Gravity is a force of attraction that acts between all objects with mass. Gravity causes all objects to accelerate towards each other at the same rate, so heavier and lighter objects fall at the same speed. The strength of gravity depends on the mass of the object, so gravity is stronger on large planets like Jupiter than on smaller bodies like the moon. Mass is a measure of the amount of matter in an object and does not change in different locations, while weight is the measure of the gravitational force acting on an object, so weight varies depending on the gravitational pull of different planets. Newton's three laws of motion describe how forces cause motion and changes in motion.

1. Chapter 10 Forces and Motion

2. Gravity Gravity is a force of attraction that acts between bodies that have a mass. Gravity was discovered by Sir Isaac Newton. Gravity acts on all objects the same. If you drop a bowling ball and a tennis ball at the same time, they will both hit the ground at the same time because gravity is a constant. The more matter something has the more gravitational pull it has. Our moon is much smaller than Earth and only has about 1/6 the amount of gravity . Jupiter’s gravity is about 2.5 times stronger than Earth’s because of its size. That is why people weigh different weights on different planets.

3. How to Measure Gravity Weight is a measure of how much Earth’s gravity pulls down on an object. Mass is the amount of matter in an object, and mass does not change from place to place. While your weight would change on the moon, your mass would not change. Mass is measured with a balance. Weight is measured with a scale. Mass and weight are two different things, so we have to use two different units for them. Mass is measured in g or kg. Weight is measured in Newtons (N). A 10g weighs 1N on Earth. So if a rock had a mass of 30g, it would weigh 3N.

4. Speed Speed is the measure of how far you move in a given time. To find speed you need to know time and distance. If you rolled a ball down a hill that was 100 meters long and it took 20 seconds to reach the bottom, its speed is 5 m/s. So the equation for velocity equals, v=d/t or distance over time. The speed of the ball throughout the trip changed. It started off slow and got faster as it rolled down. So, this calculation gives you an average speed and describes the movement for the entire trip down the street.

5. Stopping Gravity speeds you up as you go down a hill, but what makes you stop. Friction. Friction is the push or pull that opposes motion between two surfaces that are touching each other If you are roller blading, you might use the rubber stopper to cause friction against the street to help you stop. H.W. Pg. 274, ques. 1 & 2. Page 288, 1-10.

6. Newton’s Three Laws of Motion Before we talk about the laws, we have to learn about force, which is a push or a pull. Gravity is a force, as well as friction. Everytime you turn a page or open a door, you are using force to do so. Law #1: Inertia- An object at rest tends to stay at rest, and a moving object moves in a straight line until it is acted upon by an outside force. In other words, things don’t stop or start by themselves. The reasons things stop is because of friction.

7. Newton’s Second Law This law involves force, mass and acceleration. Law #2: Acceleration depends on the mass of an object and the force pushing or pulling the object. Force = mass x acceleration, F=mXa Acceleration = force/mass, a=F/m It takes less force to accelerate something with a small mass. This is why large trucks need more powerful motors to get them going from a complete stop. If you want to go faster, you have to use more force. Like having to pedal harder on a bike.

8. Newton’s Third Law Law #3: For every action there is an equal and opposite reaction. If you push on a wall, the wall is pushing back at you at the same rate.

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