Force and Motion

In physics, we come across various motions of a body. What causes the motion of an object? The answer is a force. What stops an object under motion? The answer is Force. Thus, in general Force and Motion are like two sides of a coin. Example: the interdependence of force and motion is seen in throwing a ball and catching a ball. Let us first understand the meaning of force and motion individually and then the relation between them.

ForceMotionForce and Motion RelationForce And Motion Formula

Force

We can say that force is a push or pull acting on an object or energy as an attribute of physical action or movement. This occurs when two entities are in contact. According to the universal law of gravitation, every object in this universe exerts a force on others. The force acting on an object is given by the following parameter:

• SI unit of Force: newton(N) or kg.m/s²
• Symbol of Force: F
• Type of quantity: Vector quantity
• Dimensional Formula: M¹L¹T^-2
• Other units: dyne, pound-force, kilopond, poundal, kip

For example, if there are two bodies of mass M and m, and they are kept in such a way that the body with mass m is resting over the body with mass M. In physics we say that these two bodies will exert forces on each other. So we can say that whenever there is an interaction of two or more bodies, force is a part of the interaction.

Motion

In simple words, whenever a body is moving, we say it is in motion. For instance, if a boy is walking down the street from his school to his home, we will say that the boy is in motion. But imagine if the boy stops for 5 minutes in the way and sits down. If someone asks at that time if the boy is in motion, the answer will be ‘No’. From this, we can infer that the motion of a body is dependent on time. In physics, we say that motion is the change in the position of a body with respect to time.

Force and Motion Relation

Force and motion are deeply related in nature. We can say that force is the cause of motion. Suppose something is moving, we can say that some force must be acting on it or some force must have acted on it which produced this motion. If a person is walking, and hence in motion, there must be some force acting on it which is making him move. What is this force? This force is the muscular force of his body.

Before we delve into the relationship between force and motion, let us first take a quick look into what is meant by force. Simply put, force is either a push or a pull. Force is defined as “any interaction that, when unopposed, will change the motion of an object”. By the definition itself, it is clear that force affects the state of motion of an object.

But what exactly is does state of motion mean? In physics, motion is defined as the change in position with respect to time. In simpler words, motion refers to the movement of a body. Typically, motion can either be described as

• Change in speed, or
• Change in direction.

Sir Isaac Newton was the first person to discover the relation between motion and force. From his studies in force and laws of motion we can come to the following three conclusions:

• Force can accelerate the body
• Force decelerate the body
• Force can change the direction of the moving body

Force Can Cause a Body To Accelerate

When force is applied to a body in rest, it starts to move, provided that there is no greater force opposing it. When you throw the ball towards the batsman while playing cricket, the ball starts to move forward from its initial position of rest in your hands.

When something begins to move from a state of rest, it accelerates. Acceleration can be described as a positive change in velocity over a period of time.

Similarly, if force is applied in the same direction of movement of a moving object, it accelerates. For example, if you rotate the pedal of a bicycle, it will go faster.

Force Can Cause a Body To Decelerate

On the other side of the spectrum, if force is applied on the opposite direction of a moving object, it will decelerate or slow down and eventually stop altogether if the force is continued to be applied. For example, when a goalkeeper stops a ball, he causes it to decelerate and stop.

Force Can Cause a Body To Change Directions

When force is applied on an object in an angle different to its direction of motion, it causes the object to change motion. Almost every ball game uses this principle. The speed can be maintained if the force is applied in a perpendicular angle but the velocity will change.

Force and Motion Formula

From Newton’s second law of motion, the relationship between Force and motion can be mathematically expressed as:

• Force is defined as the product of mass m and acceleration a and is given as:

• Motion is explained using laws of motion and is defined as the particle’s behaviour when in motion and is given as:

Where,

• s is the position of the particle
• s₀ is the initial position of the particle
• v₀ is the initial velocity of the particle
• a is the acceleration
• t is the time

To wrap up, force and motion are two heavily connected entities. Force can result in the acceleration, deceleration, change in direction or total cessation of motion.

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