2.4: Newton’s Second Law of Motion

What if...

• If you kept increasing the amount of applied force on an object, would its acceleration increase or decrease?

• If kept the applied force constant but increased the object’s mass, would its acceleration increase or decrease?

Consider the following:a) race car b) semi-trailer c) smart car

Newton’s Second Law of Motion

An object accelerates in the direction of the net force. The magnitude of an object’s acceleration is directly proportional to the net force acting on it

and is inversely proportional to its mass.

when is constantIn equation form:

Fnet = ma Fnet → net force (N)m → mass (kg)a → acceleration (m/s2)m a

Fnet

Does Newton’s Second Law agree with the First Law?

Second Law: Fnet = ma

If Fnet = 0, then a must also be zero.

The object must be either at rest or moving at constant velocity. Therefore, the laws are consistent.

Example 1A water polo ball of mass 0.45 kg accelerates at a rate of 130 m/s2 [W]. Calculate the applied force.

Example 2In an extreme test of its braking system under ideal conditions, a Honda CR-Z, travelling initially at 26.9 m/s [S], comes to a stop in 2.61 s. The mass of the car, with the driver, is 1.44 x 103 kg. Calculate a) the car’s acceleration

Example 2, cont’d

b) the net force required to cause that acceleration.

Homework

Pg. 133 #1-6Pg. 136 #1-6,11