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Physical Science Forces and Motion Gravity and Motion Moving on to Something New!! • In science a force is simply a push or a pull. • Scientist express force using a unit called the newton (N). Forces, Forces Everywhere!! Take a moment to consider the word force. A force is always exerted by one object on another object. Where do you see a force happening in the room right now? Which object is exerting the force, and which is receiving it? Determining Net Force • Usually more than one force is acting on an object. Net Force is the combination of all the forces acting on an object. Let’s Practice What would be the net force being exerted on this box if the person on the right pushed with a force of 25 N and the person on the left pushed with a force of 17N? What direction would the box move? Friction is a Hater on Motion Friction is a force that opposes motion between two surfaces that are in contact. Friction can cause a moving object, such as a ball, to slow down and eventually stop. Where does Friction come from? • Friction occurs because the surface of any object is rough (even if it feels smooth). • Rougher surfaces have more friction because there are more microscopic hills and valleys compared to an object with a smoother surface. Two Types of Friction Kinetic friction: the word “kinetic” means moving, and kinetic friction depends on the surface where the friction is taking place. Sliding kinetic friction Rolling kinetic friction The other type of Friction When a force is applied to an object but does not cause the object to move, static friction occurs. The word “static” means not moving. The object does not move because the force of static friction balances the force applied. Friction is Harmful and Helpful What if we had a life without friction? Write a short paragraph to describe what life would be like if friction did not exist. Be prepare to share Gravity • Gravity is a force of attraction between objects that is due to their masses. The force of gravity can change the motion of an object by changing its speed, direction, or both. The Size of Earth’s Gravitational Force • Compared with all objects around you, Earth has a huge mass. • You must apply forces to overcome Earth’s gravitational force any time you lift an object or even move your body. The Law of Universal Gravitation • This law describes the relationships between gravitational force, mass, and distance. • It applies to all objects in the universe Part 1: Gravitational Force Increases as Mass Increases • Why can’t you lift an elephant? The amount of gravity between an elephant and Earth is greater than the amount of gravity between you and Earth. Part 2: Gravitational Force Decreases as Distance Increases The Sun is larger than the Earth, so why doesn’t the Sun have a larger gravitational pull on us than Earth? Why is there such a large gravitational force between the planets and the Sun? Weight as a Measure of Gravitational Force • Weight is a measure of the gravitational force on an object. • Mass is the amount of matter in an object What’s volume again?? Acceleration and Velocity • Acceleration is the rate velocity changes over time. The acceleration of an object affected by gravity is 9.8 m/s. • Change in velocity of falling objects can be measured by the following equation: ∆v = g x t OR 9.8 m/s times the number of seconds an object falls… Gravity and Falling Objects • Objects fall to ground at the same rate because acceleration due to gravity is the same for ALL objects • Why? Acceleration depends on both force and mass. – A heavier object experiences a greater gravitational force, BUT it is also harder to accelerate. – Galileo was a genius. Air Resistance • Air resistance – the force that opposes the motion of objects through air – Amount of air resistance depends on the size, shape, and speed of the object Discussion • Why would a feather dropped from the same height as an acorn fall to Earth more slowly than the acorn? Discussion • How can a sky diver benefit from air resistance? Terminal Velocity • Terminal Velocity – when air resistance = gravity – 0 N or a balanced force! – It’s the fastest an object will fall – It’s also why rain drops don’t kill you… think about it… Free Fall • Free fall - when gravity is the only force acting upon an object – Can only happen where there is NO air resistance: SPACE (or a vacuum) • - Orbiting objects are in free fall. Orbit is caused by two motions: - Orbiting objects move forward, but are also in free fall – so are the astronauts Orbiting and Centripetal Force • Orbit is caused by two things that create an unbalanced force – Gravity provides centripetal force to the orbiting object – Centripetal force – the unbalanced force that causes an object to move in a circular path – Ex: planets around the sun or the moon around the Earth Projectile Motion • The curved path an object follows when it is thrown near the surface of the Earth • Composed of horizontal motion and vertical motion – Horizontal motion (like throwing a ball) causes the object to go forward – Vertical motion (gravity) causes the object to fall • Together they create a curved path: BUT still hits the ground the same time as a dropped object – This is why you always have to aim above a target when trying to hit it… An example of projectile motion Discussion 1. If a baseball and a cannonball are dropped from the same height at the same time, and there is no air resistance, which ball will hit the ground first? a. The cannonball lands first. b. The baseball lands first. c. The balls land at the same time. d. The ball with the larger volume lands first. 2. What feature of an object does not affect air resistance? a. its size b. its chemical properties c. its shape d. its speed 4. In what directions is a projectile accelerated? a. both vertically and horizontally b. vertically downward c. vertically upward d. horizontally forward