Topic 9: STRUCTURES Forces

Newton’s Laws of Motion Sir Isaac Newton was one of the greatest scientists and mathematicians that ever lived. He was born in England on December 25, He was born the same year that Galileo died. He lived for 85 years. Isaac Newton was raised by his grandmother. He attended Free Grammar School and then went on to Trinity College Cambridge. Newton worked his way through college. While at college he became interested in math, physics, and astronomy. Newton received both a bachelors and masters degree. While Newton was in college he was writing his ideas in a journal. Newton had new ideas about motion, which he called his three laws of motion. He also had ideas about gravity, the diffraction of light, and forces. Newton's ideas were so good that Queen Anne knighted him in His accomplishments laid the foundations for modern science and revolutionized the world. Sir Isaac Newton died in In this lesson you will develop an understanding of each of Newton's Three Laws of Motion.

The first law An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law is often called "the law of inertia".

The second law Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). An object can not create its own external force

The third law For every action there is an equal and opposite re-action.

Weight and mass What's the difference between weight and mass? As long as you stay on Earth, the difference is more philosophical than practical. Well, mass is a measurement of how much matter is in an object; weight is a measurement of how hard gravity is pulling on that object. Your mass is the same wherever you are--on Earth, on the moon, floating in space--because the amount of stuff you're made of doesn't change. But your weight depends on how much gravity is acting on you at the moment; you'd weigh less on the moon than on Earth, and in interstellar space you'd weigh almost nothing at all. scientists still like to be careful about distinguishing between the two. If you talk about the mass of an atom you're always talking about the same thing; if you talk about its weight, what you mean depends on where the atom is. UNITS Mass is measured in Kilograms and because weight is a force it is measured in Newtons

Forces There are 4 types of force Compressive (when something is in compression)– a pushing or squashing force Tensile (when something is in tension) – a pulling or stretching force Shear (when something is in shear) – a cutting force. Torsion (when something is under torque) – a twisting force

If the forces acting on an object are balanced, a stationary object remains stationary, and a moving object keeps moving at the same speed and in the same direction. If the forces acting on an object are unbalanced, a stationary object begins to accelerate in the direction of the force, and a moving object speeds up, slows down or changes direction. Acceleration depends on the force applied to an object and the mass of the object. Accelerationforcemass

Balanced and unbalanced forces You should be able to use the idea of balanced and unbalanced forces to determine the movement of an object. Balanced forces When all the forces acting on an object cancel out, we say that they are balanced. If the forces are balanced: A stationary object remains stationary. A moving object keeps on moving at the same speed in the same direction. For example, a book resting on a table has balanced forces. Its weight acting downwards is balanced by the upward force on the book provided by the table. Unbalanced forces When all the forces acting on an object do not cancel out, we say they are unbalanced. If the forces are unbalanced: A stationary object begins to move in the direction of the unbalanced force. A moving object speeds up, slows down or changes direction depending on the direction of the unbalanced force.

Forces and acceleration (Higher) You should know the equation that shows the relationship between force, mass and acceleration, and be able to use it. The equation You should see that 1N is the force needed to give 1kg an acceleration of 1m/s2. For example, the force needed to accelerate a 10kg mass by 5m/s2 is 10 x 5 = 50N. The same force could accelerate a 1kg mass by 50m/s2 or a 100kg mass by 0.5m/s2. Example A truck has a mass of 2,000kg. The driving force created by the engine is 3,000 newtons. Calculate the acceleration caused by this unbalanced force. Answer 1. Write down and rearrange the equation.force = mass x acceleration 2. Rearrange the equation.acceleration = force/mass 3. Put in the values.acceleration = 3,000N/2,000kg 4. Work out the answer and write it down.acceleration = 1.5m/s2