Linear MotionLinear Motion

There are five SUVAT equations involving motion in a straight line at a constant acceleration.

  1. hspace{10pt}v=u+at
  2. hspace{10pt}s=left(frac{u+v}{2}right)t
  3. hspace{10pt}s=ut+frac{1}{2}at^2
  4. hspace{10pt}s=vt-frac{1}{2}at^2
  5. hspace{10pt}v^2=u^2+2as

When an object is accelerating under gravity with no other force acting on it, it is said to be in free fall. The acceleration of free fall is denoted by g, whose value is 9.81ms-2.

There are various techniques and procedures used to determine the acceleration of free fall:

  1. Electromagnet and trap door: an electromagnet holds a small steel ball over a trapdoor. When the current is switched off, a timer is triggered, the electromagnet demagnetises, and the ball falls. When it hits the trapdoor, the electrical contact is broken and the timer stops. The value of g is calculated from the height of the fall and the time taken.
  2. Light gates: two light gates are attached to a clamp stand, one above the other. These detectors are attached t a timer. When the ball falls through the first beam, it interrupts the light and the timer start. When it falls through the second beam (a known distance further down), the timer stops.
  3. Taking pictures: a metal ball is dropped from rest next to a metre rule, and its fall is recorded on video or with a camera in rapid-fire repeating mode. Alternatively, a stroboscope illuminates the scene with rapid flashes, and the camera shutter is held open to produce a photograph with multiple images of the falling ball.

Car stopping distances:

  1. Thinking distance: the distance travelled between the moment when you first see a reason to stop to the moment when the brake is first applied. Affected by tiredness, intoxication and car speed. The product of speed and reaction time.
  2. Braking distance: the distance travelled from the time the brake is applied to until the vehicle stops. Affected by the conditions of the brakes, tires, and road, weather conditions and car speed.

Stopping distance: the total distance travelled from when the driver first sees a reason to stop to when the vehicle is stationary. The sum of thinking distance and braking distance.