Chapter 4 Section 1

Describing Motion

How do you know when something is moving?  You must have a reference point-something that will show if something has changed or moved.

Changing Position:

All motion involves a change of position.

1. distance-how far you went or the distance you traveled.

2. displacement-the direction and distance between the final position and the starting point.  ex. pg. 106 picture 6

speed-the distance traveled by the time needed to travel that distance.

average speed=total distance/total time

velocity-the speed in a particular direction (speed and direction must be given) velocity can change as an object moves.

acceleration-a change in velocity divided by the amount of time over which the change occurs

An object is accelerating if it is speeding up, slowing down, or turning.

Section 2 Forces

force-a push or pull-a force has size and direction

the motion of an object changes when a force acts on it

ex. pg. 110 pictures

inertia-measure an objects tendency to resist changing its motion, this also depends on the mass of the object.

balanced forces-forces that cancel out each other, if forces are balanced, an object's motion doesn't change.

unbalanced forces-forces that do not cancel each other out, the object will move.

friction-force that resists movement between two touching surfaces

friction creates heat and wear on objects

air resistance-a type of friction in the air ex. parachute opening in the sky

gravity-pull that matter exerts on other matter

gravitational pull becomes weaker as the objects move farther apart.

gravitational force is weaker if the mass of the object is less

Section 3

The Laws of Motion:  Newton’s Laws of Motion explain how forces can change the motion of an object.

1. First Law of Motion-an object will remain at rest, or keep moving in a straight line with constant speed, unless an unbalanced force acts on it.

Here on Earth, gravity and friction keep objects form moving in a straight line with constant speed.

Pg. 115 picture

16th century Italian scientist Galileo was one of the first to understand that an object in constant motion is as natural as an object at rest.

Newton’s first law means that an object can speed up-slow down-or change direction only if a force acts on it.

2. Second Law of Motion-an unbalanced force acting on an object causes the object to accelerate in the direction of the force.

Acceleration=force/mass

Force is measured in Newtons.

Newton’s second law means a larger force on an object causes a larger acceleration.

If the same amount of force is exerted, the object with the smaller mass has a larger acceleration.

3. Third Law of Motion-when a force is applied on an object; an equal force is applied by the object in the opposite direction.

Ex: when you push down on the ground, the ground pushes back on you

Force pairs-the forces exerted on an object and by the object

Only if equal and opposite forces act on the same object do they cancel out.

Ex. Pg. 120-fig. 17  pg. 121-fig. 18

According to the third law of motion, forces always act in pairs.

One force in the pair is called the action force and the other is the reaction force.

Example: You push on a wall-action force

         The wall pushes back-reaction force

Example: Hammer hitting a nail-action force

         Nail pushing back on the hammer-reaction force