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I'm curious to know why a tennis ball bounces forwards after it has made contact with the ground considering I throw it forwards and downwards at the ground.

I understand that is bounces up because of the force of the ground on the ball (Newton's third law).

The speed reduces to zero when it makes contact with the ground and then, there's friction. The force of friction acts backwards. Logically, wouldn't it bounce backwards?

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  • $\begingroup$ Note that, if the ball is spinning fast enough when it hits the ground, it can bounce backwards. That's probably not the case you're asking about, but I just wanted to point out that the direction of the bounce depends on both the velocity and the rotation of the ball. You can't treat those as independent, unless you want to neglect friction completely. $\endgroup$
    – Ilmari Karonen
    Commented Dec 5, 2015 at 16:00

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Inertia.

The ball has forward momentum, and friction with the ground can't overcome it. Keep in mind that friction only opposes motion. If the ball were to stop and reverse direction, the horizontal speed would, for an instant, be zero. No speed means no friction force, so friction can only stop the ball, not reverse its direction. Friction can never increase the speed of an object, only decrease it.

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The speed does not reduce to zero! Vertical speed component is zero for a moment - vertical speed swings from downwards to upwards during the bounce phase. But horizontal speed largely stays the same, possibly slightly lowers due to some of the kinetic energy transferring to ball rotation, but in essence, horizontal ball speed is mostly unchanged.

It's quite hard to find a slow motion of bouncing ball with some forward velocity, this is the best I could find: https://www.youtube.com/watch?v=3jI57WMOzbU

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  • $\begingroup$ If horizontal velocity never reduces to zero, would it not roll forwards for a very brief moment on the ground? $\endgroup$
    – Stefan N
    Commented Dec 3, 2015 at 8:50
  • $\begingroup$ Yes, it starts slightly rolling forward. Depending on the lenght of the contact. You can slightly see it in the video, but you can observe it yourself. If you throw non-rotating ball on the ground, with some forward velocity, it will bounce with rotation. $\endgroup$
    – airguru
    Commented Dec 3, 2015 at 9:01
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    $\begingroup$ This video is also helpful youtube.com/watch?v=yS-n_1Zdtv4, you can see how the ball changes rotation everytime it bounces. $\endgroup$
    – airguru
    Commented Dec 3, 2015 at 9:03

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