Second law in daily life

The second law explains a lot of everyday safety. The big idea: to stop something, you must change its velocity. If you stop it over a longer time, the acceleration is smaller, so the force is smaller.

Catch with soft hands. A fielder catching a fast ball pulls the hands back. The ball takes longer to stop, so a smaller force acts, and the hands are less likely to be hurt.

Airbags save lives the same way. In a crash, an airbag fills up instantly into a soft cushion. The head and chest stop over a longer time, with a smaller force. They work best together with seat belts. Bubble wrap round glass, hay in a cart, landing mats for high jumpers and sand pits for long jumpers all do the same job.

The opposite: cracking a coconut. Bring a coconut down fast onto hard ground, and it stops in a very short time. So the ground must push on it with a very large force, and the shell cracks.

Working it out: the 1500 kg car.

• Speeding up from 0 to 10 m s⁻¹ in 5 s: a = 2 m s⁻², so F = 1500 × 2 = 3000 N towards the east (the way it is going).
• Moving at a steady velocity: no net force.
• Slowing from 10 to 0 in 5 s: F = –3000 N, towards the west (against the motion).

Holding a barbell still. To hold a 30 kg barbell steady, the lifter pushes up with 30 × 9.8 = 294 N, exactly balancing its weight.

From a graph: find the acceleration from the slope of the velocity-time graph, then use F = ma. A 10 kg object with a = 2.5 m s⁻² needs 25 N.