Force and Its Types

Force is simply defined as a push or a pull felt by a body or done by a body to another body. This body could be an object, or a person, or a machine. This means, if the object is moving, force can make it stop or change the direction of its motion. If a force is applied to an object in a rest position, it can make the object move. Force may also reshape an object as in crashing a can, or bending a stick. Thus, in this case, a force can also be defined as something or a quantity which causes a change in the state of motion of an object.

The following terms are often used in the discussion of force.

Net Force – if one or more force is acting on an object, the sum of all these forces is called net force.

Unbalanced Force – the amount of two or more forces acting on the object are not equal. When forces are unbalanced, net force is not equal to zero.

Balanced Force – the amount of two or more forces acting on the object are equal. When forces are balanced, the net force is zero.

Force has also two types – contact and non-contact.

Contact Forces are forces which causes change on the state of motion of an object if it has physical contact with it. Meaning, the forces must physically pull or push something in order to exert its force and causes motion or rest to an object. Contact forces includes:

  • Applied force \((F_A)\) – force applied by to an object as in pushing the door and pulling the window. The direction of the motion is in the direction of the applied force.
  • Frictional force \((F_f)\) – the force felt by an object which is in motion against another object as in the force of the road to the wheels of the car.
  • Normal force \((F_N)\) – a force applied to a stable object which is placed to another stable object as in bag on the chair and vase on the table.
  • Air resistance \((F_a)\) – is the force felt when moving against the air.
  • Tensional force \((F_T)\) – force done on pulled strings, ropes and wires.
  • Spring force \((F_S)\) – force done in stretching or compressing a spring.

These forces require physical contact with the object for its force to be done.

 Non-contact Forces are forces which act at a distance from the object. The force is exerted even of there’s no contact with the force and the object.

  • Electrical force \((F_E)\) – forces found in electric charges.
  • Magnetic force \((F_M)\) – force exerted by magnets.
  • Gravitational force \((Fg)\) – the force exerted by gravity or the pull of gravity to the object towards itself. Gravitational force is responsible for letting us stay in our position right now. We cannot see gravity physically pulling our feet to stay on Earth but the fact that we are not flying anywhere, everywhere is a proof that there is gravitational force.

Illustrating forces acting on an object is a bit difficult. Thus, representations are made to make this easier. The representation of forces is called Free-Body Diagram. The diagram contains arrows which represents the forces and boxes representing the object to which the force is being exerted.


Here are some situations where forces are applied. We use FBD to identify the forces acting on the every object.

Example 1

Consider a flower vase on top of the table. What makes the vase stay on the table without moving or falling?

 

 

The forces responsible for the position of the vase into the table are Normal force and Gravitational Force.


Example 2

A boy is playing his toy car. What forces are acting on the car as it run against the floor? What causes it to stop assuming the boy didn’t force it to stop.

 

 

The boy applied a force so the car will start to run. Since frictional force is applied to the wheels of the car which is equal but in opposite direction to the applied force, the car will eventually come to a stop.


Example 3

Iman loves skydiving. As he jumped out of the aircraft, he felt the air as if crashing his face. Why is he falling down? Why not go up?What are the forces responsible for these experiences?

 

 

The force Iman felt on his face is the air resistance. As he goes fown, the air also exerted force upward. His motion is downward and not upward because of the gravitational force pulling him towards the Earth.


Example 4

The clothes are hanged on a rope outside the house to make them dry. What forces are acting on the clothes? Why does the rope get sagged if the clothes got stuck at the center of the rope?

 

 

The tensional force is the force existing on the pulled rope. The rope got sagged because of the gravitational force which pulls the clothes down.


Example 5.

Which of the following diagrams could be at rest or could be moving at constant speed?

a. b.

Answer:

Image a. has a net force of \(F_{net}=20\;N-15\;N=5\;N\) which means that forces are unbalanced thus object a might be in motion and is accelerating.

The sum of the forces acting on image b. is \(F_{net}=6\;N+9\;N-10\;N-5\;N=0\) meaning the forces are balanced. Therefore, object b may be at rest or moving in constant speed.