CBSE Class 8 Science Advanced Physics Force Very Short Answer Questions Set 01

Fill in the Blanks

Question 1. CGS unit of force is _______.
Answer:
Dyne
In simple words: In the CGS system force is measured in dyne.

Teacher's Note:
1. CGS uses centimetre, gram and second.
2. 1 dyne = 1 g × 1 cm/s².

Question 2. To draw water from a well we have to _______ the rope.
Answer:
Pull
In simple words: We pull the rope to bring the bucket up.

Teacher's Note:
1. Force is a push or a pull.
2. Here the force is applied by our muscles.

Question 3. Muscular force is a _______________ force.
Answer:
Contact
In simple words: Muscles must touch an object to push or pull it.

Teacher's Note:
1. Muscular force works only through touch.
2. Weightlifting, kicking and lifting a bag are examples.

Question 4. Friction force is a ______________ force.
Answer:
Contact
In simple words: Friction acts only where two surfaces touch.

Teacher's Note:
1. No touching, no friction.
2. It acts along the surfaces in contact.

Question 5. The north pole of a magnet _______ the north pole of another magnet.
Answer:
Repels
In simple words: Like poles push each other away.

Teacher's Note:
1. Unlike poles attract.
2. Magnetic force acts even without contact.

True or False

Question 6. Gravitational force is a type of contact force.
Answer:
False
It is a non-contact force.
In simple words: Gravity acts across a distance without touching.

Teacher's Note:
1. The earth pulls a falling apple without touching it.
2. Contact forces are normal force, friction and muscular force.

Question 7. Mass of a body is never zero.
Answer:
True
Every body contains some matter, so its mass cannot be zero.
In simple words: Anything that exists as matter has some mass.

Teacher's Note:
1. Weight can become zero, mass never does.
2. Mass is a scalar quantity.

Question 8. Weight of a body remains same always.
Answer:
False
Weight = mg and g changes from place to place.
In simple words: Weight changes when g changes, for example on the moon.

Teacher's Note:
1. Mass is constant, weight is not.
2. Weight is zero at the centre of the earth, where g = 0.

Question 9. Newton and kg × m/s² both are the units of force.
Answer:
True
1 N = 1 kg m/s².
In simple words: The newton is just another name for kg m/s².

Teacher's Note:
1. Force = mass × acceleration.
2. So its unit is kg times m/s².

Question 10. Normal force is a contact force.
Answer:
True
It acts between surfaces that touch, perpendicular to them.
In simple words: A surface pushes back at right angles because the objects touch.

Teacher's Note:
1. The table pushing up on a book is an example.
2. It is one of the three contact forces.

Match the Column

Teacher's Note:
1. Take the easy pairs first: mass with 'never changes' and weight with 'changes from place to place'.
2. Newton is the only unit in the list, so it goes with kg × m/s².

Very Short Answer Type

Question 12. Write the SI and CGS unit of force.
Answer:
SI unit: newton (N) = kg m/s².
CGS unit: dyne = g cm/s².
In simple words: The SI unit is the newton and the CGS unit is the dyne.

Teacher's Note:
1. Remember 1 N = 10⁵ dyne.
2. SI uses kg and m, CGS uses g and cm.

Question 13. What are the different types of non contact forces ?
Answer:
The non-contact forces are:
1. Gravitational force
2. Electrostatic force
3. Magnetic force
In simple words: Non-contact forces act without the bodies touching: gravity, electric and magnetic forces.

Teacher's Note:
1. All three can act across empty space.
2. Contact forces are the opposite: normal, friction and muscular.

Question 14. What do you mean by Resultant Force ?
Answer:
When several forces act on a body, the single force that produces the same change in its state of rest or motion as all of them together is called the resultant force.
In simple words: It is one force that does the job of all the forces together.

Teacher's Note:
1. Same direction: add the forces. Opposite directions: subtract them.
2. A resultant of zero means the forces are balanced.

Question 15. What are the gravitational units of force ?
Answer:
kilogram force (kgf) in the MKS system: 1 kgf = 9.8 N.
gram force (gf) in the CGS system: 1 gf = 980 dyne.
Also 1 kgf = 1000 gf.
In simple words: Gravitational units are the weights of 1 kg and 1 g: kgf and gf.

Teacher's Note:
1. 1 kgf is the pull of the earth on a 1 kg mass.
2. For quick work 1 kgf is taken as about 10 N.

Question 16. What measures the earth's gravitational pull on an object, its weight or mass ?
Answer:
Its weight. Weight is the force with which the earth attracts the object (W = mg), while mass is the amount of matter in it.
In simple words: The earth's pull on an object is its weight, not its mass.

Teacher's Note:
1. Weight is measured in newton and mass in kg.
2. A spring balance shows weight.

Question 17. Express 10 N force in terms of dyne.
Answer:
1 N = 10⁵ dyne.
10 N = 10 × 10⁵ = 10⁶ dyne.
In simple words: Multiply the newtons by 10⁵ to get dynes.

Teacher's Note:
1. 10 × 10⁵ = 10⁶.
2. The answer is a bigger number because dyne is a smaller unit.

Question 18. Express 50 N force in terms of g cm s⁻².
Answer:
1 N = 10⁵ g cm s⁻².
50 N = 50 × 10⁵ = 5 × 10⁶ g cm s⁻² (5 × 10⁶ dyne).
In simple words: g cm s⁻² is the dyne, so multiply 50 by 10⁵.

Teacher's Note:
1. g cm s⁻² is another way of writing a dyne.
2. 50 × 10⁵ = 5 × 10⁶.

Question 19. A force of 10 N towards the east and an unknown force F balance each other. Find the unknown force.
Answer:
Balanced forces are equal in size and opposite in direction.
So F = 10 N towards the west.
In simple words: To balance 10 N east, we need 10 N west.

Teacher's Note:
1. Net force must be zero.
2. Give the direction as well as the size.

Question 20. A particle attracts the earth with a force equal to the weight of the particle. (Yes / No)
Answer:
Yes. The earth pulls the particle with a force equal to its weight, and by the law of gravitation the particle pulls the earth with an equal force in the opposite direction.
In simple words: The pull is mutual and equal on both.

Teacher's Note:
1. Forces between two bodies come in equal and opposite pairs.
2. The earth's acceleration is tiny only because its mass is huge.

Question 21. What is the mass of an object whose weight is 49 N ? (Take g = 9.8 m/s²)
Answer:
W = mg, so m = W ÷ g = 49 ÷ 9.8 = 5 kg.
In simple words: Divide the weight by g to get the mass.

Teacher's Note:
1. Weight is a force and mass is the amount of matter.
2. Check: 5 × 9.8 = 49.

Question 22. What is normal force ?
Answer:
When two bodies touch, the contact force that acts perpendicular to the surfaces in contact is called the normal force. For a book on a table, the table pushes the book upwards and the book pushes the table downwards, both at right angles to the surface.
In simple words: It is the push of a surface at right angles to itself.

Teacher's Note:
1. It is one of the three contact forces.
2. On a horizontal table, the normal force balances the weight of the book.

Question 23. A force F gives an object with mass m an acceleration a. If this force is applied to an object of mass \( \frac{1}{4}m \), what is the new acceleration ?
Answer:
a = F ÷ m, so for the same force acceleration is inversely proportional to mass.
New a = F ÷ (m/4) = 4F/m = 4a (4 times).
In simple words: A quarter of the mass gets four times the acceleration.

Teacher's Note:
1. Same force, smaller mass, bigger acceleration.
2. Use F = ma in both cases.