Cracking the SAT Physics Subject Test

ANOTHER LOOK AT NEWTON’S SECOND LAW

When Newton first expressed his second law, he didn’t write Fnet = ma. Instead, he

expressed the law in terms of something we refer to nowadays as linear momentum.

Linear momentum is the product of mass and velocity and is symbolized

by p.

p = mv

Newton’s second law can be written as

is the same as F = mā, since ∆p/∆t = ∆(mv)/∆t = m(∆v/∆t) = mā.

1. What is the linear momentum of a car of mass 1,000 kg that is
   moving at a speed of 20 m/s? What is the momentum of a truck of
   mass 5,000 kg moving at the same speed?

Here’s How to Crack It

The magnitude of the car’s linear momentum is

pcar = mcarv = (1,000 kg)(20 m/s) = 20,000 kg · m/s.

while the magnitude of the truck’s linear momentum is

ptruk = mtruckv = (5,000 kg)(20 m/s) = 100,000 kg · m/s.

The Skinny on Momentum

Momentum is defined by the following equation: p = mv, where m is the

mass of the object and v the velocity of the object. Momentum is