Forces and Motion GHSGT Tutoring Day 4

Forces, Motion, and Gravity /A force is a push or a pull that changes motion. /Forces transfer energy to an object. /The force of gravity causes objects to have weight. /The SI unit used to measure force is the newton. (N). /One newton is the force needed to change the acceleration of a 1 kg mass by 1 m/sec 2. /The force of friction acts to slow the movement of objects.

Falling Objects /If no other forces act on an object, gravity causes it to accelerate towards the center of the earth at 9.8 m/sec 2. /At terminal velocity, the forces of friction and gravity are equal and opposite on an object. Its speed remains constant. /Projectile motion is the curved path followed by some moving objects. /Projectile motion is caused by the two forces of a horizontal push and downward gravity.

Weight is a measure of how hard the earth’s gravity pulls an object. Weight = mass x acceleration due to gravity Example: Mia has a mass of 40 kg. How much she weigh on Earth? Weight = 40 kg x 9.8 m/s 2 = 392 N

Newton’s Laws of Motion /1st Law: An object at rest stays at rest, and an object in motion stays in motion, unless acted on by an outside, unbalanced force. /2nd Law: Force = (mass)(acceleration) F = m a /3rd Law: For every force or action, there is an equal and opposite force or action.

Work = force x distance (W = F x d) When you lift a chair, you do work on the chair. Carrying the chair or holding the chair above your head does NOT do work on the chair. You can push on a heavy box all day, but unless you move the box, no work is done. Work is measured in Joules. W is in joules, F is in Newtons, and d is in meters

Calculate the joules of work done: Move a chair 2 meters using 10 newtons of force.

Mechanical advantage is the number of times a machine increases the effort force. MA = F r (resistance force) F e (effort force)

Calculate the mechanical advantage: Taylor applied 20 N of force to turn an ice cream crank. The ice cream’s resistance was 60 N. What was the mechanical advantage of the crank?

Universal Forces /Gravitational Force - is the weakest of the universal forces. It depends on the mass of the objects and the distance between them. /Nuclear Forces -The strong force holds the nucleus of an atom together by binding the protons and neutrons. The weak force holds the particles together that make protons and neutrons. /Electromagnetic Forces -may attract or repel. These are much stronger than gravity.

Motion and Energy

Frame of Reference èAn object’s movement is judged by comparing the position of the object to another object. èThe object you choose for comparison appears to be still. èWe call this comparison object your “frame of reference.” èThe most common frame of reference is the earth, but other things may also be used. èWhile inside the vehicle, you might use the inside of a moving car or a plane as your frame of reference. èAn astronaut in space might use his spacecraft, or the moon, the sun, or a star as his frame of reference to judge his motion.

Measuring Motion èSpeed is a measure of how fast you are moving. èSpeed = distance/time èVelocity is a measure of both speed and direction. èVelocity can change even if your speed doesn’t. (change of direction!) èIf an object is traveling at constant speed, then its speed does not change. èA “distance/time” graph of an object moving at constant speed will be a straight line.

Find the velocity in the following problem: Nancy runs 100 meters in 12.5 seconds.

Graphing Speed èSpeed is graphed using a “distance as a function of time” graph. èThe graph should always have a proper title. Both axes must be labeled and units must be shown. èThe distances should be plotted on the y-axis, and time should be plotted on the x-axis. èSpeed equals the slope of the line graphed. èThe steeper the slope, the faster the speed. èAverage speed = total distance / total time Distance as a Function of Time Distance (kilometers) Time (hours)

More Graphs èMany times, the line on the distance-time graph is not straight. These lines also give you information about the object’s motion. Time Distance Time Distance Time Distance Time Distance Object is moving at constant speed. Object moves at constant speed, then stops, then moves again at a different constant speed. Object is accelerating (speeding up). Object is decelerating (slowing down).

Acceleration èAcceleration is a change in velocity. This means that your speed can change, your direction can change, or they both can change. èThe formula for calculating velocity is: a = (v f – v i) /t t èPositive acceleration means “speeding up.” èNegative acceleration means “slowing down.” èChanging direction without changing speed is still acceleration. èCentripetal acceleration causes things to move in a circular path..

Calculate the acceleration in the following problem: A car accelerates from 10 m/s to 22 m/s in 6 seconds.