Introduction to Force
A force is an effort that changes the state of an object at rest or at motion. It can change an object’s direction and velocity. Force can also change the shape of an object.
Effects of Force
Some effects of force include the following:
- Force moves stationary objects
- Force stops objects from moving
- Force changes the shape of a body
- Force changes the direction of motion
Push is defined as an action of force that causes an object to move from its place. The following are examples of push:
- Opening and closing the door.
- Pushing the table.
- Pushing a car.
- Pushing off the thumb pins.
- Walking
Pull is defined as an action to make move by either tugging or dragging. The following are examples of pull:
- Plucking the string of a guitar.
- Pulling ropes while playing tug of war.
- Opening the drawer.
- Pulling the window curtain.
- Opening and closing of the doors.
What is Force?
The push or pull of an object is considered a force. Push and pull come from the objects interacting with one another. Terms like stretch and squeeze can also be used to denote force.
In Physics, force is defined as:
The push or pull on an object with mass causes it to change its velocity
Force is an external agent capable of changing the state of rest or motion of a particular body. It has a magnitude and a direction. The direction towards which the force is applied is known as the direction of the force and the application of force is the point where force is applied.
The Force can be measured using a spring balance. The SI unit of force is Newton(N).
Common symbols: | F→, F |
SI unit: | Newton |
In SI base units: | kg·m/s2 |
Other units: | dyne, poundal, pound-force, kip, kilo pond |
Derivations from other quantities: | F = m a |
Dimension: | LMT-2 |
What are the Effects of Force?
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
- Change in direction
The Force has different effects and here are some of them.
- Force can make a body that is at rest to move.
- It can stop a moving body or slow it down.
- It can accelerate the speed of a moving body.
- It can also change the direction of a moving body along with its shape and size.
Types of Force
Force is a physical cause that can change the state of motion or the dimensions of an object. There are two types of forces based on their applications:
- Contact Force
- Non-Contact Force
Contact Force
Forces that act on a body either directly or through a medium are called contact forces.
Examples of contact forces are:
- Muscular Force
- Mechanical Force
- Frictional Force
We can make use of the muscular force of animals like bullocks, horses, and camels to get the activities done. The frictional force is another type of contact force, which acts between a pair of a surface in contact and tends to oppose the motion of one surface over the other.
Non-Contact Force
Forces that act through spaces without making direct contact with the body are called non-contact forces.
Examples of non-contact forces are:
- Gravitational Force
- Electrostatic Force
- Magnetic Force
The force exerted by a magnet on other magnets is called magnetic force. Magnetic force and electrostatic force act on an object from a distance, that’s the reason they are non-contact forces. The strength of gravity is an attractive force that is exerted by the Earth on objects, which make them fall to the land. The weight of a body is the force that is pulled by the earth towards the center.
What is the Line of Action of a Force?
The line along which a force is acting on an object is called the line of action of the force. The point where the force is acting on an object is called the point of application of the force. The force which opposes the relative motion between the surfaces of two objects in contact and acts along the surfaces is called the force of friction.
Galileo experimentally proved that objects that are in motion move with constant speed when there is no force acting on it. He could note that when a sphere is rolling down an inclined plane, its speed increases because of the gravitational pull which is acting on it.
When all the forces acting on an object are balanced, the net force acting is zero. But, if all the forces acting on a body result in an unbalanced force, then the unbalanced force can accelerate the body, which means that a net force acting on a body can either change the magnitude of its velocity or change the direction of its velocity. For example, when many forces act on a body, and the body is found to be at rest, we can conclude that the net force acting on the body is zero.
Balanced and Unbalanced Forces
When balanced forces are applied to an object, there will be no net effective force acting on the object. Balanced forces do not cause a change in motion.
Unbalanced forces acting on an object change its speed and/or direction of motion. It moves in the direction of the force with the highest magnitude.
Net force
When multiple forces act on a body, they can be resolved into one component known as the net force acting on the object. The net force decides the direction of motion.
To know more about Types of Forces and Their Nature,
Frictional force
The force that opposes relative motion is called friction. It arises between the surfaces in contact.
Example: When we try to push a table and it does not move is because it is balanced by the frictional force.
First Law of Motion
A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. The First Law is also called the Law of Inertia.
What is Newton’s First Law of Motion?
Newton’s first law of motion states that
A body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.
Putting Newton’s 1st law of motion in simple words, a body will not start moving until and unless an external force acts on it. Once it is set in motion, it will not stop or change its velocity until and unless some force acts upon it once more. The first law of motion is sometimes also known as the law of inertia.
There are two conditions on which the 1st law of motion is dependent:
- Objects at rest: When an object is at rest, velocity (v = 0) and acceleration (a = 0) are zero. Therefore, the object continues to be at rest.
- Objects in motion: When an object is in motion, velocity is not equal to zero (v ≠ 0) while acceleration (a = 0) is equal to zero. Therefore, the object will continue to be in motion with constant velocity and in the same direction.
- Derivation Of Equation Of Motion
What is an External Force?
An external force is defined as the change in the mechanical energy that is either the kinetic energy or the potential energy in an object. These forces are caused by external agents. Examples of external forces are friction, normal force, and air resistance.
Understanding laws of motion with the engaging video lecture.
Let us understand the First Law of Motion by an Example
Let us take a block on a smooth surface. By smooth, we mean that there is no friction acting on the surface. The block is at rest, that is, it is not moving.
Now, let us examine the forces acting on the block. The only forces acting on the block are the force of gravity and the normal reaction by the surface. There is no force acting on it in the horizontal direction. Since the forces in the vertical direction are equal to each other in magnitude, they cancel each other out, and hence there is no external force on the block. Since this block is at rest, we can say that it confirms Newton’s first law of motion.
Thus, the first law of motion is confirmed again.
Note: Newton’s laws are valid only in inertial frames of reference.
Newton’s First Law of Motion Example in Daily Life
Wearing a seat belt in a car while driving is an example of Newton’s 1st law of motion. If an accident occurs or brakes are applied to the car suddenly, the body will tend to continue its inertia and move forward, probably proving fatal. To prevent such accidents, seat belts are used, stopping your body from moving forward in inertia and avoiding danger.
- Newton’s Second Law Of Motion And Momentum
- Newton’s Third Law Of Motion
Frequently Asked Questions – FAQs
What are the major concepts involved in the topic?
The major concepts involved are the Frame of reference, Newton’s First law of motion (Law of Inertia), Newton’s Second law of motion, and Newton’s Third law of motion (For every action, there is an equal and opposite reaction), and constraint equations.
What are the characteristics of normal force?
It is a force that always acts perpendicular to the surface of contact.
What do we understand by a free body diagram?
A free body diagram represents all the forces acting on the body. For example,
So, the FBD for sphere B will be as shown below,
What are the steps to solve a problem on Newton’s laws of motion?
Let us take the following example,
The wedge is fixed, and we need to find the acceleration of the block of mass m along the incline.
Step 1: Draw the F.B.D of the block,
Step 2: Write the force equation taking its own axis. In this case, we take two axes, one along the plane and the other perpendicular to it.
Fincline =mg × sin45°
Fnormal = N × mg cos45°
Step3: Calculating acceleration using the second law of motion
mg sin45° = ma
a = g sin 45°
What is the constraint equation?
When the motion of one body is governed by another or in other words when the motion of two or more bodies is interlinked, the equation governing such motion is known as the constraint equation. For example:
If we draw the FBD and write equations we will get,
M1* g –T = M1 * a1
2T-M2* g =M2*a2
We can see there are two equations and three unknowns: a1, a2, and T. So, we need one more equation and that will be the constraint equation.
Inertia
Basically, all objects have a tendency to resist the change in the state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. The mass of an object is the measure of its inertia.
More the mass → more inertia and vice versa.
Inertia of Rest
An object stays at rest, and it remains at rest until an external force affects it. Example: When a car accelerates, passengers may feel as though their bodies are moving backward. In reality, inertia is making their bodies stay in place as the car moves forward.
Inertia of Motion
An object will continue to be in motion until a force acts on it. Example: A hockey puck will continue to slide across the ice until acted upon by an outside force.
Second Law of Motion
In order to understand second law, we need to first understand momentum.
Momentum
Impacts produced by objects depend on their mass and velocity. The momentum of an object is defined as the product of its mass and velocity. p = mv. Vector quantity, has direction and magnitude. Some examples of momentum include: A baseball flying through the air and a bullet fired from a gun.
Second Law of Motion
The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.
Fama
F-kma
For 1 unit of force on 1 kg mass with the acceleration of 1m/s2, the value of k = 1.
Therefore, F = ma.
Conservation of Momentum
Concept of system
- The part of the universe chosen for analysis is called a system.
- Everything outside the system is called an environment.
- For example, a car moving with constant velocity can be considered a system. All the forces within the car are internal forces and all forces acting on the car from the environment are external forces like friction.
From Newtons 3rd law F_{AB} = -F_{BA}
mA UA + mBUB = mAVA + mBVB
Third Law of Motion
Newton’s 3rd law states that every action has an equal and opposite reaction. Action and reaction forces are equal, opposite and acting on different bodies.
Inertial and Non-inertial frames
- A non-inertial frame of reference is a frame of reference in which Newton’s laws of motion do not hold. A non-inertial reference frame is a frame of reference that is undergoing acceleration with respect to an inertial frame. An accelerometer at rest in a non-inertial frame will, in general, detect a non-zero acceleration.
- A frame of reference where Newton’s Laws hold is known as an inertial frame of reference.