Ultrasonic sensors are distance sensors that use sound waves to detect how far away an object is. They send out high frequency bursts of sound and listen for its echo. They then determine how far away the object is based on how long it takes for the sound to return to the sensor. This variety requires an Arduino library to operate.

NewPing Library

#include <NewPing.h>

NewPing mysensor(5, 6, 200);

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pingTime = mysensor.ping();

  int distance = mysensor.ping_in();

  int distance_cm = mysensor.ping_cm();

  Serial.println(distance);
}

Force sensors are sensors that change resistance based on how much force is applied on it. The more force applied, the lower the resistance. Force can be measured using the analog pins on the Arduino.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int val = analogRead(A0);

  Serial.println(val);
}