Real-life examples

10 real-life examples of Newton's second law (F = ma)

The net force on an object equals its mass times its acceleration. The direction of the acceleration is the direction of the net force.

  1. Kicking a football. A 0.4 kg ball reaching 25 m/s in a 5 ms contact requires an average force of 0.4 × 25 / 0.005 = 2 000 N.
  2. Pushing a shopping cart. A 30 kg cart accelerated at 0.5 m/s² needs 15 N of net push (above friction).
  3. Rocket launch. Net thrust minus weight gives the mass times acceleration. A Saturn V at takeoff had ~33 MN thrust and ~28 MN weight, leaving ~5 MN to accelerate ~2.8×10⁶ kg at ~1.8 m/s².
  4. Elevator acceleration. A passenger of weight 700 N reads 770 N on a scale when the elevator accelerates upward at 1 m/s². The extra 70 N comes from F_net = m × 1 m/s².
  5. Crash test. Doubling the impact speed quadruples the kinetic energy and (for the same crumple distance) doubles the deceleration force.
  6. Tennis serve. 0.057 kg ball reaches 60 m/s in ~4 ms: F ≈ 0.057 × 60 / 0.004 = 855 N average.
  7. Throwing a snowball. Heavier ball means lower acceleration for the same arm force — F = ma in action.
  8. Pulling a sledge over snow. Required pull = friction + (mass × desired acceleration).
  9. Drag race. Acceleration = (engine thrust − rolling resistance − air drag) / mass. Top fuel dragsters reach 5 g.
  10. Skydiver before terminal velocity. Net force = mg − F_drag; once drag balances weight, net force is zero and acceleration is zero (terminal velocity).

Where students go wrong

F is the NET force. A common error is to use only the applied force and ignore friction, weight, or normal forces.

Where this fits in the library