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Astronomy & Astrophysics
Michael Reid

Chapter 4Making Sense of the Universe41 Describing Motion Examples From Daily Life How do we describe motion y Terms used to describe motion in sciencevelocity acceleration and momentumSpeed Velocity and Acceleration y The speed of the car tells us how far it will go in a certain amount of time For example 100 kilometers per hour is a speed and it tells us that the car will cover a distance of 100 kilometers if it is driven at this speed for an hour y The velocity of the car tells us both its speed and direction For example 100 kilometers per hour going due north describes a velocity y The car has an acceleration if its velocity is changing in any way whether in speed or direction or both y In science we also say that you are accelerating when you slow down or turn not only the increase of speed y Slowing down or deceleration occurs when acceleration is in a direction opposite to the motion acceleration is negative causing velocity to decrease y Turning changes your velocity because it changes the direction in which you are moving so turning is a form of acceleration even if your speed remains constant y You can feel the effects of acceleration For example as you speed up in a car you feel yourself being pushed back into your seat As you slow down you feel yourself being pulled forward from the seat As you drive around a curve you feel yourself being pushed away from the direction of your turny In contrast you dont feel such effects when moving at constant velocity That is why you dont feel any sensation of motion when youre traveling in an airplane on a smooth flightThe Acceleration of Gravity y Gravity accelerates all objects by the same amount regardless of their mass y This fact may be surprising because it seems to contradict everyday experience a feather floats gently to the ground while a rock plummets However air resistance causes this difference in accelerationy If you dropped a feather and a rock on the Moon where there is no air both would fall at exactly the same rate y The acceleration of a falling object is called the acceleration of gravity abbreviated g 2y On Earth the acceleration of gravity is about 98 ms y For example suppose you drop a rick from a tall building At the moment you let it go its speed is 0 ms after 1 second the rock will be falling downward at about 10ms After about 2 seconds it will be falling at about 20 ms in the absense of air resisitance its speed would continue to increase by about 10ms each second until it hits the ground We therefore say that the acceleration of gravity is about 10 meters per second squaredMomentum and Force y An objects momentum is the product of its mass and velocity that is momentummass v velocity y The only way to change an objects momentum is to apply a force to it y Forces of some kind are always present For example on Earth we always feel the force of gravity and there are always electromagnetic forces acting between atomsy The net force acting on an object represent the combined effect of all the individual forces put together y There is no net force on your car when you are driving at constant velocity because the force generated by the engine to turn the wheels precisely offsets the forces of air resistance and road frictiony A change in momentum occurs only when the net force is not zero y Changing an objects momentum means changing its velocity as long as its mass remains constanty A net force that is not zero therefore causes an object to accelerate y Planets are always accelerating as they orbit the Sun because their direction of travel constantly changes as they go around their orbits We can conclude that some force must be causing this accelerationMoving in Circles y Any object that is either spinning or moving along a curved path has angular momentum y For example Earth has angular momentum due to its rotation and to its orbit around the Sun y The type of force that can change an objects angular momentum is called a torque which you can think of as a twisting force y The amount of torque depends not only on how much force is applies but also on where it is appliedHow is mass different from weight y Your mass is the amount of matter in your body y Your weight or apparent weight is the force that a scale measures when you stand on it that is weight depends both on your mass and on the forces including gravity acting on your mass y Your mass depends only on the amount of matter in your body and is the same anywhere but your weight can vary because the forces acting on you can varyy For example your mass would be the same on the Moon as on Earth but you would weigh less on the Moon because of its weaker gravity
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