Passengers sitting in a bus observe that they move forward when the bus slows down to a stop and that they move backward when it accelerates from rest - NSC Technical Sciences - Question 2 - 2023 - Paper 1

The physics law applied is Newton's First Law of Motion: "An object will remain at rest, or in uniform motion in a straight line unless acted upon by a net external force."

The free-body diagram should include:

• The weight of the car (W) acting downwards, which equals its mass multiplied by gravitational acceleration (W = mg).
• The normal force (N) acting upwards, balancing the weight.
• The tension in the rope (T) acting towards the caravan, pulling the car forward.
• The applied force (F_A) which acts in the direction of the car's movement.

To find the acceleration of the system, we apply Newton's second law:

$F_{net} = m imes a$

First, we can express the net force as the total force minus the force of tension:

For the entire system (car + caravan):

• Total mass = 900 kg + 1300 kg = 2200 kg
• Applied force = 10 500 N

$F_{net} = 10 500 N - T$

From previous calculations: $a = \frac{F_{net}}{m} \Rightarrow a = \frac{10 500 N}{2200 kg} \approx 4.77 \, m/s^2$

Using the acceleration calculated earlier: $T = F_{net} - m_{car} \times a$

For the car (mass = 1300 kg): $T = 10 500 N - (1300 kg) \times (4.77 \, m/s^2)$ $T = 10 500 N - 6201 N \approx 4299 \, N$

To find the resultant force on the falling elevator, we calculate the forces acting on it:

• Weight = mass × gravity = 1600 kg × 9.8 m/s² = 15680 N (downward)
• Retarding force = 3700 N (upward)

Resultant force: $F_{net} = Weight - Retarding \, Force$ $F_{net} = 15680 N - 3700 N = 11980 N \text{ (downward)}$

Using Newton's second law: $F_{net} = m \times a$

Rearranging for acceleration gives us: $a = \frac{F_{net}}{m}$

Where: m = 1600 kg and $F_{net} = 11980 N$. Thus, $a = \frac{11980 N}{1600 kg} = 7.49 \, m/s^2 \text{ (downward)}$

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object exerts an equal force in the opposite direction on the first.

When the apple falls to the ground, the action force is the weight of the apple acting downwards due to gravity. The reaction force is the ground exerting an equal and opposite force upwards on the apple.