## SAT Physics Subject Test

**Chapter 5 ****Linear Momentum**

When an object moves, it is necessary to account for both its mass and its velocity. Think for a minute about the difference in being hit with an inflatable beach ball traveling at 15 m/s (33 mi/h) or being hit by a motorcycle traveling at the same velocity. Ouch! Mass is definitely important. Velocity’s “impact” (pun intended) is revealed by considering the difference between a baseball hit by an eight-year-old little leaguer and one hit by an adult Major League player. In this chapter, we will discuss linear momentum, impulse, what happens when objects collide, and the center of mass of a system of objects.

**ANOTHER LOOK AT NEWTON’S SECOND LAW**

When Newton first expressed his second law, he didn’t write **F**_{net} = *m***a**. 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** = *m***v**

Newton’s second law can be written as

is the same as = *mā*, since Δp/Δ*t* = Δ(*mv*)/Δ*t* = *m*(Δv/Δ*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 and 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

*p*_{car} = *m*_{car} *v* = (1,000 kg)(20 m/s) = 20,000 kg ×m/s.

while the magnitude of the truck’s linear momentum is

*p*_{truck} = *m*_{truck} *v* = (5,000 kg)(20 m/s) =100,000 kg × m/s.

**The Skinny on Momentum**

Momentum is defined by the following equation: **p** = *m***v**, where *m* is the mass of the object and **v** the velocity of the object. Momentum is measured in units of kg m/s and is a vector (so it has both magnitude and direction). The more momentum an object has, the more you don’t want to be hit by it.

Although the car and truck have the same speed, the truck has more momentum because it has more mass.