Stopping Distance and Terminal Velocity

Stopping distance

• Thinking distance: the distance travelled during the driver's reaction time. Braking distance: the distance travelled while the brakes act.
• On a velocity-time graph of a stopping vehicle, the braking distance is the area of triangle section only.
• Thinking distance is directly proportional to speed; braking distance is proportional to speed squared.

Factors affecting stopping distance

Pressing the brake pedal harder shortens the braking distance.

A driver factor gives an increased reaction time: an increase in thinking distance and no effect on braking distance.

Forces on a falling object

• Downwards: the weight. Upwards: air resistance, the drag force; in a liquid the drag is the liquid's resistance.
• While the object accelerates downwards, draw the weight arrow longer than the air resistance arrow.

Terminal velocity

1. At the start the weight is greater than drag, so the object accelerates.
2. The drag increases as speed increases.
3. The resultant force decreases.
4. The acceleration decreases.
5. When the drag equals the weight, the resultant force is zero.
6. The object falls at terminal velocity, a constant speed.

Opening a parachute

1. The air resistance increases greatly when the parachute is opened.
2. The air resistance is greater than weight.
3. The resultant force is upwards, so the object slows down.
4. As speed decreases, air resistance decreases.
5. The resultant force becomes zero.
6. A constant speed is reached, a lower terminal velocity.