NCTB Class 9-10 Science Chapter 10 Let Us Know Force Solution

NCTB Class 9-10 Science Chapter 10 “Let Us Know Force” Solution

Bangladesh Board Class 9-10 Science Solution Chapter 10 Let Us Know Force Solution Exercises Question and Answer by Experienced Teacher.

NCTB Solution Class 9-10 Chapter 10 “Let Us Know Force”:


NCTB Bangladesh Board

9 – 10 (Nine – Ten Classes)




Chapter Name

Let Us Know Force

1) A fruit falls on the ground from the tree. Example of which type of force it is.

a) Gravitational force

b) Magnetic force

c) Electromagnetic force

d) Weak nuclear force

Answer:- a) gravitational force

2) Force-

1) Keeps the direction of motion of an object unchanged.

2) Changes the shape of an object.

3) Brings a body at test to motion.

a) I and ii b) I and iii c) ii and iii d) I, ii and iii.

Answer:-  c) ii and iii

  • By reading the following article answer questions 3 and 4.

An object is thrown on the floor with an acceleration of 4 m/S2  by applying force of 2 N. After traversing a distance the body stop moving.

3) What is the mass of the object ?

a) 200 gm b) 400 gm

c) 500 gm d) 750 gm.

Answer:- 200 gm

4) For which cause of force the body ceases to act?

a) Frictional force

b) gravitational force

c) magnetic force

d) electromagnetic force

Answer:- a) frictional force

  • Creative questions

Que 1] Swapna was going to Dhaka from kushita. The mass of the bus was 1400 kg and it was moving with an acceleration of 4 m/ S2 . When the driver applied Brake on moving bus, the passengers along with the swapna were leaning forward. Again when the bus started to move then they were bended backward.

1) What is called force in contact?

Answer:- some forces need contact for use such as lifting heavy things with crane, pushing anything or stopingany moving thing with friction contact force or force on contact. Best example for this is frictional force.

2) What is meant by force?

Answer:-push or pull of an object with mass that causes it to change its velocity.push or pull of an object is considered as force. If force is Applied on an object at rest it starts to move. While an object in motion could be stopped by applying force.

3) Calculate the amount of force acting on the bus .


Given-  m = 1400 kg

a  = 4m/S2

F= m× a

= 1400 × 4

= 5600 N

Thus the force acting on the bus is 5600 N.

4) Analyse the cause of the passengers leaning forward at first and next time the cause of bending backward?

Answer:-the tendency of an object at rest to remain at rest is called inertia of rest and the tendency of an object to remain at motion is called inertia of motion. Thus,  when the bus suddenly starts moving, the passengers of the bus bend backward due to inertia of rest and when the bus suddenly stop the passengers lean forward due to inertia of motion.

Que. 2] Turjo investigate about various incident in daily life. One day in his house sitting on a chair in front of heavy table , he began to pull the table. But he himself along with the chair moved towards the table. Next day he rolled on a marble on a smooth floor of his room with a fixed amount of force. Next time he rolled on the same marble with the same force on the puccha road outside his house.

A) What is Newton’s second law of motion?

Answer:-Newton’s second law of motion is that the rate of change of momentum of a body is proportional to the force applied. But force is defined in such a way that we can say equal instead of proportional.Newton’s second law of motion explain that how much force is to be applied on an object of a certain mass and how much change there will be in consequence.

Ma= m( v-u)/t

The applied force will be equal to F. It means

F= m(v-u)/t= ma.

B) What is meant by inertia?

Answer:-Every object tries to maintain its present state. If an object at rest, it tends to be at rest, and if a object at motion , it tends to be in motion. Until force is applied ,t he tendency of an object to remain in it’s present state is called inertia. The tendency of an object at rest to remain at rest is called inertia of rest and the tendency of an object in motion to continue motion is called inertia of motion.

C) Why did turjo along with his chair move towards the table? Explain.

Answer:-when turjo applied force on the table, the table also applied the equal and opposite force on turjo and chair. It is always true that whatever you apply a force on an object, it applies an opposite force. Thus turjoalong with his chair move towards the table.

D) Analyse the cause of traversing different distance by the marbles in two different surfaces.

Answer:-when a marble were rolling on smooth surface it was reducing friction smoothing of the surface as much as possible where the friction takes place. In smooth surface sliding friction is less. While on the other hand, when the marble were rolled on the puccha road that was rough surface thus it is increasing friction. Making two surface uneven or rough where friction takes place increase the friction.

Updated: February 8, 2021 — 6:57 pm

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