1 Force and Laws of Motion S.8.C.3.1.1,2 Unit 3 Lesson 7 The physics classroom website

2 Force A push or pull on an object. Can change the motion of an object Measured in Newtons (N) (kg*m/s 2 ) Friction and gravity are examples. Friction A force that acts against motion. Between objects that are touching Between the legs of a desk and the floor when you push a desk 1 N= 0.225lbs

3 Gravity A force that pulls matter together Between objects that are not touching Isaac Newton –“the force of gravity acts between all objects in the universe” Gravity exists between all matter –Gravity exists between you and your pencil however it is very weak The more mass an object has, the stronger its gravity Keeps objects in orbit around a star, planet or moon. Makes objects “fall”

4 Combining Forces Force Diagrams: We use arrows to show the different forces acting on objects Direction of arrow shows the direction the force is acting Length of the arrow shows the strength (size or magnitude) of the force Net force: the combination of all forces acting on an object 5N = 10N 5N 10N = = 5N 0 N = 5N

5 When two forces are the same strength (magnitude or size) but acting in opposite directions on a the same object. Cannot change an objects motion (speed and direction) An object at rest to start with balanced forces will continue at rest An object moving at 30 mph to the left with balanced forces acting on it will continue to move at 30 mph to the left Balanced Forces Forces are equal Motion remains the same If not moving, it continues to not move If moving, it continues to move exactly the same

6 Balanced Forces Forces on the person are balanced: Gravity pulls down on person, chair pushes up on person 10 N

7 Unbalanced Forces Net force of 40 N to the Right 40 N New motion depends on the starting motion Will change an objects motion (speed and direction) A net force results. Net force is calculated by adding forces in the same direction or subtracting forces in opposite directions If an object is starting at rest when a net force of 40 N  is applied, the object will move to the right If an object is already moving to the right at a constant speed when a net force of 40 N  is applied, the object will speed up.

8 Newton’s 1 st Law of Motion Law of Inertia –Object at rest will remain at rest –Object in motion will continue in motion in a straight line Unless an unbalanced force acts upon it The tendency of an object to resist a change in motion. The higher the mass of an object, the higher its inertia is. Circular motion animation car making a sharp turn

9 Inertia Video Clip Bill Nye- Inertia

10 Newton’s 2 nd Law of Motion F = MA The larger the net force on an object, the more the objects motion will change (greater acceleration). Objects with larger masses, need more force to change their motion.

11 Newton’s 2 nd Law of Motion

12 Newton’s 3 rd Law of Motion When one object exerts a force on another object, the second object exerts an equal but opposite force on the first object They are not balanced forces because the forces act on different objects 100 N

13 Newton’s 3 rd Law Action-Reaction For every action (force), there is an equal but opposite reaction (force) 3 laws Video clipVideo clip Newton’s 3 Laws