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Understanding the concept of forces is the basis for understanding Newton’s laws. A force (F) is a push or pull that one body exerts on another. The force is a vector quantity, which has both magnitude and direction. It represents the interaction of one body with another, which may be recognized by actual contact or by action at a distance. The force is characterized by its magnitude, point of action, and direction.



Manipulate the magnitude and direction of two vectors to generate a sum.

Forces Lab


Gravitational force is an example of action-at-a-distance force. It is caused by the acceleration due to gravity. We can measure the force of gravity or weight of an object with a scale. Gravity is also the force affecting the largest and smallest objects. This force controls the movement of planets of the solar system in orbit around the sun and the movement of stars in the outer space. The weight is distributed throughout the body. But we may often think of it as collected and acting through a single point called the centre of gravity. The centre of gravity is a geometric property of object. It is the average location of the weight of an object. Center of gravity is very important factor of stability. We may completely describe the motion of any object through space in terms of the translation of the center of gravity of the object from one place to another and the rotation of the object about its center of gravity if it is free to rotate.


Online Simulation of Gravitational Force

Loads Lab

Shapes Lab


Measuring the Force


The force is measured in Newton (N). One Newton is the amount of force required to give a 1-kilogram (kg) mass an acceleration of 1 meters per second square (m/s2) (Equation 6.1). We may use a simple spring scale such as those used in the market. As the scale’s pan is pulled down, the spring above is stretched and an attached pointer moves. Then all we have to do to measure forces is to calibrate the scale so that the amount of stretch measures the magnitude of the force. This concept was observed by Robert Hooke (1635-1703) and was named Hooke’s law of elasticity. Hooke’s law gives the relationship between the force applied to an unstretched spring and the amount the spring is stretched when the force is applied



Where x is the distance that the spring is stretched or compressed from its relaxed length, and k is called the spring constant for that particular spring in N/m. The spring constant is a measure of how hard it is to stretch or compress a spring. A stiffer spring has a larger spring constant because larger forces must be exerted on the ends of the spring to stretch or compress it.

The spring scale is a weighting scale often used to measure force, such as the force of gravity exerted on a mass or the force of a person’s grip or the force exerted by a towing vehicle.


The above figure shows a simple spring scale. Notice that there is a pull on both ends of the scale; one pull is from the weight hanging at the bottom of the scale and the other is from the hook attached to the ceiling from above.



Focus on Math


Vector quantities are often represented by scaled vector diagrams. Vector diagram represents a vector by use of an arrow drawn to scale in a specific direction. Observe that there are several characteristics of this diagram:

(1) a scale is clearly listed

(2) an arrow is drawn in a specified direction, therefore, the vector has a head and a tail

(3) the magnitude and direction of the vector are clearly labeled (the magnitude is 100 N and the direction is 35 degrees).




Practical Examples


An Example of Constant, Moving Force


Applications of Force


In athletics, one of the most important components of the performance is the use of force. Force is required in all actions: jumping, running, throwing, punching, and kicking. All of these actions require the use of force.

In training, usually athletes and coaches focus their training exclusively on force production and force reduction. In soccer for example, the player demands force production in order to jump up and over the defensive back and grab a high pass. However, to land in bounds and without injury, the receiver must reduce his force upon landing. In soccer, there is a constant interplay of force production and reduction, with most injuries occurring during the force reduction phase of stopping and/or rapidly changing direction.


Practical Example

Net Forces on an Airplane



Consider the following concrete beam with three forces. Could you identify the point at which the beam may crack?










Consider the following pair of pliers gripping a bolt. If the force at A = 20 N, a = 6 cm, b = 2 cm. What is the gripping force at B?



(1). 1.2 N

(2). 1.6 N

(3). 160 N

(4). 120 N