If a car stops suddenly, you feel “thrown forward.” We’d like to understand what happens to the passengers as a car stops. Imagine yourself sitting on a very slippery bench inside a car. This bench has no friction, no seat back, and there’s nothing for you to hold on to. a. Draw a picture and identify all of the forces acting on you as the car travels in a straight line at a perfectly steady speed on level ground. b. Draw your free-body diagram. Is there a net force on you? If so, in which direction? c. Repeat parts a and b with the car slowing down. d. Describe what happens to you as the car slows down. e. Use Newton’s laws to explain why you seem to be “thrown forward” as the car stops. Is there really a force pushing you forward?
SOLUTION: Step 1 of 7 (a)A person sits on a slippery bench inside a car. The forces that act on the person are, weight of the person in the downward direction due to gravitational pull of earth and the normal reaction force exerted by the bench on the person acts in the upward direction. Since, the bench is slippery, there is no action of frictional force. There are no forces along the horizontal axis and forces act only in the vertical direction. As the car moves with steady speed, the person and the bench also moves with the same constant speed. So, the acceleration of the person is zero. The equilibrium state of the person indicates that the gravitational force is balanced with the normal force in the vertical direction resulting in zero net force. The following is the figure showing the pictorial representation of the forces acting on the person in the car. Step 2 of 7 (b)The following is the figure showing the free body diagram of the person in the car. In the above figure, it is clear that the gravitational force balances the normal force in the vertical direction. So, the resulting net force on the person in the vertical direction is zero. Step 3 of 7 (c)As the car slows down, there will not be action of any new force on the person sitting inside the car. Due to the action of some frictional force, the car may slow down so that the forces acting on the car will change but not the forces on the person. The forces acting on the person remain same even though the car slows down. So, therefore there will be no change in the pictorial representation and in the free-body diagram of the person in the parts (a) and (b). Step 4 of 7 (d)The car when slows down, there will not be action of any new force on the person sitting inside the car. Due to the action of some frictional force, the car may slow down and the forces acting on the car will change but not the forces on the person. The person continues to move with the steady speed as there are no horizontal forces acting on the person.