Inertia Newton's 1st Law of Motion, aka the Law of Inertia, states that an object at rest will tend to remain at rest, and a moving body keeps moving in a straight line at a constant speed. Well - as long as any forces acting on it are balanced. If they're not, then there is a "net force" which willl change the velocity of the object - changing its speed, direction, or both. Friction often seems to deny Inertia's truth. Friction always opposes motion between surfaces, causing them to stop - or even to START - moving, just as long as the motion is not between the surfaces with friction. Tools A B 6 A man walking at 3 m/s decided to go faster. It took him 2 seconds to increase his velocity to 9 m/s. How much is his change in velocity? 3 A car accelerates from 5 m/s to 50 m/s in 15 seconds. How much is its acceleration. 4.5 An airplane taking off starts at rest and reaches 90 m/s in 20 seconds. How much is its acceleration? force A ______ is a push or a pull. FALSE Balanced forces always cause a change in motion. True / False TRUE A net force always cause a change in motion. True / False size In order for a pair of forces to be balanced, what must be equal about them? direction In order for a pair of forces to be balanced, what must be the opposite about them net Whenever forces are NOT balanced, there is a __ __ __ force inertia All objects have ____________, which means that they resist any change in their motion. friction The force that opposes motion between two surfaces is called _______. surfaces The amount of friction depends on the types of ________. weakest Gravity is the _____ of the fundamental forces. weight Your _____ is caused by the gravitational force that earth exerts on you. 29.4 Because of gravity, 9.8 newtons of force pulls downward on every kilogram of mass. How much force do you suppose pulls down on THREE kilograms of mass? ______newtons. 588 Because of gravity on earth, a 1 kg mass weighs 9.8 newtons. How much should a 60 kg man weigh? _______newtons. oppose The effect of friction is always to _________ motion between the two touching surfaces. attract The force of gravity will always cause one mass to ______ another mass.