Physics 5054/12 — October/November 2014

Walkthrough

When a coin falls, two forces act on it:

• its weight, acting downwards, which is constant;
• air resistance, acting upwards, which grows as the speed of the coin increases.

Terminal velocity is reached when these two forces balance, so the resultant force is zero and the coin no longer accelerates. For that to happen, the coin must have enough falling distance and time for the air resistance to catch up with the weight.

Because this coin falls only a short distance, it does not gain enough speed for air resistance to become equal to its weight. The weight remains always greater than the air resistance, so there is still a resultant downward force and the coin continues to accelerate right up until it lands.

Option A is irrelevant: hitting the ground does not affect whether terminal velocity is reached. Option B is what would happen if terminal velocity had been reached, not why it has not. Option C is false because weight neither increases nor decreases with air resistance. Only D correctly explains the situation.

Key Takeaways

• Terminal velocity is not about reaching a certain speed; it is about the resultant force becoming zero.
• Air resistance increases with speed, but it takes time and distance for it to grow.
• If the fall is short, the air resistance never becomes equal to the weight, so the object is still accelerating when it lands.

Common Mistakes

• Choosing A: thinking the reason is that the coin has not reached the ground. The ground is not related to terminal velocity.
• Choosing B: describing the condition for terminal velocity, which the coin has not yet met.
• Choosing C: assuming that weight changes as air resistance changes. Weight only depends on mass and gravitational field strength.

Things to Be Careful About

• "Terminal velocity" means zero resultant force, not zero speed.
• Air resistance increases with speed, not directly with distance travelled.
• The weight of an object is constant on Earth unless its mass changes.