force

When we push, pull, kick, lift, throw, flick, hit, pick, squeeze, press, inflate, open, and close an object we say a force is applied to the object. These actions are nothing but the application of force. Whatever may be the method of application of the force, they are of only two types –

• Push
• Pull

An external agent that produces motion in a body or alters the existing state of the motion in a body is called a force.

Examples of push

• We push the door to shut or open it
• We push a trolley in a store
• We push a box across the room

Examples of pull

• We lift a bucket of water
• We pull the refrigerator door to open it
• We pull a chair to sit

We use force to walk, lift any object, throw anything, move an object from its place, etc. In short, force is there in every activity we do. By applying force we usually bring anything in motion or in rest position but not always. For example, if we apply force on a wall it does not move.

The point on the body where a force acts is called the point of application of force.

A line drawn through the point of application of the force in the direction of the force is called the line of action of a force.

Effects of force

Force has many effects on the objects it is applied to. Force can be judged by the various effects it can produce on the object it is applied to.

1. The force causes motion – Force can bring a stationary object into motion provided there is no other force preventing the motion. This means that when an adequate amount of force is applied to a non-moving object then the object starts moving in the direction of the force. A change in an object’s position is called motion. For example, when we push any toy car, it moves, or a box lying on the floor can be moved by pushing it that is by applying force to it.  So, when force is applied, a stationary object comes into motion or force changes the position of a stationary object.
2. Force changes the speed – The speed of a moving body can be changed by applying force to it – by putting force on the accelerator the speed of a moving car can be increased and by applying brakes the speed can either be decreased or eventually even stop the moving car. When we apply force in the same direction as that of motion, speed increases. When we apply force in the opposite direction as that of motion, speed decreases. 
3. Force stop motion – This means when we apply force in the opposite direction of the motion, it can put a moving object in a state of rest. For example, a car in motion can be stopped by applying brakes to it. When we try to hold the ball thrown to us with force greater than the force with which it is coming, it stops. 
4. Force changes the direction – When a force is applied at an angle to a moving object, it changes the direction of the moving object. For example, in a game of tennis, when a player hits the ball back to the player on the opposite side, the force applied to the ball changes its direction. A moving car changes its direction when force is applied to its steering wheel so as to turn it. In a game of football, players change the direction of moving football by hitting the ball with their foot at an angle.
5. Force changes the shape – When force is applied to an object its shape and size change. For example, when an inflated balloon is pressed, a force applied changes its shape. When a solid block of rock is hammered, the force applied by the hammer changes its shape to make a statue. When we squeeze a plastic water bottle, a force applied changes its shape and size.

 

Characteristics of Force

• Force is due to an interaction of at least two objects
• It may change the state of motion of an object
• It may change the shape of an object
• Forces applied to an object in the same direction add to one another and resultant is the same direction
• When forces are applied to an object in the opposite direction then their resultant or net force is the difference between these opposing forces and its resulting direction is the same as that of the larger force.
•  If the two forces acting on an object are equal in magnitude but opposite in direction, then the net force acting on the body is zero.
• It is a vector quantity hence it should be specified by giving its magnitude and the direction.
• If the magnitude or the direction or both changes, then the effect of force also changes.
   

Types of forces:

• Balanced versus Unbalanced forces
• Forces acting on an object can be balanced or unbalanced.
• Balanced forces
   

Balanced forces are the ones where the result of applied forces is equal to zero. They do not cause any change in the state of the object it is applied upon i.e. the object in which force is applied the state doesn’t change from motion to rest or vice versa, however, balanced forces can change the shape and size of an object. Balanced forces are equal in magnitude but opposite in directions. Balanced forces are considered to be in a state of equilibrium.

For example, in arm wrestling where there is no winner, the force exerted by each person is equal, but they are pushing in the opposite direction. The resulting force (net force) is zero. Or, in a tug of war, if there is no movement in the rope, the two teams are exerting equal but opposite forces that are balanced. Again, the resulting force (net force) is zero.

When forces are balanced there is no change in direction.

• Combined forces that are balanced are always equal to zero.
• Balanced forces cannot change the motion or direction of an object.
• A balanced force keeps an object moving at a constant velocity

A book on the table is an example of a balanced force. The force of the book’s weight is counteracted by the normal force (support force) of the table. The two forces are completely equal and opposite.

An example of a balanced force that keeps an object moving at a constant velocity is the cruise control on a car which attempts to equal the forces of friction with a forward force. Once constant velocity is obtained the two sets of forces are completely equal and opposite.

 

Unbalanced forces

Unlike balanced forces, unbalanced forces are the ones where the resultant applied force is greater than zero. The forces acting on the object are not equal and they always cause the motion of an object to change the speed and/or direction that it is moving in.

When two unbalanced forces are exerted in opposite directions, their combined force is equal to the difference between the two forces. The magnitude and direction of the net force affect the resulting motion. This combined force is exerted in the direction of the larger force. For example, if in a tug of war, one team pulls harder than the other, the resulting action (net force) will be that the rope will change its motion in the direction of the force with the greater strength/magnitude.

When unbalanced forces are exerted in the same direction, the resulting force (net force) will be the sum of the forces in the direction the forces are applied. For example, if two people pull on an object at the same time in the same direction, the applied force on the object will be the result of their combined forces.

When forces act in the same direction, their forces are added. When forces act in opposite directions, their forces are subtracted from each other.

Unbalanced forces also cause a nonmoving object to change its motion

If there is no net force acting on the object, the motion does not change. If there is a net force acting on an object, the speed of the object will change in the direction of the net force.
 

Contact versus Non-contact forces

Depending upon the interaction between a force and an object, forces are classified as contact and non-contact forces.

Contact forces: A force that can be applied only when it is in contact with an object is called a contact force. All mechanical forces are contact forces e.g. muscular force, and frictional force. 

Types of contact forces:

• Applied force – Force due to the action of muscles is called applied force. It is also called muscular force. We apply muscular force during the basic day to day work of our life such as breathing, digestion, lifting a bucket, pulling or pushing some object.
• Normal force – It is a contact interaction force between surfaces. It always acts perpendicular to the surfaces and out of the surface.
• Frictional force – When two objects slide over each other they rub and push against each other. This pushing force is called frictional force. For example, lighting a matchstick or stopping a moving ball comes under frictional force.
• Tension force – An applied force where the force is applied through a string, cable, rope, etc. A tension force can only pull, it cannot push. We usually assume that the tension in a cable is the same everywhere on the cable.
• Air resistance force – This is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. The force will usually be neglected due to its negligible magnitude. It is most noticeable for objects that travel at high speeds (e.g. a skydiver or a downhill skier).  
• Spring force - Force exerted by a compressed or stretched spring is ‘spring force’. The force created could be a push or pull depending on how the spring is attached.

Non-contact forces: A force that can be applied without any contact with two bodies is called a non-contact force e.g. magnetic force, electrostatic force, gravitational force. 

Types of non-contact forces

• A gravitational force is a force that tries to pull two objects toward each other. Anything which has mass also has a gravitational pull. The more massive an object is, the stronger its gravitational pull is. Earth’s gravity is what keeps you on the ground and what causes objects to fall. Water droplets falling down from the sky is an example of gravitational force. Apple falling down from the tree is the best and most popular example of a non-contact force.
• The magnetic force is the force exerted by a magnet on magnetic objects. They exist without any contact between two objects. For example, Iron pins are attracted when in the vicinity of a magnet without any physical contact.
• The electrostatic force is the force exerted by all electrically charged bodies on other charged bodies. It can be both attractive and repulsive in nature based on the charge of the body. For example, the charging of the hair and attraction of paper bits towards it.
• Nuclear force is the force that holds atoms and their particles together.

Force field

• Region or space in which non-contact forces such as magnetic force, gravitational force, and electrostatic force act is called a force field.
• The region surrounding a magnet, where a magnetic substance experiences force is called the magnetic field.
• The region surrounding an electric charge, where an electric substance experiences force is called the electric field.
• Thus a force field is a sphere of influence of non-contact forces.

Force = Mass × Acceleration

Acceleration - The change in the speed of an object is called acceleration. When an object gains speed, its acceleration is positive; when speed is lost, acceleration is negative.

Mass - Every object is made up of matter. The more matter an object has, the bigger it is, and the more mass it has.

According to Newton’s second law of motion, Push on an object of a certain mass, and it accelerates based on the amount of force and mass. A small force with a large mass results in a slow acceleration and a large force with a small mass gives a fast acceleration. This means a force of zero on any mass gives zero acceleration. If the object is standing still, it remains still; if it’s moving, it continues to move at the same speed and direction.