A block is pushed along a horizontal surface and comes to rest after 4.0 m. Which best explains why it stops even though no force is applied once released?

Question

BlackTom AI · Accepted Answer

To understand why the block stops, we must consider what forces are acting on it after release and how those forces affect motion.
Option a: 'The block has no inertia, so it naturally slows down' is incorrect. Inertia is the property of an object that resists changes to its motion; a block on a frictionless surface would continue at constant velocity once released. The statement wrongly claims no inertia, which contradicts Newton's first law.
Option b: 'Friction exerts a force opposite to motion, reducing velocity to zero' is correct. On a horizontal surface, kinetic friction acts opposite to the direction of motion, removing kinetic energy and slowing the block until it comes to rest, assuming no other driving force remains. This aligns with the work–energy view where friction does negative work equal to the initial kinetic energy, bringing the object to rest.
Option c: 'Kinetic energy is converted into gravitational potential energy' is not applicable here. On a horizontal surface with no vertical displacement, there is no change in gravitational potential energy; energy dissipates as heat due to friction, not by lifting the block.
Option d: 'The normal force acts upward, canceling motion' is a misunderstanding. The normal force balances the weight in the vertical direction and does not cancel horizontal motion; it has no direct role in stopping motion along the surface unless it changes the friction level, which is not the primary mechanism described.
In summary, the only option that correctly identifies the mechanism—friction opposing motion and reducing velocity until rest—is the one describing frictional force opposite to motion producing deceleration and energy dissipation as heat, rather than any of the other proposed mechanisms.

SPH4U - Physics 12 (2025-26) - A

A block is pushed along a horizontal surface and comes to rest after 4.0 m. Which best explains why it stops even though no force is applied once released?

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