Mission CG3 Centripetal Force and Inertia
A car makes a left-hand turn. The front-seat passenger claims that she feels a sensation of being pulled outwards. This is best explained by the fact that ____.
Inertia is the property of an object that describes its natural tendency to keep on doing whatever it is doing.
Inertia is simply the tendency of any object to keep on doing whatever it is doing. It is the natural tendency of an object to stubbornly maintain the same speed and the same direction that it has at any given moment. The direction that you are moving at all points along the curve is in a straight line directed tangent to the curve. You would feel a strong inertial tendency to continue in motion along this straight line unless acted upon by an unbalanced force. The presence of an unbalanced force on your body pushes you towards the center of the circle so that you can make the turn rather than travel straight ahead. This force is supplied by the outside door, by the seat belt and harness or by friction between you and the seats. If this force were somehow removed, there would no longer be an unbalanced force on your body and you would fly out of the car and tangent to the circle. An object in motion would continue in motion in the same direction when not acted upon by an unbalanced force. You don't need a force to sense this feeling of flying out of the car. The sensation is explained by inertia - your tendency to keep on moving in a straight line in the same direction when not acted upon by an unbalanced force.
Wrong ideas do not die easily. The belief in an outward force is so strong among some students that they will do nearly anything to keep it. They will even redefine the adjective centripetal to mean outward when its true meaning is inward. But don't be fooled! There is no such outward force. Forces are not mysterious or ghostly. A force requires a person or thing to do the pushing or pulling. Can you think of any physical object which pushes you outward as you travel along a curve? No. The sensation of being pushed outward is simply the tendency which you have to move in a straight line at all times. Draw a circle and put a dot at the three o'clock position. Draw a straight line tangent to the circle at this position (it would be directed straight down or straight up). Observe that the tangent line is directed away from the center of the circle. This is the direction which you tend to move at all times. But you would never need a force to move in a straight line; it happens naturally. To stay on the circular path, you would need a force directed towards the center.
