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A force is…. a push or a pull. Friction, Drag, Gravity, and Weight are forces…. Newtons. Newton- the SI (international system of units) unit for force. Symbol - N. Force. Its not always easy to tell what is exerting the force and what is receiving the force. Forces in the Same Direction.

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## A force is…

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**Aforce is…**a push or a pull. Friction, Drag, Gravity, and Weight are forces…**Newtons**Newton- the SI (international system of units) unit for force. Symbol - N**Force**Its not always easy to tell what is exerting the force and what is receiving the force.**Forces in the Same Direction**Two students will now come to the front of the room to demonstrate. Net Force 25N + 20N = 45N To the direction of the moving object.**Forces in Opposite Direction**Two students will come to the front of the class to demonstrate Tug of War. Net Force 12N - 10N = 2N In the direction of the winner.**Forces**There are many different forces which act on objects. If forces are balanced the object doesn’t move If forces are unbalanced the object will do one of the following things: • Begin to move • Stop moving • Change direction • Speed up • Slow down • Change shape**Balanced Forces**The ship is floating on the water, the forces are balanced. Which forces are acting on the ship? Because Gravity and Upthrust are equal the ship stays afloat. (Water’s) Upthrust Gravity**Balanced Forces**The vase is resting on the table. It is not moving, therefore the forces must be balanced. Which forces are acting on the vase? Have a look around the classroom and name some of the balanced forces in action on different objects. Gravity Table’s Upthrust**Unbalanced Forces**Air resistance is pushing upwards The man and the parachute are slowly falling to the ground. Which forces are acting on the parachute? Which force is bigger? How can you tell? Gravity is the bigger force. We can tell this because the direction of movement of the man and the parachute is the same as the direction of the force of gravity. Gravity is pulling the man to the ground**Unbalanced Forces**Air resistance is pushing upwards The same forces are acting on the elephant but without a parachute there is much less air resistance. Gravity is a much bigger force. Because there is a bigger difference between the force pulling downwards and the force pushing upwards the elephant falls much more quickly than a man with a parachute. Gravity is pulling to the ground**Friction**• A force that opposes motion between two surfaces that are in contact. • Depends upon two factors: • Force pushing the surfaces together. • Roughness of the surfaces.**From Glencoe Physical Science**Force pushing the surfaces together.Roughness of the surfaces.**Kinetic Friction**• Friction between two moving surfaces. • Kinetic means moving. • Surfaces can slide past one another. • Surfaces can roll over other surfaces. • This type is used in transportation • The force of sliding kinetic friction is greater than the force of rolling kinetic friction.**From Glencoe Physical Science**Static Friction • When a force is applied to an object, but does not cause the object to move. • “static” means not moving. • The force of static friction balances the force applied. • Static friction disappears as soon as the object starts moving, then kinetic friction takes over.**What is Gravity?**• Gravity is a force of attraction between two objects that is due to their masses.**Gravity: A Force of Attraction**• All matter has mass. • Gravity is the result of mass. • Because of gravity you are being pulled toward your desk, pencil, and paper. • So why don’t I notice the objects moving towards each other? • The mass of most of the objects are too small to cause a force large enough to move the objects towards each other.**Sir Isaac Newton (1643-1727) an English scientist and**mathematician famous for his discovery of the law of gravity also discovered the three laws of motion.**Newton**• Legend is that Newton made a connection as he saw an apple fall from the tree. • Unbalanced forces were needed to change the motion of objects • The unbalanced forces on the apple made it fall • He reasoned that unbalanced force on the moon kept it moving circularly around the earth.**Law of Universal Gravitation – any two masses exert an**attractive force on each other. The amount of attraction depends upon two things: the mass of the objects and the distance between the objects.**Weight as a measure of Gravitational Force**Mass- the amount of matter in an object has. Matter- something that has mass and takes up space. Weight- is a measure of the gravitational force on an object.**Units of Weight and Mass**• SI unit of Force is the Newton. • Gravity is force and weight is the measure of gravity, so weight is also measured in Newtons. • The SI unit of mass is the kilogram.**Unbalanced Forces**The paperclip is jumping up to the magnet. Which forces are acting on the magnet? Which is the greater force? The magnetic force is pulling the paperclip upwards The magnetic force is greater than the force of gravity therefore the paperclip moves in the direction of the biggest force. Gravity is pulling the paperclip to the ground**Forces**Each team is exerting a force – what happens if the pulling force produced by each team is equal? What would happen if the team on the left were able to produce a bigger pulling force than the team on the right?**What is a force?**• A force is a push or pull that causes an object to move, stop, or change direction • In physics a force is anything that makes an object accelerate**Force**• Forces come in pairs • Forces have a magnitude and a direction Magnitude: 5N Direction: north (up) 5N, north (up)**Balanced and Unbalanced Forces**• Forces occur in pairs and they can be either balanced or unbalanced**Balanced Forces**• Balanced forces do not cause change in motion • They are equal in size and opposite in direction**Unbalanced Forces**• An unbalanced force always causes a change in motion • When unbalanced forces act in opposite directions you can find the net force • Net force • Magnitude • The difference between the two forces • Direction • Direction of the largest force**Unbalanced Forces**3 N, right – 6 N, left = 3N, left**Unbalanced Forces**4 N, left – 10 N, right = 6N, right**Unbalanced Forces**5 N, right + 10 N, right = 15N, right**A combination of all the forces acting upon an object.**Net Force**Net Force**Thrust Drag 100 N 120 N The difference is 20 N, to the left. The car will speed up.**Inertia = Mass**Bigger masses have more… Inertia Smaller masses have less inertia.**Inertia = Resistance to…**• changes in motion. • net forces. • changing velocity. • changing direction.**Which can accelerate easily?**Small masses change motion easily.**Inertia = resistance to changes in motion.**Large masses resist net forces and do not accelerate much. A large mass resists changing its velocity.**What if we put rockets on each creature?**Small masses change motion easily. Large masses resistchanging motion.**Is a force needed to keep an object in motion?**No! An object in motion stays in motion at constant velocity forever! (Unless a new net force acts…)**If no net force…**Fnet = MA 0=MA No Acceleration Constant Speed!**Inertia**Would this experiment work well using foam plates and plastic cutlery?**stays at rest unless acted on…**by a net force.**Inertia = resistance to changes in motion.**Which is harder to move? The gorilla has more inertia.

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