Notes On Force - CBSE Class 9 Science
 If an object does not change its position with respect to time and the surroundings, it is said to be at rest, else it is said to be in motion. Force is that which changes or tries to change the state of rest or of motion of an object by a push or pull. The magnitude of force on an object is given by the product of the mass of the object (m) and its acceleration (a). Mathematically it is expressed by the equation, F = ma. CGS unit of force is dyne and the SI unit is newton (N). The line along which a force acts on an object is called the line of action of the force. The point where the force acts on an object is called the point of application of the force. When a number of forces act simultaneously on an object then their equivalent is the net force on an object. If the net force is zero the forces are said to be balanced which results in zero acceleration, else the forces are said to be unbalanced which results in acceleration of the object. The force that opposes the relative motion between the surfaces of two objects in contact and acts along the surfaces in contact is called the force of friction or simply friction. According to the concepts developed by Galileo Galilei and Isaac Newton, if a body is either at rest or in uniform motion along a straight line path, then it is said to be in its natural state. When the forces acting on an object are balanced, the net force or the resultant force acting on the body is zero. In such cases, the body continues to be in its natural state. If all the forces acting on a body result in an unbalanced force, then the unbalanced force can accelerate the body. It means that a net force or resulting force acting on a body can either change the magnitude of its velocity or change the direction of its velocity. For example, when many forces are known to be acting on a body, and the body is found to be at rest, then we can conclude that the net force acting on the body is zero. Sometimes, balanced forces can cause a change in the shape of a body. The SI unit for force is the “newton,” and its CGS unit is the “dyne”. Gram–centimetre per Second Square is known as “dyne”. One newton is equal to 105 dyne.

#### Summary

 If an object does not change its position with respect to time and the surroundings, it is said to be at rest, else it is said to be in motion. Force is that which changes or tries to change the state of rest or of motion of an object by a push or pull. The magnitude of force on an object is given by the product of the mass of the object (m) and its acceleration (a). Mathematically it is expressed by the equation, F = ma. CGS unit of force is dyne and the SI unit is newton (N). The line along which a force acts on an object is called the line of action of the force. The point where the force acts on an object is called the point of application of the force. When a number of forces act simultaneously on an object then their equivalent is the net force on an object. If the net force is zero the forces are said to be balanced which results in zero acceleration, else the forces are said to be unbalanced which results in acceleration of the object. The force that opposes the relative motion between the surfaces of two objects in contact and acts along the surfaces in contact is called the force of friction or simply friction. According to the concepts developed by Galileo Galilei and Isaac Newton, if a body is either at rest or in uniform motion along a straight line path, then it is said to be in its natural state. When the forces acting on an object are balanced, the net force or the resultant force acting on the body is zero. In such cases, the body continues to be in its natural state. If all the forces acting on a body result in an unbalanced force, then the unbalanced force can accelerate the body. It means that a net force or resulting force acting on a body can either change the magnitude of its velocity or change the direction of its velocity. For example, when many forces are known to be acting on a body, and the body is found to be at rest, then we can conclude that the net force acting on the body is zero. Sometimes, balanced forces can cause a change in the shape of a body. The SI unit for force is the “newton,” and its CGS unit is the “dyne”. Gram–centimetre per Second Square is known as “dyne”. One newton is equal to 105 dyne.

#### Activities

 Activity 1 learningcenter.nsta has developed an interactive online simulation which explains how force accelerates the bodies. This simulation allows to compare the accelerations produced on the ships. Go to Activity Activity 2 Physicsclassroom has developed an interactive online simulation which explains how force accelerates the a wooden block. This simulation plots a Velocity - Time graph that is useful to analise the motion of the block. Using this mass of the block and applied force and surface friction can be changed to relate them with the acceleration caused in the block. Go to Activity Activity 3 amrita.olabs  has developed an interactive online simulation which explains how force accelerates the a wooden block attached to a spring balance. Mass of the block and applied force and roughness of the table can be changed to relate them with the acceleration caused in the block. Go to Activity

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