Forces and Motion

1. Forces and Motion Chapter 2 – Gravity, Motion, and Light

2. Focus activity • You are an astronaut working on the outside of the international space station. You do not have a rocket back pack. Your line breaks and you begin to float away from the space station. All you have is your tool box which is loaded with tools. Are you able to get back to the station. If so how?

3. Matter • Anything that has shape and takes up space. • All matter is composed of Atoms • Atoms have several subatomic particles. • _________&_________ – In the nucleus • __________ – Orbit around the nucleus • All matter, and subsequently atoms, are subject to four forces of Nature.

4. Forces of Nature • – The force of attraction that is between two bodies and is generated by their masses. • More later • – The force arising between electrically charged particles or between charges of a magnetic field. (holds electrons to the nucleus) • More later • force – Only important on a subatomic scale. Holds the protons to the neutrons in the nucleus. • force – Only important at an atomic scale and is responsible for radioactive decay.

5. Gravity • _______gives the universe its structure. • It acts on all objects. • Every particle is drawn toward every other particle by its pull. • – The tendency for an object at rest to remain at rest, and a body in motion to remain in motion in a straight line at a constant speed. • Inertia was first demonstrated by Galileo, but describe in to the law we know today by Isaac Newton.

6. Newton’s first law of motion • The law of inertia • Objects at rest remain at rest unless a force is acted upon it. Objects in uniform motion continue moving in a straight line unless other forces act upon it. • Newton’s first law explains how planets move around the sun. • Video

7. Activity • Use your stop watch to time how long it takes the ball to hit the ground when thrown. • Use your stop watch to time how long it takes the ball to hit the ground when dropped. • What was the vertical height of the ball when thrown? • What was the vertical height of the ball when dropped? • What forces were acting on the dropped ball? • What forces were acting on the thrown ball? • Repeat the experiment using your lab table and car. (Use a jacket to make a cushion for your car.) • Are there any similarities in your results?

8. Determining the rate of drop • Velocity is how fast an object is moving in one direction. • Velocity = • V=d/t so • T = d/v • On Earth gravity causes objects to fall at 9.8 m/s2 • Notice that an objects mass has little to do with its rate of fall in this example. • However, an objects mass has a lot to do with how quickly an object can accelerate

9. Newton’s second law of motion Newton’s second law deals with a change in direction or the acceleration of an object. • – A change in an object’s velocity. (a deviation from uniform velocity). • Acceleration can be positive - speeding up • Acceleration can be negative - slowing down • A change in direction is also a change in acceleration. • – The amount of acceleration that a force can produce, depends on the mass of the object being accelerated.

10. The math behind Newton’s 2nd law • __________ = object’s mass x acceleration. • F = ma • Force is measured in Newtons, mass in grams, and acceleration in meters/sec. • Shortly you will measure the amount of force necessary to move two objects across two different surfaces. • Newton was able to deduce the law of gravity using the moon’s motion and applying his second law of motion.

11. The law of gravity • Every mass exerts a force of attraction on every other mass. The strength of the force is directly proportional to the product of the masses divided by the square of their separation. • OR simply : Gravity = GMm/r2 • G is a constant = 6.67x10-11 • We will not be calculating the force of gravity between two objects

12. The effects of Gravity • Gravity is the force that keeps us “attached” to the Earth. • It also keeps the moon orbiting around the Earth. • The moon’s gravitational pull is what causes the tides. • We will discuss the tides later in the semester. • Newton then used is first two laws to state his third law – action-reaction.

13. Newton’s third law of motion • When two bodies interact, they create equal and opposite force on each other. See fig 2.8 in book. • When I asked how would you return to the space station with only a tool box in your possession? • We relied on Newton’s third law. The force you applied to the tool box was equal and opposite to the force that the tool box applied on you, thus it would force you to move in the opposite direction.

14. Pulling g’s • The term pulling g’s refers to the gravitational attraction at a planet’s surface. • Otherwise known as surface gravity. • Surface gravity (g) is VERY important. • It determines your: weight • The celestial body’s shape • The presence &/or the components of the atmosphere • The formula for determining surface gravity is: g = GM/R2. • G = constant 6.7x10-11 • M = The mass of the attracting body • R = Radius of the attracting body

15. Determine your weight on the Moon • Mass of the moon = 7.3x1022 • Radius of the moon = 1.7x106 • G= 6.7x10-11 • Formula for g = GM/R2 • The g on earth = 9.8 • divide 9.8 by gmoon and that is how many times different your weight is on the moon than on earth • Multiply your weight by that number to get your lunar weight.

16. Escape velocity • Escape velocity = √2GM/R • G = constatnt (6.7x10-11) • M= Mass of the body be escaped from • R= Radius of the body to be escaped from • The escape velocity from the moon is 2.4 km/second • The escape velocity from the Earth is 11 km/second. • Why is it easier to escape the moon than Earth? • Does escape velocity have an effect on the ability of a planet to sustain life?