· Lightning, if strikes on living beings, can take their lives.

These damages can be prevented if preventive guidelines against lightning is taught to each and every individual. The preventive measures can be propagated to people using different mediums such as books, T.V., radio, etc. Awareness programmes should be conducted in schools and societies to educate people and students about the wrath of lightning and measures to fight it. Following are some safety measures which should be told and followed by individuals when lightning strikes:
If outdoor:
1. Avoid taking shelter under trees.
2. Avoid being near to any kind of metal objects which may include metal wires, fences, machinery and power appliances.
3. Take shelter inside a fully enclosed car with all the windows shut.
If indoor:
1. Turn off all the electrical appliances such as television and refrigerator.

7th std Science chapter 6 Measurement of Physical Quantities l Exercise

 YADNYA e-Learning

Science Studies -  7th Standard  

6. Measurement of Physical Quantities

 

Question & Answer

 

1. Write answers to the following questions in your own words.

(a) Why is the weight of the same object different on different planets ?

Ans: Weight of an object, W is given as
W=mg where, m = mass of the object and g = acceleration due to gravity of a place
We know that acceleration due to gravity is different at different planets. Hence, the weight of same object differs on different planets.

 

(b) What precautions will you take to make accurate measurements in day-to-day affairs ?

Ans: Precautions to be take to make accurate measurements in day-to-day affairs are

· The device used for measurement should be appropriate and properly working.

· The device should be used properly.

· It should be checked that the device in use should have stamp of standardization by the department of weights and measure.

· 3-4 readings should be taken for achieving accuracy in measurement

(c) What is the difference between mass and weight ?

Ans:

Mass	Weight
Mass is the amount of matter contained in a body.	Weight is the force exerted on a body due to the gravitational pull of another body such as Earth, the sun and the moon.
Mass is an intrinsic property of a body.	Weight is an extrinsic property of a body.
The mass of a body remains the same everywhere in the universe.	The weight of a body depends on the local acceleration due to gravity where it is placed.
The mass of a body cannot be zero.	The weight of a body can be zero.
The SI unit of mass is kilogram (kg).	Since weight is a force, its SI unit is newton (N).
The mass of a body can be measured using a beam balance and a pan balance.	The weight of a body can be measured using a spring balance and a weighing machine.

 

2. Who is my companion?

 

Group ‘A’	Group ‘B’ (Ans)
Velocity	Meter/second
Area	Square metre
Volume	litre
Mass	kilogram
Density	Kilogram/cubic metre

 

3. Explain giving examples.

(a) Scalar quantity

Ans: A quantity which has only magnitude and no direction is known as scalar quantity. Distance, work, speed, time, energy, etc. are all scalar quantities. These quantities can be expressed completely by their magnitude alone.

 

(b) Vector quantity

Ans: A quantity which has magnitude as well as direction is known as vector quantity. Force, velocity, displacement, etc. are all vector quantities. These quantities are expressed completely only when their directions are defined along with their magnitude.

 

4. Explain, giving examples, the errors that occur while making measurements.

Ans: Various types of errors can occur while taking measurement. These are as follows:

· Instrumental errors: These errors arise when the instrument used for measurement has become faulty. For eg, ruler with rough and broken edge can cause instrumental error.

· Observational errors: These errors arise when the reading in the instrument is noted incorrectly. These can also arise if the instrument is used in incorrect method. For example, if the tip of pencil does not coincide with zero of the scale, then we will get error in measurement of length of pencil.

· Theoretical errors: These are caused by simplification of the model system. For example, a theory states that the temperature of the system surrounding will not change the readings taken when it actually does, then this factor will begin a source of error in measurement.

· Environmental errors: These errors arises due to the instruments working getting affected due to some environmental factors such as temperature, pressure, etc.

 

5. Give reasons.

(a) It is not proper to measure quantities by using body parts as units.

Ans: It is not proper to measure quantities by using body parts as units because these are not reliable and unique. These units of measurement vary from person to person.

(b) It is necessary to get the weights and measures standardized at regular intervals.

Ans: Because of difference in physical conditions from place to place, the reference chosen as units for weights and measures varies. So, it becomes necessary to get the weights and measures standardized at regular intervals.

 

6. Explain the need for accurate measurement and the devices to be used for that.

Ans: We need not be very accurate every time we take a measurement. The degree of accuracy in measurements vary according to the situations. For example,

· when someone asks about the time of the day, we do not have to be specific up to the seconds of time, because there is no need to be that accurate in the given situation. On the other hand, for laboratory purposes or for scientific experiments, we have to measure the time even up to milliseconds accurately.

· while measuring mass of precious substances, such as gold, silver, etc., we need to be accurate in our measurement.

· in competitions and events, a difference of second can make you lose or win the game. Thus, accurate time measurement in such cases is very important.

· shopping in our day to day life, like for groceries, milk, flour etc. requires lot of alertness at our part so that we do not get cheated. We should always check whether the measurement is done properly or not by the vendors selling these products.

The devices used for accurate measurement are:

· For time measurement, stop watch can be an accurate device

· For mass measurement, electronic balance can be an accurate device.

· For length measurement, ruler or measuring tape can be the accurate devices.

· For temperature measurement, thermometers can be an accurate device.