What is Force?

Jay Gregorio, Amanda Robb, Elaine Chan
  • Author
    Jay Gregorio

    Jay Gregorio has taught Physics and STEM courses for over 11 years. He has a degree in Education with specialization in Physics from the Philippines and a STEM leadership certificate from Columbia University, New York. He is currently taking his doctorate degree in Organizational Development at the Southeast Asia Interdisciplinary Development Institute.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

  • Expert Contributor
    Elaine Chan

    Dr. Chan has taught computer and college level physics, chemistry, and math for over eight years. Dr. Chan has a Ph.D. in Chemistry from U. C. Berkeley, an M.S. Physics plus 19 graduate Applied Math credits from UW, and an A.B. with honors from U.C .Berkeley in Physics.

Understand what force is and get to know the force equation. Learn about various types of forces, units of force, force formula, and the concept of net force. Updated: 03/04/2022

Table of Contents


Force Definition: What is Force?

A force is a push or a pull. When a grocery cart is pushed through the aisle of the supermarket, the hands are exerting force on the cart. When the arrow on a bow is pulled, there is also a force applied on the bow. When a ball is thrown in the air, it comes down because gravity pulls on the ball. These examples show that a force can move objects. Specifically, a force can change the object`s speed, direction, or shape.

Forces help bring about a change of motion. When a skater moving at a constant speed is given a push from behind, the skater will move faster towards their original direction. The same skater may slow down when they encounter a hump on the road. The amount of force given to an object determines its motion. However, there are some situations where a force is applied, and the object does not move. For the discussions in this lesson, we will consider objects that move after a force is applied.

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  • 0:00 What Is Force?
  • 1:01 Force Formula
  • 2:00 Solving for Other Variables
  • 3:34 Net Force
  • 6:14 Lesson Summary
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Unit of Force: What is Force Measured In?

The International System of Units (SI) defines a unit of force as the Newton with the symbol N. The Newton is related to three base units: the meter (m), kilogram (kg), and second (s). A meter is a unit of length, a kilogram is a unit of mass, and a second is a unit of time.

Using these base units, we find that:

{eq}1\;N=1\;\frac{kg\;m}{s^{2}} {/eq}

There are many other equivalent units of force that can be used, especially in the English system. SI units are commonly used as the system has only one unit for each quantity.

Force Equation: Force and Motion

The standard force formula is derived from Isaac Newton's second law of motion which states that the net external force on an object is equal to the product of its mass and acceleration. More succinctly, force equals mass times acceleration. In symbols, this can be written as follows.

{eq}F = ma {/eq}

where F is the force, m is the mass, and a is acceleration. Acceleration is defined as the rate of change in the object's velocity. Mass is the amount of matter in an object. Force and acceleration can be described by both magnitude (represented by a number) and a direction in space. Likewise, mass is described only in terms of magnitude.

We can calculate the net force if the mass of an object and the acceleration are known. When a 0.62 kg basketball is dropped from a height and accelerates at 10 m/s^2, we can use the formula {eq}F=ma {/eq} and find that the net force acting on the falling basketball is 6.2 kg m/s^2 or 6.2 N.

A free-body diagram is an illustration that represents all forces acting on an object. These forces are represented by arrows where the length shows the magnitude and the arrowhead points to the direction of the force.

A free-body diagram is a representation of all forces acting on an object.

An example of a free-body diagram showing an object with two forces acting on it in opposite directions.

Types of Forces

There are a variety of forces that can be described based on how they interact with an object. A contact force requires that two objects are touching. When a button on the computer is pressed, the force exerted is a contact force. Likewise, there are forces that do not need contact to see their effects. Using a previous example, the basketball dropped from a height will fall to the ground because of the force of gravity even though there is no contact between the Earth and the ball as it falls. The four fundamental forces are the gravitational force, electromagnetic force, weak force, and strong force.

Gravitational Force

Gravitational force is one of the most familiar forces in nature. Classical physics defines gravitational force as the attraction between two objects. The saying `what goes up, must come down` describes the consequence of gravitational attraction between an object and the Earth. The Earth`s gravity is massive and therefore, smaller objects on its surface tend to be attracted towards its center. Weight is an example of a force due to gravity. It is caused by the Earth pulling our bodies towards its center. This force prevents us from flying off into outer space.

Electromagnetic Force

Electromagnetic force describes the attraction and repulsion between charged particles such as positively charged protons and negatively charged electrons. As the name suggests, the electromagnetic force covers both electric and magnetic forces. The electrical property of the force is displayed when charged particles are stationary and electric field lines fill up the space around them. Once set in motion, these charged particles create a magnetic field around them. This explains why some wires become magnetic when they are used to charge a device.

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  • Activities
  • FAQs

Calculate force

Acceleration a, assumed constant in time, is ( v - vo )/ t = a.

Here v is the velocity at time t and vo is the velocity at time equal zero. We can solve this equation for time t = ( v - vo )/ a .

The position x of an object at time t is x = xo + vav t.

Here xo is the initial position and vav is the average velocity, which for an object at constant acceleration is ( vo + v )/2.

Using average velocity and time in the equation for the position results in the equation v2 = vo2 + 2 a ( x - xo ).v%

Question :

What constant net force is required to stop a 1500 kg car to rest from a speed of 100 km / hour within a distance of 55 m ?

Answer :

Use Newton's second law and determine the acceleration. The acceleration is constant because the net force is constant. The initial velocity is 100 km /h = 1000 m/ 36 s = 27.78 m/s. The final velocity is zero. Use the formula v2 = vo2 + 2 a ( x - xo ). to solve for a, with x - xo = 55 m. a= (0 - (27.78)2 )/(2 (55)) = -7.016 m/s2 .

Multiply by the mass, to obtain -10524 Newtons of force in the direction opposite the initial velocity.

What is a force in science?

In science, the simplest definition of force is a push or a pull. This definition can vary depending on what type of force is being described.

What is the basic equation for force?

The basic equation of force is F = ma which states that the net force acting on an object is equal to the product of mass and acceleration. In short, it is force equals mass times acceleration.

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