Class 9
Chapter – Force and Laws of Motion
Activity (Inertia and Force)
Case-Based Questions:
- Two children push a motorcar of mass ( \displaystyle 1200\text{ kg} ) with equal effort, moving it at uniform velocity. What does this indicate about the forces involved?
- When a third child joins and the car accelerates, what changes in the forces and motion?
- Which law of motion is demonstrated by this activity?
Assertion–Reason:
Assertion: An object remains at rest or in uniform motion unless a force acts on it.
Reason: This is the law of inertia or Newton’s First Law of Motion.
Options:
- Both assertion and reason are true and reason is the correct explanation.
- Both assertion and reason are true and reason is not the correct explanation.
- Assertion is true but reason is false.
- Assertion is false but reason is true.
Activity (Measuring Force of Friction)
Case-Based Questions:
- A stone of ( \displaystyle 1\text{ kg} ) is rolled on an icy surface and comes to rest after some distance. What force causes it to stop?
- How can you measure this force experimentally?
- How does friction affect the motion of objects?
Assertion–Reason:
Assertion: Friction opposes motion between two surfaces in contact.
Reason: Frictional force acts in the opposite direction to the applied force.
Options: Same options as above.
Activity (Calculating Force and Acceleration)
Case-Based Questions:
- A truck of mass ( \displaystyle 1500\text{ kg} ) slows down from ( \displaystyle 90\text{ km/h} ) to ( \displaystyle 18\text{ km/h} ) in ( \displaystyle 4\text{ seconds} ). Calculate the acceleration.
- What force is needed to cause this acceleration?
- How does mass affect the force required to accelerate an object?
Assertion–Reason:
Assertion: Force applied on an object depends directly on its mass and acceleration.
Reason: This is Newton’s Second Law of Motion.
Options: Same options as above.
Activity (Action and Reaction – Playing Catch on Carts)
Case-Based Questions:
- Two children standing on separate carts throw a sandbag to each other. What happens to the carts?
- Explain the forces acting on the carts when the bag is thrown and caught.
- Which law of motion does this illustrate?
Assertion–Reason:
Assertion: For every action, there is an equal and opposite reaction.
Reason: Action and reaction forces act on different bodies.
Options: Same options as above.
Activity (Change in Momentum)
Case-Based Questions:
- Two objects of given masses collide and stick together. How do you calculate the velocity after impact?
- What principle is used to solve this problem?
- How does mass affect momentum?
Assertion–Reason:
Assertion: Total momentum before and after collision remains constant.
Reason: Momentum is conserved in an isolated system.
Options: Same options as above.
Answers
Activity (Inertia and Force)
Answers:
- The motorcar moves with uniform velocity because the net force on it is zero — the applied force by children equals the frictional force opposing the motion.
- When the third child pushes, the total force increases, resulting in acceleration as per Newton’s Second Law.
- This activity illustrates Newton’s First Law (Law of Inertia) — an object continues in its state unless acted upon by an external force.
Activity (Measuring Force of Friction)
Answers:
- Frictional force between stone and icy surface opposes motion and causes it to stop.
- The force can be measured using a spring balance while pulling the stone on the surface.
- Friction always acts opposite to the direction of motion, slowing moving objects.
Activity (Calculating Force and Acceleration)
Answers:
-
First, convert velocities to ( \displaystyle \text{m/s} ):
- ( \displaystyle 90\text{ km/h} = 25\text{ m/s} )
- ( \displaystyle 18\text{ km/h} = 5\text{ m/s} )
Acceleration:
[ \displaystyle a = \frac{v – u}{t} = \frac{5 – 25}{4} = -5\text{ m/s}^2 ]
(negative indicates deceleration). -
Force required:
[ \displaystyle F = m \times a = 1500 \times (-5) = -7500\text{ N} ] - Greater mass requires more force to produce the same acceleration.
Activity (Action and Reaction – Playing Catch on Carts)
Answers:
- When the bag is thrown, the throwing cart moves backward due to the reaction force; the catching cart moves backward when catching the bag for the same reason.
- The forces acting are equal in magnitude but opposite in direction on the two different carts.
- This illustrates Newton’s Third Law of Motion.
Activity (Change in Momentum)
Answers:
-
Momentum before collision = ( \displaystyle m_1 u_1 + m_2 u_2 ), velocity after collision:
[ \displaystyle v = \frac{m_1 u_1 + m_2 u_2}{m_1 + m_2} ] - Principle used is conservation of momentum.
- Heavier objects (greater mass) have more momentum at the same velocity.
Assertion–Reason Question Answers
- a) Both assertion and reason are true and reason is the correct explanation.
- a) Both assertion and reason are true and reason is the correct explanation.
- d) Assertion is true but reason is false because action and reaction forces act on different bodies, not the same.
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