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Forces Part 1: What are Forces? What Is A Force? A force is either a push or a pull. Forces are examples of vectors. They have a magnitude and a direction. What Do Forces Do? Forces can have 4 effects on an object. Make it start moving. Make it stop moving. Make it change direction. Make it change shape. Names of Forces Contact Forces Distance Forces Normal Gravity Buoyancy Electrical Friction (+ Air Resistance) Magnetic Tension Spring Applied Measuring Forces Forces are measured using a spring scale. The strength of a force is measured in newtons (n). One newton (1 n) is the roughly amount of force used to pick up an apple off the floor. Interacting Forces Forces usually act together. When 2 forces act in the same direction, the forces add together. 80 40NN Interacting Forces Forces can be balanced or unbalanced. When 2 forcesforce, act inor equal and opposite The resulting net force, is zero. they are balanced. The directions, object’s motion does not change. 15 N no net force Interacting Forces Forces can be balanced or unbalanced. Butthe when forces unequal, Since net two force is notare zero, the object thein forces are unbalanced. will move the direction of the net force. 20 N 5N net force of 5 N to the right Drawing Forces Forces are shown with arrows. The length of the arrow is the size of the force NORMAL The direction of the arrow is the direction of the force APPLIED FRICTION GRAVITY Free-Body Diagrams Forces are drawn using free-body diagrams. Free-body diagrams show an object as a empty box + the forces acting on it. What forces are acting on this crate? FNormal FGravity Free-Body Diagrams The skydiver is accelerating downward. F Air resistance FGravity Free-Body Diagrams The car is moving at a constant speed left. FNormal FApplied FFriction FGravity Forces Part 2: Friction Introduction Friction is caused when 2 surfaces rub together. Friction is a force that acts in the opposite direction of a moving object. Friction will cause an object to slow down or stop moving completely. How Friction Works Friction is caused by rough surfaces. No surface is perfectly smooth. Under a microscope, most surfaces have rough edges When surfaces rub together, these edges catch on each other, creating friction! Showing Friction Friction can be shown on a force diagram: Showing Friction Friction can be shown on a force diagram: FNormal FApplied FFriction FGravity Types of Friction The ear is divided into 3 major sections: 1. Static Friction Occurs between objects that aren’t moving. Types of Friction The ear is divided intoFriction 3 major sections: 2. Sliding When solid objects slide over a surface. Types of Friction The ear is divided intoFriction 3 major sections: 3. Rolling When objects roll over a surface. Types of Friction The ear is divided into 3 major sections: 4. Fluid Friction When an object moves through a fluid. Fluid Friction = Air Resistance The fluid friction created when an object moves through the air is called air resistance. friction FApplied FAir Resistance Fluid Friction = Air Resistance friction The fluid friction created when an object moves through the air is called air resistance. FAir Resistance FGravity Good & Bad Friction Friction can be helpful or unhelpful. “Good” Friction “Bad” Friction Brakes Engine damage Shoes Tire wear Pencils Running on sand Sand paper Reducing Friction Friction always occurs, but it can be reduced. There are 2 ways to reduce friction between surfaces. Make the surfaces smoother, such as using sand paper. Reducing Friction Friction always occurs, but it can be reduced. There are 2 ways to reduce friction between surfaces. Add a liquid lubricant to make the surfaces smoother and slide easier. Example: oil Forces Part 3: Mass, Weight, and Gravity Weight If you wanted to know how much you weigh, you would just step on a scale. Metric scales give measurements in several units: Milligrams (mg) Grams (g) Kilograms (g) A Problem With Mass There is a problem with our everyday wording. When we describe how much an object weighs, we actually are measuring its mass. Mass is a measure of how much matter something is made out of. A Problem With Mass We say the apple “weighs” 100 g. But we should really say the apple has a mass of 100 g. “Weight” A Minute! What does weight mean, then? Weight is a force. Weight is measured in newtons (n). Weight is a measurement of the force of gravity pulling on down on an object. “Weight” A Minute! This apple has a mass of 100 g. And it has a weight of 1 n! 1n Another Example Take this soda can and put it on a scale: The scale shows a mass of 350g. Take the same soda can and put it on a spring scale: The meter shows a weight of 3.5 n. Remember! Mass is a measure of how much matter something is made out of. It is measured in mg, g, kg and t. Weight is a measure of the force of gravity acting on something. It is measured in newtons (n). Losing Weight! The reason why we have weight on Earth is because Earth’s gravity pulls on our mass. Einstein has a mass of 50 kg on Earth. Which equals a weight of 500 n. But this situation is different on Mars… Losing Weight! Instant weight loss. Cool! Einstein still has a mass of 50 kg: that doesn’t change! But his weight will be less, since the force of gravity is less. His weight is closer to 400 n on Mars! That Soda Can, Again! Our soda can has a weight of 3.5 n on Earth. But it will weigh different on other planets! Earth 3.5 n Mars 2.5 n Jupiter 7.0 n Pluto 1.7 n The space shuttle has a mass of 4,500 kg! But in space, it is weightless. Why? Any Questions?