Sectieoverzicht

  • Understanding a force as the result of an object’s interaction with another object allows us to classify forces according to two broad categories:

    • Contact forces result from contact interactions, in other words when two objects touch each other.
    • Non-contact forces result from objects interacting with each other at a distance, without direct contact between them. These interactions are due to the fields around objects interacting with each other.

    In the next two activities, you will have an opportunity to explore different types of forces in each category. Being able to correctly identify the types of forces acting on an object will help you to draw force diagrams.

    To identify the types of force acting on an object, always ask how the object is interacting with its surroundings.


    • Leerlingen moeten
      Als voltooid aanduiden

      Activity 1: Investigate non-contact forces

      Time required:

      5 minutes

      What you need:
      • a (non-fragile) object made of wood, plastic or metal
      • two magnets
      • some metal paper clips or drawing pins
      • a plastic ruler
      • a dust cloth
      • small pieces of paper

      What to do:
      1. Hold the wooden, plastic or metal object above the ground. Then release your grip. What do you observe?
        Holding a box
      2. Put the paper clips or drawing pins on a flat surface. What happens when you hold the magnet over them? 
        Magnets above pins
      3. Take the two magnets and bring them towards each other. What do you feel and observe?
        Holding magnets near each other
      4. Rub the ruler vigorously with the dust cloth. Then hold the ruler over the pieces of paper. Describe your observations.
        Rubbing ruler

      What did you find?

      From your observations you will be able to describe examples of non-contact forces.

      Part 1:

      When you released the object you held above the ground, it fell downwards. As you know by now, the motion or shape of an object changes when an unbalanced force acts on it. Therefore, a force must have been acting on the object to make it fall down.

      The force at work here was a gravitational force, often simply called gravity. We can write it as FG. Gravity is a non-contact force that exists between any two objects with mass because they attract each other. This is known as the law of universal gravitation.

      The effect of gravitational force is most notable when one of the objects is much more massive than the other. This is what we experience on Earth: the Earth’s gravitational force is much bigger than other objects on Earth, and so all objects close to the Earth’s surface are attracted downwards, towards the Earth.

      Part 2:

      When you held the magnet above the paper clips or pins, you would have noticed that they were attracted towards the magnet, without direct contact exerting a force.

      Part 3:

      Similarly, when you brought the two magnets towards each other (whether with opposite or like poles facing each other), you would have felt either a pull or a push between them, again without their touching each other. Both these effects are caused by the magnetic force, which results from the items interaction with the magnetic field.

      Part 4:

      When you rubbed the plastic ruler with the dust cloth, the ruler became electrostatically charged. This means it had an excess of one type of charge. When you held the charged ruler above the pieces of paper, they were attracted to the ruler. This is due to the interaction of electric fields between the ruler and the paper, which caused an electric force. The size of the field around the charged ruler was stronger than that around the pieces of paper, and therefore the pieces of paper moved in the direction of the ruler.

      Note
      • If there is a magnet near an object, there will be a magnetic force.
      • If there is a charged object near an object, there will be an electric force.
      • If there is a planet (or other massive body) near an object, there will be a gravitational force. All objects on Earth are therefore acted on by the Earth’s gravitational force.
    • Leerlingen moeten
      Als voltooid aanduiden

      Watch: What are NON-CONTACT Forces? - Part 1

    • Leerlingen moeten
      Als voltooid aanduiden

      Watch: What are NON-CONTACT Forces? - Part 2