Inertial and Non inertial Frames of Reference Frame of Reference is a place from which motion is observed. An inertial frame of reference is one in which Newton's First Law is valid. Ex: When there is no motion or When there is uniform motion (Constant speed in straight line) Example: Consider tossing an apple in a parked car or in a car moving at a fixed speed down a straight level road. The motion of the apple to you was identical in each case. Newton s First Law makes it all clear. When the apple leaves your hand, it has an upward motion which you gave it and a horizontal motion due to the motion of the car. As stated in the first law, the apple has a tendency to maintain this motion because there is no external unbalanced force on it. It therefore comes back to your hand very nicely because it and your hand have had the same horizontal fixed speed. Whether the car is at rest or moving as described, it will be an inertial frame of reference. Example: If you are in a jet plane that is flying at 1000 km/hr in a straight line, any motion you observe will be identical to the motion if you viewed it while the plane was parked on the runway. Coffee can be poured straight into your cup, you could play a game of ping pong if there were room, and, if you toss something into the air, it doesn't go streaking to the back of the plane instead it comes right back to your hand. The frame of reference of the plane is an inertial frame of reference. Any motion you observe in the frame of reference of the plane will obey Newton's first law. 1
A Non Inertia frame of reference is one where Newton s First Law is NOT Valid. Ex: When there is acceleration involved in the frame of reference. Imagine that you are sitting on a bus that is traveling at a fixed speed in a straight line. A ball is in the aisle of the bus near your feet. The ball is at rest with respect to the floor of the bus, and will remain at rest with respect to the floor, and with respect to you as you sit in the seat AS LONG AS THE BUS MOVES WITH A FIXED SPEED IN A STRAIGHT LINE. Now let s say the bus driver touches the brake ever so slightly not enough for you to notice. The ball will roll forward with respect to you. You will see the ball move for no reason at all! After all, objects at rest should remain at rest. WHILE THE BUS CHANGES SPEED, IT IS NO LONGER AN INERTIAL FRAME OF REFERENCE. From your point of view sitting in the bus the ball did not obey Newton's first law. As far as you could tell, the ball moved even though no external force was applied. The ball moved because the bus decelerated. 2
Newton s First Law of Motion has several significant implications: 1. An external, unbalanced force is required to change the velocity of an object. Internal forces have no affect on an object s motion. 2. The external force must be unbalanced if it is to affect the motion of the object. 3. Objects at rest will not move unless an external, unbalanced force acts upon it. 4. Objects in motion will not stop unless an external, unbalance force acts upon it. For example, a car moving into a flat icy curve will tend to continue in a straight line, off the side of the road. 3
1. An object is remaining perfectly stationary. Which statement is correct? A ) There is no net force acting on the object. B ) There could be a small net force acting on the object. C ) There is a large unbalanced force acting on the object. D ) All of the above are correct. 2. Which object has the greatest INERTIA? A ) A 2 kg rock is moving at 30 m/s. B ) A 0.1 kg bullet is moving at 400 m/s. C ) A 5 kg concrete block is sitting stationary on the ground. D ) A 1 kg ball is accelerating at 1 m/s 2. 4
3. Newton's First Law of Motion BEST explains which observation? A ) Gravity affects all objects. B ) When you let the air out of a balloon it tends to fly around. C ) A force tends to accelerate your bicycle. D ) Curling stones tend to move at constant speeds in a straight line. 4. Which choice represents a NON INERTIAL frame of reference? A ) a bicycle traveling at 20 km/h [West]. B ) a race car going around a circular track at 200 km/hr. C ) a balloon moving at constant velocity straight upwards. D ) a spacecraft sitting on the surface of Mars. 5
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