Introduction to Forces Guided Discussion Student notes are shown in blue.

Force Defined 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.

Force Defined 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.

Net Force The combination of all forces on an object. When two or more forces act on an object at the same time, the forces combine to form the net force.

Balanced and Unbalanced Forces In a tug of war, each side exerts a force on the rope. If the opposing forces are equal, they are balanced and the rope does not move. If one force is greater than the other, the forces are unbalanced and the rope moves in the direction of the greater force.

Balanced Forces Do not change motion. Cancel each other. Combine to produce a net force of zero. Unbalanced Forces Do not have the same strength (size). Cause acceleration.

Friction A force that opposes the sliding motion of two surfaces. Rule of friction – Friction always acts in a direction to oppose motion. Depends on the materials the surfaces are made from and the roughness of the surfaces.

Friction 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.

Friction

Static Friction – The frictional force that prevents two surfaces, in contact from sliding past each other. Sliding Friction – The force that acts in the opposite direction to the motion of a surface sliding on another surface.

Air Resistance 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.

Air Resistance The flying squirrel increases its area by spreading out. This increases air resistance and decreases the speed of its fall. Air resistance, not an object’s mass, is why feathers, leaves, and sheets of paper fall more slowly than pennies, acorns, and apples.

Terminal Velocity As an object falls, the downward force of gravity causes the object to accelerate. As the speed of the object increases, the upward force of air resistance also increases. When the upward air resistance force equals the downward force of gravity, terminal velocity is reached – the velocity becomes constant.

Terminal Velocity Depends on the size, shape, and mass of the object. Terminal velocity is reached for the sky divers when air resistance balances the downward force of gravity.