Newton’s Laws of Motion Newton’s 1st Law: An object maintains its velocity unless acted on by a force Newton’s 2nd Law: The acceleration of an object is equal to the net force acting on it divided by its mass: a = F / m rd Newton’s 3 Law: For every force F applied to object B by object A, there is a force –F applied to object A by object B The Big Question: What causes an object to accelerate (i.e. change its state of motion)? How come in football (and in life) you can never hit without being hit back? What this builds on: The idea of motion developed in the previous two units. You know how to solve motion problems given the acceleration, now you will learn where the acceleration comes from and how to find its value. Fundamental Concepts: • An object will not change its state of motion unless a net force acts on it. • If no net force acts on an object the object remains at constant velocity or at rest. • The force of gravity and your weight are the same thing and equal mg • Your mass does not change as you move to other planets, etc.; mass is equivalent to how many atoms an object (or person) contains. • Net force means the addition of all the forces in each of the 3 possible space directions (x,y,z). • Newton’s 3rd law states for every force there is an equal but opposite reaction force. This means if I punch the wall, my hand exerts a force on the wall and the wall exerts a force in the opposite direction on my hand. Fundamental Equation: r r ∑ F = ma this is Newton’s 2nd law and the only formula you need to solve force and acceleration type problems. The vector sign means it’s actually 3 equations (one each for x,y,z): ∑ Fx = max (add up all the forces acting on an object in the x-direction and this equals the objects mass multiplied by its acceleration in x- direction) ∑ Fy = ma y (same as above, but for the y-direction) ∑F z = maz (same, but we usually don’t include the 3rd dimension) Steps to Solving a Force/Acceleration Problem: 1) Identify which is the object of interest. This is the object we care about. 2) Identify all the forces acting on the object. We don’t care about forces (like reaction forces) that act on other objects in the problem. 3) Identify which forces or which pieces of forces are in the x-direction and y-direction and which are at an angle (i.e. in both directions) 4) Draw the Free-body diagram 5) Break the forces that are at angles into their x-parts and y-parts using the angle and SOHCAHTOA. 6) Then simply add the x-forces to the x-forces and the y-forces to the y-forces. 7) Divide both by the mass of the object and you have the acceleration in the x-direction and the acceleration in the y-direction. The different forces and what they are: • Fg = Force of gravity. The force of gravity is often called ‘weight’ and is equal to the mass of the object multiplied by the acceleration of gravity (mg) • FN = Normal Force. Normal means perpendicular and true to its name the normal force is always perpendicular (and outward) to the surface that the object is resting on. • Fspring = Force of a spring. The force of the spring equals the strength of the spring (k) multiplied by the distance it is stretched or compressed (i.e. Fspring=k∆x). The units of k are N/m. • FT = Force of tension. This is the force that a rope pulls on an object. • Fs = Force of static friction. This is the force of friction on an object that is not moving. The force of friction is always parallel to the surface that the object is resting on. Static friction force equals the coefficient of static friction multiplied by the normal force (i.e. Fs = µsFN) • Fk = Force of kinetic friction. This is the force of friction on an object that is moving. Kinetic friction force equals the coefficient of kinetic friction multiplied by the normal force (i.e. Fs = µkFN) Reminder: Room 313 homework parties are every day after school and at lunch on Wednesdays. Dr. Philhour’s site at www.siprep.org has links to Java applets for physics and other tools for learning this material interactively. Check it out … maybe seeing the material from a new angle will help you!
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