## Table of Contents

- Force Definition: What is Force?
- Unit of Force: What is Force Measured In?
- Force Equation: Force and Motion
- Types of Forces
- Concept of Net Force
- Lesson Summary

- Force Definition: What is Force?
- Unit of Force: What is Force Measured In?
- Force Equation: Force and Motion
- Types of Forces
- Concept of Net Force
- Lesson Summary

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.

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.

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,

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.

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 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 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.

The strong force is also called the strong nuclear force or the strong nuclear interaction. It lives up to its name because it is trillions of times stronger than the gravitational force. It is the force that binds the proton and the neutron at the nucleus of an atom. The protons in the nucleus repel each other but a tiny fraction of the strong force keeps them together. This means that the strong force is present at an extremely close distance between subatomic particles. When there is an attempt to separate the particles, the strong force increases its strength. A process called nuclear fission happens when the force pushing the nucleus apart overcomes the strong force. This happens in a nuclear bomb explosion where trillions of the atoms' nuclei are forced to split an atom into smaller nuclei.

The weak force bears its name because of its lower intensity compared to other fundamental forces. It is a force that is unseen in daily life and exists at the subatomic level. The weak force accounts for the radioactive decay of certain subatomic particles. The weak force also plays an important role in the process called nuclear fusion where hydrogen fuses into helium. When two protons are very close to each other, the repulsion is strong, and overcoming this force requires an extremely high temperature. The weak force starts the process of burning hydrogen in the Sun that it uses to power itself. The weak force is, therefore, not weak after all. It helps produce the energy needed by most life forms on Earth.

A **net force** is the vector sum of all the forces acting on an object. Sometimes this is referred to as a resultant force. It shows how the combination of a force's magnitude and direction will affect the object it is acting on.

In a free-body diagram where a box experiences a force of 10 N to the left and 20 N to the right, the vector sum of the forces is:

{eq}F = -10\;N+20\;N \\ F = 10\;N {/eq}

The net force or resultant force is 10 N to the right. The use of a positive sign for the rightward direction and a negative sign for the leftward direction are common conventions. Suppose the box weighs 10 kg, the acceleration of the box can be calculated as:

{eq}F=ma \\ a=\frac{F}{m} \\ a=\frac{10\;N}{10\;kg} \\ a=1 \frac{m}{s^{2}} {/eq}

Look at the second free-body diagram in the image. This time a box is experiencing a 15 N force to the right and another 30 N force in the same direction. The vector sum of the forces is:

{eq}F = 15\;N + 30\;N \\ F = 45\;N {/eq}

The net force or resultant force is 45 N. If the acceleration of the box is 4.5 m/s^2, the mass can be calculated as:

{eq}F=ma \\ m=\frac{F}{a} \\ m= \frac{45\;N}{4.5 \frac{m}{s^{2}}} \\ m=10\;kg {/eq}

A **force** is a push or a pull that can change the object's state of motion. The International System of Units (SI) unit of force is the **Newton** (N) where {eq}1\;N=1\;\frac{kg\;m}{s^{2}} {/eq}. The standard force formula states that the net external force on an object is equal to the product of its mass and acceleration. The force equation is {eq}F = ma {/eq} where * F* is the force,

There are a variety of forces that can be described based on how they interact with an object. The four fundamental forces on Earth include the gravitational force, electromagnetic force, weak force, and strong force. Weight is an example of a force due to gravity. It is the force that pulls us down, towards the Earth. In solving Force problems, a **free-body diagram**, a diagram showing an object and all forces acting on it as arrows, is helpful for analyzing the magnitude and direction of the forces acting on an object. Once the **net force** or vector sum of all forces is determined, we can rearrange the force equation such that {eq}F=ma {/eq} can be {eq}a=\frac{F}{m} {/eq} or {eq}m=\frac{F}{a} {/eq}.

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- Activities
- FAQs

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%

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 ?

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.

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.

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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