Guided Discussion Student notes are shown in blue. » Any influence that can cause a change in an object’s motion. A bat strikes the ball with a force that causes the ball to stop and then move in the opposite direction. » A push or pull that one object exerts on another » Has a size and direction » Measured in newtons (N) You have to apply a force of about 3 N to lift a full can of soda. Forces are measured in Newtons. One newton is equal to the amount of force needed to accelerate a 1 kg mass 1 m/sec2. In other words, a kilogram mass exerts about 9.8 newtons of force. To put that in perspective, about half of one of Isaac Newton's apples would exert 1 N of force. Newtons are relatively small. http://chemistry.about.com/od/chemistryglossary/g/N ewton-Definition.htm When multiple forces act on the same object at the same time, the forces are combined to form the net force. » The combination of all forces on an object Balanced Forces: No Acceleration In a tug of war, each side exerts a force on the ground. If the opposing forces on the ground are equal, they are balanced and the people do not move. Unbalanced Forces: Acceleration If one of the forces on the ground is greater than the other, the forces are unbalanced and the people move in the direction of the greater force. » Do not change motion » Cancel each other » Combine to produce a net force of zero » Do not have the same strength (size) » Cause acceleration » Friction is a force that stops objects from sliding against each other. » The amount of friction between two objects depends on what the materials are made of. » Rule of Friction – Friction always acts in a direction to oppose motion. Examples: If you push a solid block along the floor to the right, the force of friction on the block will be to the left. A boat propelled to the east by its motor experiencing water friction to the west When an object falls downward through the air, the force of friction (air drag or air resistance) acts upward. PUSH FRICTION AIR RESISTANCE WEIGHT Unbalanced forces: acceleration Balanced forces: constant speed Balanced forces: no motion A When a car is accelerating, the forces are unbalanced. The force moving the car forward is greater than the opposing force of motion. B When a car is cruising at constant speed, the forces moving the car forward is balanced by the force of friction. C This car does not roll because the force of friction between the brakes and the wheels balances the force of gravity. » Static Friction – The frictional force that acts on stationary objects when trying to move them. » Sliding Friction – The frictional force that acts on a moving object, causing it to slow down. » Rolling Friction – The static frictional force between the ground and the wheel as it rolls. » Fluid Friction – The frictional force an object experiences as it moves through a gas or liquid. » A type of frictional force that opposes the motion of objects that move through the air » Causes objects to fall with different accelerations and different speeds » Acts in the direction opposite to the velocity of an object moving in air » Size of the force depends on the size and shape of the object The flying squirrel increases its area by spreading out. This increases air resistance and decreases the speed of its fall. It appears that a pieces of paper falls slower than a book because it is smaller, but the real reason is the piece of paper is effected by air resistance. Objects fall because gravity is pulling them down to the ground. Because gravity accelerates objects, objects will keep increasing in speed. The faster the objects travel, the more they are effected by air resistance. » When the upward air resistance force equals the downward force of gravity, terminal velocity is reached – the velocity becomes constant. » Affects objects differently based on the objects size, shape, and mass of the object Terminal velocity is reached for the sky diver when air resistance balances the downward force of gravity.