Q1: What is the SI unit of force? (a) Joule (b) Newton (c) Watt (d) Pascal
Solution:
Ans: (b) The SI unit of force is newton, symbolised as N. It is named after Isaac Newton who formulated the laws of motion.
Q2: A spring balance is used to measure which of the following? (a) Mass of an object (b) Volume of an object (c) Magnitude of a force (d) Temperature of an object
Solution:
Ans: (c) A spring balance measures the magnitude of a force. It was earlier used to measure weight, which is the gravitational force acting on an object.
Q3: When two forces of 10 N and 6 N act in opposite directions on an object, what is the net force if 10 N acts towards the right and 6 N towards the left? (a) 16 N towards the right (b) 16 N towards the left (c) 4 N towards the right (d) 4 N towards the left
Solution:
Ans: (c) When forces act in opposite directions, the net force equals the difference of magnitudes. Here, 10 N - 6 N = 4 N towards the right.
Q4: According to Newton's first law of motion, what happens to an object moving with constant velocity when the net force acting on it is zero? (a) It immediately stops moving (b) It continues to move with the same constant velocity (c) It accelerates in the direction of motion (d) It changes direction randomly
Solution:
Ans: (b) Newton's first law states that an object in motion continues moving with constant velocity unless a net force acts upon it.
Q5: A weightlifter holds a barbell with total mass 30 kg steady. What upward force must she apply? (Take g = 9.8 m s⁻²) (a) 30 N (b) 98 N (c) 196 N (d) 294 N
Solution:
Ans: (d) Gravitational force = mg = 30 kg × 9.8 m s⁻² = 294 N. To keep it steady, she applies 294 N upward to balance this.
Fill in the Blanks
Q1: Force is a physical quantity for which we need to specify both magnitude and _____.
Solution:
Ans: direction
Q2: When two forces are equal in magnitude but opposite in direction, they are called _____ forces.
Solution:
Ans: balanced
Q3: The force of friction acts on an object in the direction _____ to its direction of motion.
Solution:
Ans: opposite
Q4: One newton of force produces an acceleration of 1 m s⁻² on an object of mass _____ kg.
Solution:
Ans: 1
Q5: The acceleration due to gravitational force by the Earth is denoted by the symbol _____.
Solution:
Ans: g
True or False
Q1: A force can change the shape of an object.
Solution:
Ans: True Force can change the shape of an object, such as squeezing a lemon causes it to change its shape.
Q2: Smoother surfaces have more friction than rougher surfaces.
Solution:
Ans: False Smoother surfaces have less friction compared to rougher surfaces. Friction depends on the nature of the surfaces in contact.
Q3: The value of acceleration due to gravity (g) depends on the mass of the falling object.
Solution:
Ans: False The acceleration due to gravitational force by the Earth does not depend on the mass of the object near Earth's surface.
Q4: According to Newton's third law, action and reaction forces act on different objects.
Solution:
Ans: True The pair of equal and opposite forces as per Newton's third law acts on two different objects, so they do not balance.
Q5: Galileo Galilei explained that a continuous force is required to keep an object moving at constant velocity.
Solution:
Ans: False Galileo explained that a force is not needed to keep an object moving at constant velocity if no resistance like friction is present.
Match the Following
Column A
Column B
1. Force that opposes motion between surfaces
A. Newton's third law of motion
2. An object at rest remains at rest unless acted upon by a net force
B. F = ma
3. Mathematical form of Newton's second law
C. Friction
4. Equal and opposite forces acting on different objects
D. Newton's first law of motion
5. The value is approximately 9.8 m s⁻² near Earth's surface
E. Acceleration due to gravity
Solution:
Ans:
1 - C: Friction is the force that opposes motion between surfaces and acts in the direction opposite to motion.
2 - D: Newton's first law states an object at rest remains at rest unless a net force acts upon it.
3 - B: The mathematical form of Newton's second law is F = ma, relating force, mass, and acceleration.
4 - A: Newton's third law explains equal and opposite forces acting simultaneously on two different objects during interaction.
5 - E: Acceleration due to gravity is denoted by g and has a value of approximately 9.8 m s⁻² near Earth's surface.
Short Answer Questions
Q1: Explain the difference between balanced and unbalanced forces with one example each.
Solution:
Ans: Balanced forces are equal in magnitude but opposite in direction, resulting in zero net force. For example, in a tug of war with equal pulling force, the rope does not move. Unbalanced forces are not equal, creating a non-zero net force. For example, if one team pulls harder, the rope moves in that direction.
Q2: State Newton's second law of motion and write its mathematical form.
Solution:
Ans: Newton's second law states that when a net force acts on an object, it accelerates in the direction of the net force. The acceleration magnitude is proportional to the net force and inversely proportional to the object's mass. The mathematical form is F = ma, where F is net force, m is mass, and a is acceleration.
Q3: Why does a coconut break when it hits the ground but a cricket ball caught by pulling hands back does not injure the player?
Solution:
Ans: A coconut stops in very little time when hitting the ground, producing a large force that breaks it. When catching a cricket ball, pulling hands back increases the stopping time, which reduces the force applied on the hands. This prevents injury. Both situations relate to how stopping time affects the force experienced during impact.
Q4: How does a rocket launch and achieve motion according to Newton's third law?
Solution:
Ans: During rocket launch, the rocket engine fires gases downward with great force. According to Newton's third law, the gases exert an equal and opposite force on the rocket in the upward direction. This upward force pushes the rocket into space. The same principle applies to achieve controlled landing by firing gases opposite to motion direction.
Q5: Define one newton of force.
Solution:
Ans: One newton of force is defined as the amount of force that produces an acceleration of 1 m s⁻² on an object of mass 1 kg. Using the formula F = ma, we get F = 1 kg × 1 m s⁻² = 1 kg m s⁻² = 1 N. This is the standard unit for measuring force.
Long Answer Questions
Q1: A student pushes a stationary block of mass 25 kg on a horizontal floor. The maximum force of friction is 50 N. Calculate the displacement of the block in 2 seconds if the student applies (i) 50 N and (ii) 55 N force in the forward direction.
Solution:
Ans:
(i) When applied force equals friction (50 N), the forces are balanced and net force is zero. The block remains stationary, so displacement is 0 m.
(ii) When applied force is 55 N, net force = 55 N - 50 N = 5 N. Using Newton's second law, a = F/m = 5/25 = 0.2 m s⁻². Using s = ut + ½at² with u = 0, t = 2 s: s = 0 + ½ × 0.2 × 4 = 0.4 m in forward direction.
Q2: Explain Newton's first law of motion and describe how the position-time and velocity-time graphs look for an object at rest and an object moving with constant velocity.
Solution:
Ans: Newton's first law states that an object at rest remains at rest and an object in motion continues moving with constant velocity unless a net force acts upon it. For an object at rest, the position-time graph is a horizontal straight line as position does not change, and the velocity-time graph is horizontal at v = 0. For constant velocity motion, the position-time graph is a straight line inclined to the time axis, and the velocity-time graph is horizontal at a constant non-zero value.
Q3: When a bullet is fired from a gun, both experience equal and opposite forces. Explain why the gun recoils with much smaller acceleration than the bullet using Newton's second and third laws.
Solution:
Ans: According to Newton's third law, when a bullet is fired, the gun exerts a forward force on the bullet and the bullet simultaneously exerts an equal and opposite backward force on the gun. These forces are equal in magnitude. However, according to Newton's second law (a = F/m), acceleration is inversely proportional to mass. Since the gun has much larger mass than the bullet, it experiences much smaller acceleration, causing a small recoil. The bullet has very small mass, so it experiences very large acceleration.
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