Case Study 1: Pushing a Trolley
A student pushes an empty shopping trolley with
a certain force. The trolley starts moving and its
speed increases. When the same force is applied
to a loaded trolley, the trolley accelerates more
slowly. When the student stops pushing, the trolley
gradually comes to rest due to friction.
Questions:
1. Why does the trolley start moving when the student
pushes it?
2. What happens to the acceleration when the mass of the
trolley is increased while the applied force remains the same?
3. Which force brings the trolley to rest after the student
stops pushing?
4. According to Newton’s second law, write the relation
between force, mass and acceleration.
Answers:
1. An unbalanced force acts on the trolley.
2. Acceleration decreases.
3. Frictional force.
4. F = ma
Case Study 2: Football Game
During a football match, a stationary football is kicked
by a player. The ball starts moving rapidly. Another player
stops the moving ball by applying force with his foot.
Sometimes, a player changes the direction of a moving
ball by kicking it from the side.
Questions:
1. What effect of force is demonstrated when the stationary ball starts moving?
2. What happens to the motion of the ball when the second player stops it?
3. How can force change the direction of a moving object?
4. Is force required to change the velocity of an object?
Answers:
1. Force changes the state of rest of the ball.
2. Its velocity decreases and becomes zero.
3. By applying force in a direction different from its existing motion.
4. Yes. A change in velocity requires an unbalanced force.
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Case Study 3: Bus Starting Suddenly
A passenger is standing inside a bus. When the driver suddenly
starts the bus, the passenger tends to fall backwards. When the
moving bus suddenly applies brakes, the passenger tends to move forward.
Questions:
1. Why does the passenger fall backwards when the bus starts suddenly?
2. Why does the passenger move forward when the moving bus stops suddenly?
3. Which property of matter is responsible for this behaviour?
4. State the law of motion associated with this phenomenon.
Answers:
1. Due to inertia, the passenger's body tends to remain at rest while the bus moves forward.
2. Due to inertia, the body tends to continue its state of motion.
3. Inertia.
4. Newton’s First Law of Motion.
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Case Study 4: Cricket Ball
A cricket ball of mass 0.2 kg is initially at rest. A batsman
applies a force of 10 N on the ball for a short time. The force makes the ball move forward.
Questions:
1. Calculate the acceleration produced in the ball.
2. What would happen to the acceleration if the same force were applied to a ball of greater mass?
3. What type of force changes the state of motion of the ball?
4. Which Newton’s law gives the mathematical relationship between force, mass and acceleration?
Answers:
1. Using F = ma
\[
a=\frac{F}{m}=\frac{10}{0.2}=50\,m/s^2
\]
2. Acceleration would decrease.
3. Unbalanced force.
4. Newton’s Second Law of Motion.
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Case Study 5: Rocket Launch
A rocket is launched from the Earth. Hot gases are expelled downward at very high speed. As a result, the rocket moves upward. The upward motion of the rocket occurs even though there is no solid surface for the rocket to push against.
Questions:
1. Which Newton’s law explains the upward motion of the rocket?
2. What is the action force in this situation?
3. What is the reaction force?
4. Are the action and reaction forces equal in magnitude?
5. Do action and reaction forces act on the same object?
Answers:
1. Newton’s Third Law of Motion.
2. The rocket exerts a force on
the gases downward.
3. The gases exert an upward force on the rocket.
4. Yes, they are equal in magnitude and opposite in direction.
5. No. They act on two different objects.
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