Why is the Pentagon interested in UFOs?

U.S. Navy pilots and sailors won't be considered crazy for reporting unidentified flying objects, under new rules meant to encourage them to keep track of what they see. Yet just a few years ago, the Pentagon reportedly shut ...

West Africa warms but airborne dust keeps the Red Sea cool

The Red Sea is located between North Africa and the Arabian Peninsula, the world's largest dust source regions. Summer winds pump dust from the Sahara and Arabian deserts down a narrowing mountain-fringed passage, causing ...

The conspicuous absence of women in India's labour force

India's rapid economic growth has been accompanied by falling fertility rates and higher educational attainment among women. These advances often lead to an increase in women entering the labour force, but there has been ...

What happens when a raindrop hits a puddle?

Have you ever taken a walk through the rain on a warm spring day and seen that perfect puddle? You know, the one where the raindrops seem to touch down at just the right pace, causing a dance of vanishing circles?

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In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. Force can also be described by intuitive concepts such as a push or pull. A force has both magnitude and direction, making it a vector quantity. Newton's second law, F=ma, was originally formulated in slightly different, but equivalent terms: the original version states that the net force acting upon an object is equal to the rate at which its momentum changes.

Related concepts to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque which produces changes in rotational speed of an object. Forces which do not act uniformly on all parts of a body will also cause mechanical stresses, a technical term for influences which cause deformation of matter. While mechanical stress can remain embedded in a solid object, gradually deforming it, mechanical stress in a fluid determines changes in its pressure and volume.

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