Newton’s Second Law

• In this chapter we investigate the effect of a net force on a mass. Newton’s Second Law is:

• Whenever an unbalanced force acts on a body, it produces in the direction of the force an acceleration that is directly proportional to the force and inversely proportional to the mass of the body.

• F = ma

Newton’s Second Law

Variation of acceleration with forceWhen force is increased, the acceleration increases proportionally.

Variation of acceleration with massWhen mass is increased, the acceleration is related inversely

Mass and Weight

• Mass is a universal constant that indicates the amount of matter in an object

• Weight is the gravitational force that acts on an object, it varies with location

• W = mg

Mass and Weight

• Find the mass of a body whose weight on earth is 100 N. If this mass were taken to a planet where g = 2.0 m/s2, what would its weight be on that planet?

• m = W/g = 100 N/9.8 m/s2 = 10.2 kg • Weight on planet = (10.2 kg)(2.0 m/s2) = 20.4 N

Applications of Newton’s 2nd Law

• A force of 29 N acts on a 7.5 kg mass in an east direction. What is the resulting acceleration?

• A = F/m = 29 N/7.5 kg = 3.87 m/s2

Horizontal Forces

• Horizontal Forces typically oppose frictional forces. When the applied horizontal force equals the friction force, the object moves at constant speed.

Fapplied

Ffriction

Vertical Forces

When an object is subjected to an upward force, the downward pull of the object’s weight acts against this force. The net force is the difference of the upward, tension force and the downward weight

Ft

W

Fnet = Ft - W

Mass connected to a pulleyConsider the forces that act on the two masses connected by a string over a pulley. The mass on the table has a vertical weight balanced by the normal force. The tension T acts in the horizontal direction.The hanging mass has an upward T, and downward weight, the net force is the difference between these two forces