# Understanding the Force in Physics

A man was driving a car in a street. He pushed the brake right in front of his house. Turned off the machine and pulled the hand brake. He then opened the car door, took his bag and got down. He slammed the car’s door to close it. He grabbed the key from his pocket. He opened the door and entered to take a rest.

The story above tells us that we frequently push or pull something. When we push or pull something we doing force on it. We apply force to move a box from one place to another. The force resulted from the engine combustion is used to move the car. The force from chemical source is used by the horse to pull the cart. And many other examples of force in our daily live.

Force is defined as the change of momentum of an object per unit time. Momentum is the product of mass and velocity. When momentum of an object changes, its velocity or its mass changes. Sometimes both of them are change, for example in a rocket launching. The total mass of the rocket is gradually reduced due to the fuel combustion in thrusting the rocket.

In a constant mass system, the force applied will change the velocity. The force causes acceleration or deceleration. Sometimes force is defined as the product of mass and acceleration. Force and acceleration are vector quantities. They have direction in addition to their magnitude. The acceleration resulted by the force is in the direction of the force itself. The object’s velocity is increased if the net force is in the direction of the movement. The object undergoes acceleration. On the contrary, if the net force is in the opposite direction with the movement, the object’s velocity is decreased. This time the object undergoes deceleration.

Sometimes, when we push a heavy object, the object will not move. What’s wrong? The resultant force applied to the object is zero. The force from our hand is opposed by the frictional force. Unless the pushing force greater than this friction force, the object will not move. It is the friction force that prevents an object from moving. This is called static friction force.

It is easier to push a box using wheels than without them. Using wheels reduces the friction when moving. A friction force also exists when an object is moving. This is called kinetic friction force. Kinetic friction force is less than static friction force.

Friction force depends on two factors. The first is the normal force. The second is the roughness of the contact surfaces. We called the degree of roughness as coefficient of the friction force. The normal force is the force at the interface of the objects in contact. The heavier object gives greater normal force.

Sometimes friction forces disadvantage us. They can wear the materials in contact out. Frictions reduce the system’s performances. We have to replace our car brakes after a certain period. We find our shoes wear out. In machinery, friction reduces the efficiency and need to be minimized.

Nevertheless, friction still gives us benefits. Do you know why it is difficult to walk on a slippery surface? The friction force is very small in a slippery surface. Swinging our left foot forward, we push our right foot backward. The pushing force from our right foot will pull our body backward. The backward force prevents us to move forward. Fortunately, the frictions from the shoes eliminate this force. We have a forward net force from our left foot to move.

I gladly give you another example. It is hard driving car on a muddy road. Its slippery surface gives only small friction force. When the engine rotates the wheels, they push the road backward. The road surface reacts by pulling the wheels forward. This forward force pulls the car forward. In a muddy road, the small friction force prevents the car movement.

A kind of force makes an object moves in a circular motion. This force keeps the object moves curvedly. We name it centripetal force. This force’s direction is to the centre of the path. Without this force, the object will not be bent to a circular path.

The centripetal force keeps the moon orbiting the earth. The force comes from the attractive force between them. According to the Newton’s law of gravitation, two bodies have mutual attractive forces.

When we drive our car around a curved road, we experiences centripetal force. The centripetal force keeps our car in curved path. This centripetal force comes from the friction between the road and the wheels.

Only attractive force exists between two masses. Both attractive and repulsive forces can be found in magnetism as well as in electrostatic. Two alike magnetic poles repel each other. Two unlike magnetic poles attract each other. Two homogenous charges repel each other. Two opposite charges repel each other.

We benefit the magnetism repulsive force in machinery. This repulsive force rotates the moving part. The moving part provides energy that can be used. The attractive and repulsive forces in the electrostatic produce electromagnetic waves. Electromagnetic wave is useful for communication.

Buoyant force in the water makes vessel floats. This is an upward force exist in gases or liquids. The force is first realized by Archimedes. Buoyant force is caused by the difference pressure of fluid at the different depth. Buoyant force in the air makes balloon floats. Balloon was an attractive transportation utility in its time.

The force is important in our life. There are many of our activities that are involved forces. Understanding the concept of force can make us wiser to take the advantage of it.