Chapter 3 Forces http://www.nsf.gov/news/special_reports/olympics/speedskating.jsp

Newton’s 1st Law The Law of Inertia: An object in motion will stay in motion unless acted on by another force. An object at rest, will stay at rest unless acted on by another force.

Sec. 1 Force, Mass & Acceleration For any object, the greater the force applied to it, the greater its acceleration. The acceleration of an object depends on its mass as well. Force, mass, and acceleration are connected.

Newton’s 2nd Law Newton’s 2nd law of motion—the net force on an object causes the object to accelerate in the direction of the net force. Equation: acceleration = net force mass a = F/m -or- F = ma F = ma F = 1000kg x .05m/s2 F = 50 kg m/s2

Units The SI unit for mass is the kilogram (kg) The unit for acceleration is meters per second squared (m/s2) So the unit for force is kg x m/s2 The kg x m/s2 is called the newton (N)

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Friction Friction—the force that opposes motion between 2 surfaces that are touching each other. The amount of friction depends on 2 factors: The kinds of surfaces The force pressing the surfaces together The areas where bumps on the surfaces stick together are the source of friction. The stronger the force, the more friction.

Types of Friction Static Friction—the friction between 2 surfaces that are NOT moving past each other. Sliding Friction—the force that opposes the motion of 2 surfaces sliding past each other. Rolling Friction—the friction between a rolling object and its surface.

Air Resistance When an object falls, it is pulled downward by gravity. Air resistance—a force that acts on objects as they fall through the air. The amount of resistance on an object depends on the speed, size, and shape of the object. A feather will fall more slowly than an apple

Terminal Velocity As an object falls, it accelerates. The force of air resistance increases with speed until it becomes large enough to cancel out the force of gravity. Then the forces on the object are balanced. Terminal velocity—the highest velocity a falling object will reach.

Terminal Velocity

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Sec. 2 Gravity The Law of Gravitation—states that any 2 masses exert an attractive force on each other. The amount of attractive force depends on the mass and distance between them. Gravity is a long-range force. All stars in a galaxy exert a gravitational force on each other. This is what gives a galaxy its shape.

Gravitational Acceleration Near Earth’s surface, the acceleration due to gravity is 9.8 m/s2. When an object falls towards earth it is called “free fall” To calculate the force of an object in free fall F = m x 9.8 m/s2.

Weight Weight—the gravitational force exerted on an object Represented by the symbol W. You can use Newton’s 2nd Law (F=ma) to calculate weight Weight = mass x acceleration due to gravity. W = m x 9.8 m/s2

Weight and Mass Weight and mass are not the same. Weight is a force—mass is the amount of matter. Weight and mass ARE related The more mass an object has, the more it will weigh in the same location. Weight is usually determined for Earth. An object will have a different weight on the moon.

Projectile Motion If a ball is thrown, it does not always travel in straight lines—they curve downward. Anything thrown or shot is called a projectile. Projectiles curve because of Earth’s gravitational pull and their own inertia. They have both horizontal and vertical velocities.

Horizontal and Vertical Motion When you throw a ball, you give it horizontal motion. After you let go, gravity starts to pull it downward—giving it vertical motion. The ball appears to travel in a curve, even though the horizontal and vertical motions are independent of each other.

Centripetal Force Centripetal acceleration—acceleration toward the center of a curved or circular path. Centripetal Force—a force directed toward the center of a circle for an object moving in circular motion. Centripetal force keeps these riders moving in a circle.

Sec. 3 Newton’s 3rd Law Newton’s 3rd law of motion—when an object exerts a force on a second object the second one exerts a force on the first that is equal in size and opposite in direction. To every action there is an equal and opposite reaction.

Action & Reaction When you jump on a trampoline, you exert a force downward; the trampoline exerts an equal force upward—sending you back in the air. If forces are equal, how does anything ever happen? Action-reaction forces are acting on different objects! They are equal, but they are not balanced

Momentum Momentum—the product of an object’s mass and its velocity. Represented by the symbol p Momentum = mass x velocity p = m x v The unit for momentum is kg m/s An archer’s arrow can have a large momentum because it has a high velocity even with a small mass. An elephant may have a low velocity, but has a large momentum because of its large mass.

Newton’s 2nd Law and Momentum The force of changing momentum: F = (mvf – mvi) / t Law of conservation of momentum—states that total momentum is neither created or destroyed, but it is transferred in collisions. Example: when you hit a cue ball for the first time in a game of pool, it has all of the momentum. When it strikes the other pool balls, it transfers some of its momentum to them. http://www.nsf.gov/news/special_reports/olympics/slapshot.jsp