Throwing Your Arm Out
Many parents mistakenly believe they can restrain an unbelted child in a sudden stop simply by throwing their arm in front of the child. Newton's second law explains why this won't work. Newton discovered that the faster an object moves, the greater the force required to change its motion. Stopping an unbelted 50 lb child in a 45 mph crash by throwing an arm in front of the child would be like trying to stop a 1000 lb steer from trotting past you at 10 mph with an extended arm. Both feats require the same amount of energy!
Infant in the Lap
One of the most dangerous places for an infant to ride in a car is in the lap of an unbelted mother. In a crash, the infant has little chance for survival. Imagine a 120 lb woman holding a baby in her lap in a car that crashes into a stationary object at a mere 30 mph. When the car stops, the baby and mother keep traveling forward at 30 mph. The baby smashes into the dashboard at that speed but the worse is yet to come. At 30 mph, the mother then smashes the baby into the dashboard from behind with a force of over 1,200 lbs! Even if the mother is belted in, she will not be strong enough to hold the baby in her arms. In a 30 mph crash, a twenty pound infant will pull away with a force of over 200 pounds.
Most crash deaths occur within 25 miles of home at speeds under 40 mph. Seat belts should be used on every trip, no matter how short.