The document Very Short Answers - Force and Laws of Motion, Science, Class 9 Class 9 Notes | EduRev is a part of the Class 9 Course Class 9 Science by VP Classes.

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**Q1. Define force. Ans. **It is a push or pull on an object that produces acceleration in the body on which it acts.

**Q2. What is S.I. unit of force? Ans.** S.I. unit of force is Newton.

**Q3. Define one Newton. Ans. **A force of one Newton produces an acceleration of 1 m/s

**Q4. What is balanced force? Ans. **When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.

**Q5. What is frictional force? Ans. **The force that always opposes the motion of an object is called force of friction.

**Q6. What is inertia? Ans. **The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

**Q7. State Newton’s first law of motion. Ans. **An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

**Q8. State Newton’s second law of motion. Ans. **The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

**Q9. What is momentum? Ans. **The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The S.I. unit is kg m/s. (p = mv)

**Q10. State Newton’s III law of motion. Ans. **To every action, there is an equal and opposite reaction and they act on two different bodies.

**Q11. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg? Ans.** A body of mass 20 kg will have more inertia.

**Q12. Name the factor on which the inertia of the body depends. Ans**. Inertia of a body depends upon the mass of the body.

**Q13. Name two factors which determine the momentum of a body. Ans.** Two factors on which momentum of a body depend is mass and velocity.

Momentum is directly proportional to the mass and velocity of the body.

**Q14. What decides the rate of change of momentum of an object? Ans. **The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

**Q15. The diagram shows a moving truck. Forces A, B, C and D are acting on the truck. Name the type of forces acting on a truck. Ans.** The forces A, B, C and D acting on the truck are:

. The forces A, B, C and D acting on the truck are:

A → driving force

B → reacting force

C → frictional force

D → weight/gravitational force

**Q16. A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner? Ans.** m = 68 kg, F = 59N, a = ?

F= ma,

59 = 68a

a= 59/68 = 0.87 m/s^{2}

**Q17. A trolley is dragged across an ice covered lake. The box accelerates at 0.08 m/s ^{2} and is pulled by a 47 N force. What is the mass of the box? Ans. **a = 0.08 m/s

F=ma,

47 = m(0.08)

m= 47/0..8 = 587.5 kg**Q18. A tennis ball, 0.314 kg, is accelerated at a rate of 164 m/s ^{2} when hit by a professional tennis player. What force does the player’s tennis racket exert on the ball? Ans.** m = 0.314 kg, a = 164 m/s

F = ?

F = ma = (0.314)(164) = 51.50 N

**Q19. A coolie exerts a force of 628 N on a luggage trolley. Later, the mass of the trolley is increased by a factor of 3.8. If the coolie exerts the same force, how does the new acceleration compare to the old one? Ans. **a = (F/m)

⇒ a = 628N/3.8

= 165.26 N

**Q20. What is the mass of a runner whose weight is 648 N? Ans**. g = 9.80 m/s

W=mg

m= 66.12 kg

= m= W/g

= m=648/9.8**Q21. When you jump, the Earth is pushed away from you. Use a formula to explain why this movement of the Earth isn’t noticed. Ans.** a = (F/m) the Earth has lot of mass that its acceleration is tiny.

**Q22. Compare the force on a cricket ball by a bat with the force exerted by the ball on the bat. Ans.** The force has the same magnitude but opposite direction.

**Q23. A cart is being moved by a certain net force. If more force is applied on the cart, doubling its mass, state by how much the acceleration changes. Ans**. It will be half.

**Q24. A hammer is hitting a nail. Compare the force on the nail; with the force on the hammer. Ans. **The forces acting on nail and on hammer are equal in magnitude (strength) but opposite in direction.

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