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Transcript
Newton’s First Law of Motion INERTIA First Law of Motion also called the “law of inertia” “If the net force acting on an object is zero, the object will maintain its state of rest or constant velocity.” in other words, unless the forces change, the object will keep doing what its doing! Inertia a property of matter that causes an object to resist changes in its state of motion it is proportional to the mass of the object ie: the greater the mass, the greater the inertia an object possesses, the harder it is to change the object’s motion interaction between a frictionless ball and frictionless ramp **Force is not needed to keep an object in motion!** example: standing on the bus as the bus starts to move, your body wants to stay at rest because of inertia, so you might fall towards the back of the bus once the bus reaches a constant velocity, you have no trouble standing because you are also moving with a constant velocity if the bus slows or stops unexpectedly you will likely fall towards the front of the bus because your body wants to keep moving forward Net Forces the net force is the sum of all the forces acting on an object as long as the forces acting on an object are balanced (offset), then the motion of the object will not change Applications of the 1st Law Seat Belts ◦ when a car suddenly stops, you continue to move forward until an object exerts a net force on you (the dashboard!!) ◦ a seat belt is designed to exert a force on you to slow you down ◦ in order to allow you to move about freely with your seat belt on, the belt is designed to activate upon sudden decreases in velocity only the belt strap is attached to a spool which is attached to a gear beneath the gear is a pendulum that swings freely when the car suddenly stops, the pendulum swings forward due to inertia and locks the gear other applications Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator. The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. When drinking hot coffee in the car, and sudden acceleration will cause you to spill coffee into your lap. To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrust downward at high speeds and then abruptly halted. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions. While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard. Summary 1. 2. 3. 4. objects at rest remain at rest unless acted upon by a net force objects in motion remain in motion unless acted upon by a net force if the velocity of an object is constant (or zero), the net external force acting on it must be zero if the velocity of an object is changing either in magnitude, direction or both, the change must be caused by a net external force on the object