Freefall describes any object that is free to move near Earth's surface. If air resistance is negligible, all objects in freefall will experience a constant vertical acceleration towards Earth of 9.81ms -2. Any sideways motion of the object will not be affected by its vertical acceleration. So an object thrown in a horizontal direction will hit the ground at the same time as an object dropped from the same height. The horizontal component of its motion will remain at the same value throughout its flight.

Any projectile motion can be predicted by considering the horizontal component of its velocity separately to its vertical component, using the equations of motion with zero as the horizontal acceleration value and 9.81ms -2 as the vertical acceleration value. The time of flight will be the same in both horizontal and vertical directions. The path traced out by any projectile motion is called a parabola (or a parabolic path). Note that the displacement-time graph for this motion in the vertical direction will be a similar shape.

If the Space Shuttle and the International Space Station are falling freely without power, why don't they fall back to Earth? Sir Isaac Newton understood the answer this problem in the 17th century. He even published a picture to illustrate the principle. Sir Isaac Newton and his orbit diagram If you go to the top of a tall mountain and fire a cannon horizontally, the cannon ball falls to the ground in a curved path. The more powerful the cannon, the faster the cannon ball, the straighter the path, and the farther the cannon ball travels before it hits the ground. If you could shoot the cannon ball with sufficient speed, the curvature of its path would match the curvature of the Earth, and it would travel endlessly in a circular orbit around Earth. The Moon travels around the Earth in the same manner.

Isaac Newton – again (proves how important he is!) If all forces are balanced (there is no resultant force) there will be no change in motion If there is a resultant force then there will be a change in motion. The bigger the force the greater the rate of change.

Newton’s 1 st Law If there is no resultant force on an object then if it is at rest, it will stay at rest; if it is in motion, it will keep on moving in a straight line at a steady speed. The object is in equilibrium. (Its reluctance to change velocity is called its inertia) Newton’s 2 nd Law The resultant force on an object is proportional to its rate of acceleration. (The change in motion takes place in the direction of the force) (mathematically this is written like this …. F = ma and you need to remember this) Newton’s 3 rd Law If an object A exerts a force on an object B, then B exerts an equal but opposite force back on object A. (the opposite force is often called the reaction force or normal contact force)

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