Chapter 6 Forces & Motion Section 2 Newton’s Laws of Motion pages 158 to 165

1st Law of Motion (aka Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

Newton’s First Law (law of inertia) INERTIA is a property of an object that describes how much it will resist change to the motion more ____ means more ____ mass inertia

Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

1st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

1st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

2nd Law Force = mass • acceleration F = ma

NEWTON’S SECOND LAW OF MOTION The acceleration of an object depends on the mass of the object and the amount of force applied to it. Equation: F = m∙a Force = mass times acceleration

NEWTON’S SECOND LAW OF MOTION In other words… Force causes an object to accelerate, while the object’s mass resists the acceleration. The larger the object (the more mass it has) the harder it is to accelerate.

NEWTON’S THIRD LAW OF MOTION For every action there is an equal and opposite reaction. ACTION-REACTION FORCE PAIRS DO NOT ACT ON THE SAME OBJECT! See Figure 7 & Figure 8 on p. 164.

Other examples of Newton’s Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

Action: tire pushes on road Reaction: road pushes on tire

Newton's Laws 1stlaw: Homer is large and has a lot of mass, therefore he has a lot of inertia. Friction and gravity oppose his motion. 2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force. 3rd law: Homer pushes against the ground and it pushes back.