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Newton's Laws of Motion

Newton's Laws of Motion problem 16

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With the help of Newton's second law, explain the following phenomena:

We are standing in a bus when it suddenly starts. Why can we lose our balance? In which direction can we fall if we face the direction of travel? How do we prevent this?
A weight is hanging on a rope attached to the ceiling of a car. In which direction will it deviate during acceleration and in which direction during braking?
We are sitting in a car. We have a container of water in our hands. We observe the water level. How do we know when a car is accelerating, braking, or driving steadily?
We lower a 15 kg sand container uniformly with a rope. How much force do we feel in our hands? Do we feel more or less force while dropping the container on the ground?
Explain how the seat belt works during a sudden stop. What does the force with which the belt acts on the driver when it stops depend on?
When a plane accelerates along the runway we feel it sticking us to the backrest. Explain the phenomenon.

Show solution

The bus accelerates due to the force of the engine acting on the total mass of the bus. The force is not transmitted to a passenger who is not sitting or standing and holding the pole, so in fact, he is still at rest - he remains in his position. The bus accelerates, which means that the stationary passenger is moving towards the back of the bus relative to the bus. Depending on the direction of travel, he may fall backwards. He prevents a fall by sitting down, holding the pole, or setting his legs apart for better stability.
Since the weight is not rigidly attached to the car, but via a flexible rope, the force that accelerates the car is not transmitted to the weight at the beginning of the acceleration. The weight is stationary because it remains in its position while the car accelerates. Depending on the movement of the car, the weight is deflected backwards.

During braking, the phenomenon is reversed. The car slows down, but the braking force is not transmitted to the weight via the rope at the beginning of braking. The weight wants to maintain its original speed and the bus brakes, so the weight swings forward in the direction of travel.
During acceleration, the water moves towards the back of the container, similar to a standing passenger in an accelerating bus - see part (a) above. The water level is tilted so that the water behind is higher. The reverse is true during braking. In this case, the level is higher in the part of the container facing the direction of travel. The explanation for why this happens is the same as in parts (a) or (b).
If the surface of the water in the container is horizontal, no additional force acts on the water. The movement is steady.
During a steady descent, the resultant of all forces acting on the container is zero. The weight of the container acts downwards and the force of the arm acts upwards. The forces balance each other and the body moves uniformly - see the theory, Newton's First Law of Motion for KS4. We feel the weight of 150 N of the container in our hands. When dropping the container on the ground, we need an additional force that will slow down the container. Because of this extra force, the total force we feel in our hands is greater.
During a sudden stop, the vehicle is subjected to strong braking forces, which cause a negative acceleration (deceleration). The seat belt ensures that the driver stops with the same deceleration. The seat belt transmits the braking force to the driver. The force with which the seat belt acts on the driver is proportional to the driver's mass and deceleration. Without a seat belt, the driver would be pushed towards the steering wheel.
A plane accelerates due to the force of the engine acting on the plane. The passengers in the seats also move with the same acceleration. The force required to accelerate the passengers is transmitted to the passengers via the backrest of the seat.

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