Case Study:

On a highway construction site, two trucks are used to transport materials. Truck A is empty, while Truck B is fully loaded with sand. When both drivers try to start their trucks from rest using the same engine force, Truck B takes noticeably longer to begin moving. However, once both trucks are in motion, Truck B continues to roll forward for a longer distance even after the engine is turned off.

To analyze this, engineers recorded that both trucks were pushed with equal force for 10 seconds. Truck A reached a speed of 8 m/s, while Truck B reached only 4 m/s in the same time. After cutting off the engine, Truck A stopped within 20 meters, whereas Truck B continued moving for nearly 50 meters.

This observation raises an important question: Why does the loaded truck resist starting but continues moving for a longer distance once in motion?

Using the concepts of work done and kinetic energy, engineers concluded that although the same force acts on both trucks, the mass of the truck affects how energy is gained and utilized during motion.

This is a classic example of energy in action, but if you're wondering how this compares to energy at rest, take a look at our breakdown of Kinetic vs. Potential energy.

If you’re still getting a grip on the basics, check out our guide on understanding the difference between positive, negative, and zero work to see how force and direction change everything.

CASE-BASED QUESTIONS

MCQ

Q1. Why does Truck B take more time to start moving compared to Truck A?

A. It has less friction
B. It has more mass
C. It has less kinetic energy
D. It has more speed

Q2. Which truck has more kinetic energy while moving?

A. Truck A
B. Truck B
C. Both have equal kinetic energy
D. Cannot be determined

Assertion - Reason

Q3. Assertion (A): Truck B continues moving longer after the engine is turned off.
Reason (R): Truck B has greater kinetic energy due to its larger mass.

A. Both A and R are true, and R is correct explanation of A
B. Both A and R are true, but R is not correct explanation
C. A is true, R is false
D. A is false, R is true

Application-Based Question

Q4. If both trucks are subjected to the same force, why does Truck A achieve a higher speed than Truck B?

Q5. Explain why Truck B travels a longer distance after the engine is switched off.

Data/Logic-Based Question

Q6. Given:

• Truck A speed = 8 m/s
• Truck B speed = 4 m/s
• Truck B has double the mass of Truck A

Which truck has greater kinetic energy? Justify using logic.

ANSWER KEY WITH EXPLANATION

A1. B -  It has more mass
Explanation: Greater mass means more inertia, so more force (or time) is required to change its state of rest.

A2. B - Truck B
Explanation: Kinetic energy depends on mass and velocity. Despite lower speed, higher mass gives Truck B greater kinetic energy.

A3. A - Explanation: Larger mass results in greater kinetic energy, allowing Truck B to continue moving longer.

A4. Truck A has less mass, so for the same force, it accelerates more (Newton’s Second Law), achieving higher speed.

A5. Truck B has greater kinetic energy due to higher mass, so it takes longer for friction to stop it.

A6. Truck B has greater kinetic energy.
Explanation: K.E. ∝ mass × velocity². Even though velocity is lower, doubled mass compensates and increases total kinetic energy.

Remember, energy isn't lost; it's just transformed. You can see more of this principle in our article on the Law of Conservation of Energy with real-life examples. Also, if you’re curious about how engines handle this load efficiently, explore why we say machines save us effort and time.

HOTS EXTENSION QUESTIONS

1. If Truck B had the same speed as Truck A, how would their kinetic energies compare? Explain with reasoning.

2. Suggest one method to make Truck B start faster without reducing its load. Explain using physics concepts.

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