Question

An office worker smacks a table, bringing his hand to rest in 0.0025 seconds from an initial speed of 4.00 m/s. How much force is exerted on the table, taking the effective mass of his forearm and hand to be 1.5 kg?

Answer 1

The force is (according to Newton's second law):

$$F=ma.$$

The acceleration is how quickly speed changes:

$$a=\frac{v_f-v_i}{t}.$$

Thus, the force is

$$F=m\frac{v_f-v_i}{t}=1.5\frac{0-4}{0.0025}=2400\text{ N}.$$