Ch. 4 Forces and Motion PS2

What is Force? Force: A push or a pull that is exerted on an object. Forces can: Stop motion. Slow motion. Changes direction.

Units of Force Gravity is a force. Gravity pulls objects down toward the Earth’s center and gives objects weight. W = mg (a version of Newton’s 2nd Law). Weight = mass * acceleration due to gravity. Mass, m is in the unit of a kilogram, kg. Acceleration due to gravity, g = 9.8m/s^2 on the Earth.

Units of Force Acceleration due to gravity varies on different planets. The unit of force is the Newton, N. 1 N = 1kg * m/s^2. Ex: m = 5kg, What is the weight of the object on the Earth? Ans: W = (5kg)*(9.8m/s^2) = 49 N

Units of Force Ex: m = 5kg, What is the weight of the object on the Moon? To answer this problem, you need to find the value of the acceleration due to gravity for the Moon which is 1.6m/s^2. Ans: W = (5kg)*(1.6m/s^2) = 8 N If you compare the value on the Earth, 49N to the value on the Moon, 8N; what does it mean?

Multiple Forces/Mass Forces can be either added together, if they are going in the same direction; or subtracted from each other, if they are going in opposite directions. Mass: is a measure of the amount of matter in an object. The mass of an object remains constant, ITS WEIGHT DOESN’T. More force is needed to move an object with a larger mass.

Force of Gravity The Universal Law of Gravitation states that in the universe, every mass attracts every other mass. All objects exert a gravitation force or pull on other objects.

Force of Gravity FG = Gm1m2 / d2 FG is the force of gravity in Newtons, N. G is the Universal Gravitation constant. G = 6.67 * 10-11 N*m2/kg2 The two masses, m1m2 are in kg, kilograms. Distance, d is in meters and it must be squared!

Force of Friction Friction is a force that resists motion. Greater friction is caused by rougher surfaces or heavier objects. Friction also happens when objects move through air and water. Friction causes some energy to be wasted during an energy transformation. Heat…..

The Effect of Friction Friction can be bad, but it can also be good. Bad: Sliding a box across a floor. Bad: Moving engine or mechanical parts. Good: Car tires on the road. Especially when conditions are wet and icy! Good: Walking without slipping and sliding.

Reducing Friction Ways to decrease friction: Lubricating the contact areas between surfaces. Oil, grease, graphite. Using ball bearings. Used in car wheels and roller skates/skateboards. Polishing surfaces. Playground slide. Using wheels. Using a cart or a dolly to move something.

What is Motion? Motion is a change in the position of an object relative to another object, which is assumed to be at rest. Uniform motion is when an object moves in a straight line at a constant speed. If the object changes speed or direction, it doesn’t have uniform motion.

Speed and Velocity Speed is the distance traveled per unit time. Ex: 55mph; 42m/s. Velocity is the speed of an object in a specific direction. Ex: 55mph West; 42m/s South. Note: Both speed and velocity use the same equation! Speed & velocity equation: v = d/t

Acceleration Acceleration is the rate of change in velocity. Acceleration describes the increase or decrease in velocity. Equation: a = vf – vi /t Note: ∆v is vf – vi The units of acceleration are m/s^2 which means m/s for every second you’re moving. Speed/velocity and acceleration equations can be used in substitutions.

Acceleration If you have acceleration, it means your speed is increasing or decreasing. Ex: getting your car to move/increase speed after you stopped for a stop sign or as you get your car to slow down to a stop for a stop sign. If you don’t have acceleration, it means you are not speeding up or slowing down. Ex: the posted speed limit is 35mph and you have your car maintaining that speed.

Motion of a Falling Object Gravity cause objects to fall towards the Earth’s surface. Air resistance or air drag is a force that affects falling objects. It can increase the time it takes for an object to hit the ground. Ex: feather vs. a coin falling.

Motion of a Falling Object In a vacuum, air drag doesn’t affect falling objects. All objects fall at the same rate because the force of gravity is the only force affecting them. We use acceleration due to gravity, g and use the equation: v = a * t for the velocity of a free-falling object.

Motion of a Falling Object The distance of a free-falling object travels is calculated by this equation: d = a * t^2 /2. Check out page 115, Table 4-3 and compare the values listed under the columns of time, acceleration due to gravity, velocity, and distance.

Newton’s Laws of Motion 1st Law of Motion: an object at rest will remain at rest and an object in motion will remain in motion unless acted on by an outside force. Newton’s 1st Law of Motion is also known as the Law of Inertia. Ex: Sudden starting of a car from a stop &/or the sudden stopping of a car from moving and you have a seat belt on. (What happens to you?)

Newton’s Laws of Motion 2nd Law of Motion: the relationship between force, mass, and acceleration is expressed by the equation; F = ma. A large force that acts on a given mass will cause a greater acceleration than would a small force that acts on the same mass. A small mass acted on by a force will have a greater acceleration than would a large mass acted on by the same force.

Newton’s Laws of Motion 3rd Law of Motion: for every action there is an equal and opposite reaction. Ex: Kicking a ball, releasing a balloon and it goes in the opposite direction, rockets.

Important Theories of Physics Mechanics: refers to how bodies change position with time. Special Theory of Relativity: 1905 Einstein described how space and time are affected when the speed of light is approached. General Theory of Relativity: 1916 Einstein added on more to his work and it describes the physics associated with very massive bodies and gravity.

Important Theories of Physics Newtonian Mechanics could not adequately explain the physics of some objects but Einstein’s theories of Relativity could. Theory of Quantum Mechanics: describes the physics of matter and radiation on the atomic level and was developed during the 1920s through the works of several physicists.

Important Theories of Physics Quantum Mechanics helps to explain the wave-particle duality of electromagnetic radiation that Newtonian Mechanics cannot. Newtonian Mechanics is the basis for understanding everyday physical phenomena. It can explain the behavior of objects larger than an atom and ones moving slower than the speed of light.