A ball is hanging from a long string that is tied to the ceiling of a train car traveling eastward on horizontal tracks. An observer inside the train car sees the ball hang motionless. Draw a clearly labeled free-body diagram for the ball if the train is speeding up uniformly. Is the net force on the ball zero in either case? Explain.

Crates $A$ and $B$ sit at rest side by side on a frictionless horizontal surface. They have masses $m_A$ and $m_B$, respectively. When a horizontal force $F$ is applied to crate $A$, the two crates move off to the right. Draw clearly labeled free-body diagrams for crate $A$ and for crate $B$. Indicate which pairs of forces, if any, are third-law action–reaction pairs.

Boxes $A$ and $B$ are in contact on a horizontal, frictionless surface (Fig. E$4.23$). Box $A$ has mass $20.0$ kg and box $B$ has mass $5.0$ kg. A horizontal force of $250$ N is exerted on box $A$. What is the magnitude of the force that box $A$ exerts on box $B$?

World-class sprinters can accelerate out of the starting blocks with an acceleration that is nearly horizontal and has magnitude $15$ m/s². How much horizontal force must a $55$-kg sprinter exert on the starting blocks to produce this acceleration? Which body exerts the force that propels the sprinter: the blocks or the sprinter herself?