Page number 128, 129 and 130.

In physics, force is an external agent that can cause an object with mass to change its velocity. While you can't directly see a force, you can observe its effects, such as the change in an object's motion or its shape. For example, you can't see the force of gravity, but you can see the effect it has on a falling object. 

Here's a more detailed breakdown:

• Force as an Influence:

  A force is an interaction that can cause an object's velocity to change, unless it's counteracted by other forces. 

  
  

• Forces are Vector Quantities:

  Both the magnitude (how strong the force is) and the direction of the force are important. 

  
  

• Examples of Forces:

  • Contact Forces: These require direct physical contact between objects (e.g., pushing a door, friction, tension in a rope). 

  • Non-Contact Forces: These can act over a distance (e.g., gravity, magnetism, electrical forces). 

    
    

• Seeing the Effects of Force:

  You can observe the effects of a force by:

  • Change in Motion: Seeing an object start, stop, or change direction. 

  • Change in Shape: Stretching a rubber band, compressing a spring, etc. 

  • Change in Momentum: The momentum of an object (its mass multiplied by its velocity) changes when a force is applied.