Newton’s First Law

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Goals & Objectives

• Competency Goal 6: The learner will conduct investigations, use models, simulations, and appropriate technologies and information systems to build an understanding of motion and forces. – 6.03 Evaluate motion in terms of Newton's Laws:

• The force of friction retards motion. • For every action there is an equal and opposite reaction. • The greater the force, the greater the change in motion. • An object's motion is the result of the combined effect of all forces acting on

the object: • A moving object that is not subjected to a force will continue to move at a

constant speed in a straight line • An object at rest will remain at rest.

NC STANDARD COURSE OF STUDY

Force• A soccer ball sits on the ground, motionless,

until you kick it. • A force is a push or pull.

– Example: when throwing a ball your hand exerting force on the ball and the ball accelerates forward until it leaves your hand. After the ball leaves you hand, the force of gravity causes its path to curve downward. When the ball hits the ground, the ground exerts a force, stopping the ball.

– A pull, gravity, or picking something up– Combining forces– Balanced and unbalanced forces

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Combining Forces• More than one force can act on an object at

the same time.• The combination of all the

forces acting on an object is the net force.

• When more than one force Is acting on an object, the net

force determines the motion of the object.

• If forces are acting together in the same direction then they add together to form the net force.

• If two forces are in opposite directions, then the net force is the difference between the two forces, and it is in the direction of the larger force.

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Balanced and Unbalanced Forces

• A force can act on an object without causing it to accelerate if other forces cancel the push or pull.

• When two or more forces exerted on an object cancel each other and they do not cause change in the object’s motion it is called balanced forces.– Net force = 0

• If two or more forces acting on an object do not cancel

and cause the object to accelerate then it is called an unbalanced force.– Net force ≠ 0 and the motion

of the object changeshttp://quest.arc.nasa.gov/aero/virtual/demo/aeronautics/tutorial/images/LiftWeight.gif

Newton’s First Law of Motion

• If you stand on a skateboard and someone gives you a push, then you and the skateboard will start moving. You and the skateboard would stay at rest until an unbalanced force acted on you ante skateboard such as someone pushing you.

• Because of Galileo, Isaac Newton better understood the nature of motion.

• Newton explained the motion of objects in three rules known as Newton’s Laws of Motion.

• Newton’s First Law states that if the net force acting on an object is zero, the object remains at rest, or if the object is already moving, continues to move in a straight line with constant speed.

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Friction• Everyday you see

objects come to a stop.• The force that brings

nearly everything to a stop is friction.– Friction is the force that

acts to resist sliding between two touching surfaces.

• Friction is the reason why you never see objects moving with constant velocity unless a net force is applied.

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Friction Opposes Sliding

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• If two objects are in contact, frictional forces always try to prevent one object from sliding on the other object.

• If you rub your hand against a tabletop, you can feel the friction push against the motion of your hand.

• Friction always will slow a moving object

Older Ideas About Motion• In an air hockey game, the puck

floats on a layer of air, so that the friction is almost eliminated. As a result, the puck moves in a straight line with nearly constant speed after it’s been hit.

• People thought that the natural state of an object was at rest.

• If friction could be removed, an object in motion would continue to move in a straight

line with constant speed.

Static Friction

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• Have you ever watched the World’s Strongest Man competition?

• When they first pull the plane you might discover that nothing happens. The force on the plane and the ground has a net force of zero. The friction that prevents an object from moving when force is applied is called static friction. Then as they pull harder and harder, the plane suddenly starts to move.

• The static friction causes the surfaces to stick or weld

together where they are in contact.

– As the surface gets rougher and the object gets heavier, the

force of static friction will be larger.

Sliding Friction

• While static friction keeps an object at rest, sliding friction slows down an object that slides.

• A force must be applied to move the rough areas of one surface past the rough areas of the other.

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• A sliding friction force is produced when the brake pads in a car’s brakes rub against the wheels.

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Rolling Friction

• Rolling friction is needed to make a wheel or tire turn.

• It occurs between the ground and the part of the wheel touching the ground.

• Rolling friction keeps the wheel from slipping on the ground.

• Rolling friction between the wheel and the ground is less than the sliding friction.

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