2. Forces

3. Forces affect Motion A force is a push or a pull that changes an object’s motion. This means that forces make objects speed up, slow down, or change direction. Forces also transfer energy from one object to another object like when you hit a ball with a bat. The SI unit used to measure force is the newton. (N).  One newton is the amount of force needed to accelerate a 1 kg mass by 1 m/sec 2.

4. Gravity and Friction are Forces  The force of gravity on an object is called weight.  Weight is different than mass.  Mass is the amount of matter in an object and weight measures the force of gravity on that mass.  Mass does not change on different planets, but weight does, depending on how strong the force of gravity is!  The force of friction acts to slow objects that are already moving or make them stop.  Friction is caused when objects rub together.  There are three kinds of friction: sliding, rolling, and fluid friction.

5. Types of Friction  Sliding friction - when two objects slide over one another; determined by object’s weight and types of surfaces  Fluid friction - when an object moves through a fluid; the smallest force of the three types of friction for similar objects  Rolling friction - when an object rolls over a surface; a smaller force than sliding friction for similar objects

6. Falling Objects /If no other forces act on an object, gravity causes it to accelerate towards the center of the earth at 9.8 m/sec 2. /At terminal velocity, the forces of friction and gravity are equal and opposite on an object. Its speed remains constant. / Projectile motion is the curved path followed by some moving objects. / Projectile motion is caused by the two forces of a horizontal push and downward gravity.

7. Balanced and Unbalanced Forces According to Newton’s 3rd Law, forces always act in pairs. Forces may be balanced (equal in amount and opposite in direction). Balanced forces cause no change in an object’s motion. Forces can also be unbalanced (opposite in direction but unequal in amount). Unbalanced forces do change an object’s motion (The racquet’s force makes the ball reverse direction!

8. Net Force Net force is the sum of all the forces acting on an object. If the forces are acting in the same direction, add them together to find the net force. Which way will the boats be pulled? (Hint: Don’t forget direction.)  If the forces are acting in the opposite direction, subtract to find the net force. Net force on B = _____ balanced or unbalanced? Net force on C = _____ balanced or unbalanced? 15 N10 N 16N 4 N 12 N C B Net force on A = _____ balanced or unbalanced? N S EW 5 N 15 N AA

9. Momentum and Circular Motion /Momentum is a way to measure “inertia of motion.” It tells how hard it is to stop or start an object’s movement. / An object moving in a circle has centripetal acceleration. / Centripetal force is the force needed to make an object move in a circle. /Momentum = (mass)(velocity)

10. Universal Forces There are four forces that work together to control the universe as we know it. Some of these forces work over very large distances and some can only be detected at very small distances. The four forces are: 3. Strong Force - holds the nucleus of an atom together by binding the protons and the neutrons. 2. Electromagnetic Force - may attract or repel. These are much stronger than gravity. 1. Gravitational Force - is the weakest of the universal forces. It depends on the mass of the objects and the distance between them. 4.Protons, neutrons, and electrons are made of smaller particles called “quarks.” Weak force - glues the quarks together. Quarks stuck together to make a proton