Question

At the local farm you buy a flat of strawberries and place them on the back seat of the car. On the way home, you begin to brake as you approach a stop sign. At first the strawberries stay put, but as you brake a bit harder, they begin to slide off the seat. What is a complete explanation for this?

A. The strawberries have inertia and tend to move at the same speed (Newton's 1st Law) but the frictional force opposes their motion. When you hit the break harder the frictional force changes from static to kinetic.

B. From the point of view of the driver, the static frictional force pulls the flat of strawberries forward and keeps it on the seat. The acceleration of the strawberries is due to this frictional force. When the acceleration of the car increases, the frictional force increases, too. As the static frictional force has an upper value, it cannot provide more than a maximum acceleration a_max. When you hit the brakes hard, the backwards acceleration is greater than the a_max that the static force could provide. The friction between the seat and the flat of strawberries becomes kinetic friction, the strawberries slide.

C. From the point of view of the driver, the static frictional force pulls the flat of strawberries backwards and keeps it on the seat. The acceleration of the strawberries is due to this frictional force. When the acceleration of the car increases (backwards, causing the car to slow down), the frictional force increases in the backward direction, too. As the static frictional force has an upper value, it cannot provide more than a maximum acceleration a_max. When you hit the brakes harder, the backwards acceleration is greater than the a_max that the static force could provide therefore the car slows down faster than the strawberries. The friction between the seat and the flat of strawberries becomes kinetic friction, the strawberries slide.

D. The strawberries have inertia and tend to move at the same speed (Newton's 1st Law) but the frictional force opposes their motion. When the breaking force is bigger (when you hit the break harder) there is enough force to beat the frictional force and the strawberries slide from the seat.

Answer 1

The strawberries which are at rest inside the vehicle at rest are moving along with the vehicle. When it stops all of a sudden, only the lower part of strawberries comes to rest whereas the upper part continues to be in motion because of the Inertia of Motion. The static friction force opposes this motion. But with harder break strawberries overcome the static frictional force and slides forward with kinetic friction. Hence option(A) i.e. The strawberries have inertia and tend to move at the same speed (Newton's 1st Law) but the frictional force opposes their motion. When you hit the break harder the frictional force changes from static to kinetic.