Warm-Up

»Which of the following statements can be explained by Newton’s first law?»A) When your car suddenly comes to

a halt, you lunge forward»B) When your car rapidly accelerates,

you are pressed backwards against the seat.

Warm-Up

»An object is moving at a constant velocity. Which statement cannot be true?»A) No forces act on the object»B) A single force acts on the object»C) Two forces act on the object

simultaneously»D) Three forces act on the object.

Warm-Up

»Which of the following graphs, if any, represent a situation in which the net force is 0?

Newton’s 2nd Law

»Newton’s 1st Law explains motion when no net force acts on an object.»Newton’s 2nd Law deals with what

happens when a net force does act on an object.

Newton’s 2nd Law

»When a net external force ΣF acts on an object of mass m, the acceleration a that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass.»The direction of a is the same as

the direction of the net force.

Newton’s 2nd Law

» In a nutshell:

» a = or ΣF = ma

»SI Units of Force: newton (N)» 1 N = 1 kg*m/s2

Example 1

» EX: (page 91, Example 1 Cutnell): Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car. The other person applies a force of 395 N in the same direction. A third force of 560 N also acts on the car, but in the direction opposite to that in which the people are pushing. This force arises because the friction of the pavement opposing the motion of the tires. Draw the free body diagram and then find the net force and acceleration of the car.

a = +0.059 m/s2

Example 2

»EX: If two people are on a tandem bicycle which is careening wildly down a hill and accelerating at 5 m/s2 , find the force on each person if Person A has a mass of 70 kg and Person B has a mass of 40 kg.

Person A = 350N, Person B = 200 N

Example 3

»Example : Forces in “one-dimension”»An applied force of 50 N is used to

accelerate a 10kg object to the right across a frictional surface. The object encounters 10 N of friction. What is the acceleration of the object. (Neglect air resistance.)

Example 4

»Example 2: Using kinematics AND F = ma together»A jet catapult on an aircraft carrier

accelerates a 2000-kg plane uniformly from rest to a launch speed of 320 km/h in 2.0 s. What is the magnitude of the net force of the plane? Convert units to m/s, and use kinematics to find ‘a’.