What Is Force? Understanding Motion and States with Simple Examples
Force is the cause of change in the state of motion of a body or an object. It is a quantitative description of an interaction that causes a change in an object's motion. Force can cause an object to move or accelerate, to slow down or decelerate, to stop, or to change its direction. The applied force can be a push, a pull, or dragging of an object.
Examples of Force in Everyday Life
Applying brakes to stop a vehicle
Lifting a load
Pushing and pulling a door
Kneading and rolling the dough
Kicking a football
Stretching a spring or a rubber band
Attracting paper bits with an electrostatically charged comb
Force exerted by our muscles while moving the limbs
Throwing a stone in the air and it’s coming down.
A magnet attracting iron nails
The State of Motion and Causes of Change in Motion
Motion is the change in position of a body with respect to its surrounding environment, within a given interval of time. An object is said to be in motion if its position changes with time, with reference to a fixed frame.
A chair cannot move its own, what do you do to make a chair move?
We often say that a force has been applied to the chair when it is pushed.
The motion of an object is explained by its speed and direction of motion. If an object is at rest, the state is considered to be in the state of zero. By applying force, we can change the position of the object or can say that the object is in motion.
While taking a penalty kick in football, before being hit, the ball was at rest. Then, its speed was zero. The player applied force on the ball. This applied force sets the ball in motion towards the goal.
Suppose, the ball hits the goal or the goalkeeper dives and saves the goal. In both conditions, the speed of the ball changes. Force can also cause an increase or decrease in motion if it is applied in the same direction or the opposite direction respectively.
Many times, an applied force may not result in any change in the state of motion.
What will happen when you push a wall with the maximum force that you can exert?
No effect of force is observed.
How can Force Change the State of Motion?
1. The Applied Force can Cause Acceleration
The change in motion is equivalent to a change in velocity. A change in velocity applies that there will be an acceleration. The force causes a change in motion. So it produces acceleration too.
If an object is stationary in the beginning, it accelerates when it starts to move. Likewise, if an object is already moving and a force is applied in the same direction, the object will accelerate as long as the force is applied to the object. If the force is removed, the acceleration will also stop.
For example, James was walking towards the north at a speed of 10 metres per second. James speeds up and now begins running towards the west at 20 metres per second after 5 seconds. In this case, James has accelerated his velocity by 2 m/s2 i.e his velocity has increased by 2m/s every second.
Another example is of an apple falling down. It starts falling at zero metres per second. At the end of the first second, the apple is moving at 9.8 metres per second. The apple has accelerated. This acceleration here is caused by gravity.
2. The Applied Force can Cause Deceleration
If an object is moving and a force applied to it in the opposite direction of the motion, the object will decelerate or slow down.
Suppose, a cricketer hits the ball high up. It will slow down as it travels upwards due to the force of gravity. Likewise, a boat decelerates due to wind flowing opposite to the direction of motion of the boat.
Decelerating force can put a moving body to rest.
For example, when a car driver applies brakes, it begins to decelerate.
3. Force can Cause a Change in the Direction of Motion
A change in either the speed of a moving body or its direction or both are referred to as a change in its state of motion. Thus, the force can change the direction of motion.
For example- In a cricket match, a bowler bowls the ball towards the batsman with some velocity(u). The batsman hits the ball and it travels in a different path with another velocity(v). This is because the batsman applies force on the ball and changes the direction of the ball.
Newton’s Laws of Motion
Newton the great physicist gave laws of motion that are useful in everyday lives. The three laws of motion are described below.
1. Law 1:- The first law of motion tells that an object will remain at the state of rest or continue to move at a certain speed unless an external force is applied on the object, which will violate the equilibrium of the system. The first law of motion by Newton is also called the “law of inertia” and it explains the concept of inertia, application of the force, and inertial frame of reference.
2. Law 2:- Newton’s second law is the quantitative description of the changes that force may induce in the object on which the force is applied.
This entails that when a force is applied on a given object of constant mass, the rate of change of the speed of the object will be directly proportional to the total force applied on the object. In simpler words, the acceleration produced in the object due to the application of the force will be directly proportional to the force applied.
i.e. Force applied ∝ acceleration produced
F ∝ a F = Ka and K here is the mass so
F = ma or
Force applied = (mass of the object) (Acceleration produced in the object)
3. Law 3:- The third law of Newton states that for every action or force, there will be equal but opposite reactions or force. If you push the wall, the wall does not move as it exerts an equal and opposite reaction force. This law is commonly called the law of action and reaction in physics.
Example:- If a book is lying on a table, which can be interpreted as the book is applying force equal to its weight on the table, according to this law, the table applies an equal and opposite force on the book which will counter the force of weight applied by the table.
FAQs on Force, Motion, and State of Motion: Physics Concepts & Examples
1. What is the fundamental definition of force in physics?
In physics, a force is defined as a push or a pull upon an object resulting from its interaction with another object. A force is a vector quantity, meaning it has both magnitude and direction. It has the ability to change an object's state of motion (i.e., start moving, stop moving, or change direction) or change its shape. The SI unit of force is the Newton (N).
2. How is the 'state of motion' of an object described?
An object's state of motion is described by its velocity—that is, its speed and its direction of movement. An object is considered to be in a state of rest (zero velocity) or a state of uniform motion (constant velocity). Any change in this state, such as a change in speed or direction, indicates that the object is accelerating, which is caused by a net external force.
3. How does an unbalanced force affect an object's state of motion?
An unbalanced force is a net force that is not zero, causing a change in an object's state of motion. Specifically, it causes the object to accelerate. This means the object will either:
- Start moving from a state of rest.
- Speed up if the force is in the direction of motion.
- Slow down if the force is opposite to the direction of motion.
- Change its direction of movement.
For example, kicking a stationary football applies an unbalanced force, causing it to move and accelerate.
4. What is the relationship between force, mass, and acceleration as described by Newton's Second Law?
Newton's Second Law of Motion describes a direct relationship between force, mass, and acceleration. It states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. This is mathematically expressed by the formula F = m × a, where 'F' is the net force, 'm' is the mass of the object, and 'a' is the acceleration produced.
5. Could you provide some real-world examples of different types of forces?
Certainly. Forces are acting all around us in daily life. Some common examples include:
- Gravitational Force: The force that pulls objects towards the Earth's center, giving them weight.
- Frictional Force: The force that opposes motion between two surfaces in contact, like a car's brakes slowing it down.
- Normal Force: The support force exerted by a surface on an object resting on it, like a book on a table.
- Tensional Force: The force transmitted through a string, rope, or cable when it is pulled taut.
- Applied Force: A force that is directly applied to an object, such as pushing a door open.
6. What is the difference between balanced and unbalanced forces?
The key difference lies in their effect on an object's motion. Balanced forces are two or more forces of equal magnitude acting in opposite directions on an object. They cancel each other out, resulting in a net force of zero and no change in the object's state of motion. In contrast, unbalanced forces have a non-zero net force, which causes a change in the object's motion, i.e., acceleration.
7. Can an object be in motion even if no net force is acting on it?
Yes, an object can be in motion without a net force acting on it. According to Newton's First Law of Motion (the Law of Inertia), an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This is known as uniform motion. For example, a satellite in the vacuum of space continues to move at a constant velocity because there is no significant external force to change its state of motion.
8. If action and reaction forces are always equal and opposite, why don't they cancel each other out?
This is a common point of confusion. While action and reaction forces (as per Newton's Third Law) are indeed equal and opposite, they do not cancel each other out because they act on different objects. For cancellation, forces must act on the same object. For example, when you push a wall (action), the wall pushes back on your hand (reaction). The action force acts on the wall, while the reaction force acts on your hand. Since they act on two separate bodies, they cannot cancel out.
9. What are some key types of motion, with examples?
Motion describes a change in an object's position over time. The main types of motion studied in physics are:
- Translational Motion: The object moves from one point to another without rotating. This can be rectilinear (a car moving on a straight road) or curvilinear (a thrown ball).
- Rotational Motion: The object spins around a fixed axis. Examples include a spinning top or the blades of a fan.
- Oscillatory Motion: The object moves back and forth repeatedly about a central position. Examples include a swinging pendulum or a vibrating guitar string.
