14Facts of FrictionIn addition to force that moves objects, there is a force called friction that can slow down objects. Friction resists, or ?ghts, motion between objects that are touching.Anytime two objects are touching, there is friction between their surfaces. Have you ever rubbed your hands together to keep warm on a cold day? You were producing friction. You sensed that the friction would produce heat, and you were correct! Friction changes kinetic energy (motion) to heat.When you’ve walked on a gymnasium ?oor, you’ve probably noticed a big difference if you’ve been wearing leather-soled shoes instead of sneakers. The sneakers have rubber soles that increase friction and keep you from slipping on the smooth wooden ?oor.Anything we use with brakes, such as cars and bicycles, use friction. The brakes have rubber pads that press against a moving surface to cause friction.Tires help us ride smoothly, but they have to generate some friction so we can stop when we need to. Surfaces can wear out due to friction, and sometimes we want objects, such as gears, to move smoothly without friction. We can make surfaces smooth or add machine oil to reduce friction when we need to.Think of a baseball player racing to get to a base. He doesn’t want too much friction to stop him. He slides on the ground, using the smoothness of his baseball uniform and his body to keep him moving as quickly as possible.MAIN IDEA AND DETAILS Give an example of how friction works.Tires have to be able to stop on wet roads. If a tire is too smooth, it is not safe to ride on. That’s why car tires should be inspected for wear every 10,000–15,000 miles.A smooth, waxed wood ?oor helps a bowling ball move quickly.Summary
Position is where an object is located. Motion is the change of position of an object. Objects move in different directions and at different speeds, and we can
measure their movement. Force also causes motion. The amount of force needed to move an object depends on the object’s mass. There are forces at work all around us,
including natural forces and gravity.
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Glossaryacceleration (ak?sel?er?AY?shuhn) Any change in the speed or direction of an object’s motion (7, 8, 9)
force (FAWRS) A pull or push of any kind (8, 9, 10, 11, 12, 13, 14, 15)friction (FRIK?shuhn) A force that resists motion between objects that are in contact (14, 15)gravity (GRAV?ih?tee) The force of attraction between Earth and other objects (12, 13, 15)inertia (in?ER?shuh) The property of matter that keeps an object at rest or keeps it moving in a straight line (11)motion (MOH?shuhn) A change of position of an object (2, 3, 7, 8, 11, 14, 15)
position (puh?ZISH?uhn) The location of an object (2, 3, 4, 15)speed (SPEED) The measure of an object’s change in position during a unit of time (4, 5, 6, 7, 8, 15)
velocity (vuh?LAHS?uh?tee) The measure of the speed and direction of an object’s motion (6, 7)weight (WAYT) A measure of the gravitational force acting on a object (12, 13)16Think and Write 1. How can we ?nd the speed a train is traveling, if we know how far it went and how long it took to get to the end of its journey?2. COMPARE AND CONTRAST Why does an object look smaller when we are farther away from it?3. CAUSE AND EFFECT What two things affect velocity when they change? 4. Expository Writing Write a short story that has to do with an object that has to be moved but is dif?cult to move. Describe different ways that are used to apply force to the object. Make the story lively and humorous. Hands-On ActivityWalk around your classroom or a room in your home. Use your own force to move different objects. Pick things up, put things down, or push and pull objects. Notice which things are easy to move and which things are dif?cult to move.School-Home ConnectionInterview people in your family or neighborhood to ?nd out what their understanding of gravity is. Share your understanding of the topic with them.
move it
