Chapter 10 Forces - Section 3: Newton’s First and Second Laws What is Newton’s first law of motion? What is Newton’s second law of motion? Key Concepts:

Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.unless acted upon by an unbalanced force

So, an object at rest will remain at rest... and

An object in motion will remain in motion at a constant velocity…………

Unless acted upon by an unbalanced force  An unbalanced force will cause an object to speed up, slow down, or change direction.

Inertia  Inertia is the tendency of an object to resist a change in motion.  Newton’s first law of motion is also called the “law of inertia.”  If you don’t want to move, someone may call you “lazy” or “inactive”, this is what inertia means in Latin.

Inertia  Inertia depends on mass…..the greater the mass, the greater the inertia.

Inertia

Newton’s Second Law  The force exerted on an object is equal to the acceleration multiplied by the mass of the object on which the force is acting.”

Newton’s Second Law of Motion  Acceleration depends on the net force acting on the object and the object’s mass.  More mass – More Force needed.  Less Mass – Less Force needed.  Formula is: Net Force = Mass x Acceleration

Newton’s 2nd Law Basics:  Acceleration is measured in meters per second per second (m/s 2 )  Net force is measured in newtons (N)  Mass is measured in kilograms (kg)  Acceleration = Net Force Mass Mass  Net Force = Mass x Acceleration  1 Newton is the force required to give 1 kg mass an acceleration of 1 m/s 2

Calculating Force A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s 2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s 2 Part 1

Calculating Force Plan and Solve What quantity are you trying to calculate? The net force F = ? What formula contains the given quantities F = m x a Perform the calculation. F = m x a = 55 kg x 2.0 m/s 2 F = 110 kg m/s 2 F = 110 N Part 2 Problem: A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.

Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ? F = m x a F = 1,000 kg x 3 m/s 2 F = 3,000 kg. m/s 2 F = 3,000 N