Transcript Forces
Forces Ch 7 6th grade 7.1 Vocabulary • Force • Net force How are Forces Described? • A force is described by its strength and by the direction in which it acts. • The direction and strength of a force can be represented by an arrow. • The strength of a force is measured in the SI unit called the newton (N). How do forces Affect Motion? • The net force determines if and how an object will accelerate. • A nonzero net force causes a change in the object’s motion. • When forces are moving in the same direction, add all of the forces together to find the net force. • When two forces are moving in opposite directions, subtract the smaller force from the larger force. The net force is the same direction as the larger force. 7.2 Vocabulary • • • • Friction Gravity Mass Weight What factors Affect Friction? • Types of surface: smooth surfaces produce less friction than rough surfaces. • Hardness that surface is pushed together: friction increases when surfaces push harder against each other • Without friction, a moving object will not stop until it strikes another object. • Sliding friction: when two solid surfaces slide over each other that slows objects down. • Static friction: acts between objects that aren’t moving • Fluid friction: occurs when a solid object moves through a fluid (such as water or air) • Rolling friction: when an object rolls across a surface What factors Affect Gravity? • Law of Universal Gravitation: the force of gravity acts between all objects in the universe that have mass. – You are attracted not only to Earth, but also to all planetary bodies and all objects around you. – You do not notice the attraction because these forces are extremely small compared to the force of Earth’s attraction. – Gravitational force between some objects is stronger than the forces of others. You only observe the effects of the strongest gravitational forces. – The more mass, the greater the gravitational force. – Gravitational force also depends on the distance between them. – Your mass is the same on Earth as it would be on any other planet, but weight varies with the strength of the gravitational force. 7.3 Vocabulary • Inertia Newton’s First Law of Motion • An object at rest with remain at rest unless acted upon by a nonzero net force. • An object moving at a constant velocity will continue moving at a constant velocity unless acted upon by a nonzero net force. Newton’s First Law • All object, moving or not, resist changes in motion. So this law is sometimes called the Law of Inertia. • The greater the mass of an object, the greater its inertia, and the greater the force required to change its motion. • Inertia Newton’s Second Law of Motion • An object’s acceleration depends on its mass and on the new force acting on it. • Acceleration = Net Force Mass OR: Net Force = Mass (Acceleration) • Acceleration is measured in meters per second per second (m/s2). • Mass is measured in kilograms (kg). • So, force is measured in kilograms times meters per second per second (kg· m/s2), or more simply the newton (N). • One newton is the force required to give a 1-kg mass an acceleration of 1 m/s2. Newton’s Third Law of Motion • If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. • For every action, there is an equal but opposite reaction. Action-Reaction Pairs • Pairs of action and reaction forces are all around you. As you walk, your feet push on the ground as an action force, but the ground pushes with an equal and opposite reaction force. Detecting Motion • When gravity pulls an item downward, Newton’s third law states that the item is also pulling upward on the Earth. Since the Earth has such a large mass, its acceleration is so small, that we can’t see it. Cancellation? • Equal and opposite forces cancel each other out, but only when applied to same objects. • Action and reaction forces do not cancel out because they act on different objects. • If you pushed on a friend’s hand with your hand with the same amount of force, neither of you would move. But if each of you push on the other’s shoulder, there would be action and reaction forces that do not cancel out. • 7.3 Review