Transcript Net Force
Net Force Normal Force Frictional Force Free Body Diagrams Forces Review Yesterday you learned Newton’s three laws: 1. 2. 3. An object with zero or constant velocity will remain at zero or with a constant velocity unless acted upon by an outside NET force. An object experiencing an unbalanced net force experiences an acceleration in the direction of the net force. (F = ma) For every force there is an equal and opposing force. These laws are the basis for the study of dynamics Zero Velocity For example a book sitting on a table, is not moving and does not move unless acted upon by an unbalanced force. When it isn’t moving there are two forces: gravity and the table. The table’s force on the book is known as a NORMAL FORCE Since the two forces are balanced then the book isn’t moving. The NET FORCE is zero If you remove one of the forces and the net force becomes unbalanced then the book accelerates. FNET FNET 0 Constant Velocity Constant velocity can be a little more difficult. When a car is travelling down the highway at a constant speed the forces balance and you have a zero net force. If you want to accelerate then you need to increase the force applied by the engine of the car which creates an unbalanced NET FORCE. FNET 0 Frictional Force Frictional force depends entirely on the normal force applied to an object. Depending on how the two types of materials interact decides what the force of friction is. Force of friction can be calculated using the formula: F f k FN Materials in Contact Coefficient of Static Friction* S Coefficient of Kinetic Friction * K Wood on wood 0.5 0.3 Waxed ski on snow 0.1 0.05 Ice on ice 0.1 0.03 Rubber on concrete (dry) 1.0 0.8 Rubber on concrete (wet) 0.7 0.5 Glass on glass 0.94 0.4 Steel on aluminium 0.61 0.47 Steel on steel (dry) 0.7 0.6 Steel on steel (lubricated) 0.12 0.07 Teflon on steel 0.04 0.04 Teflon on Teflon 0.04 0.04 Synovial joints (in humans) 0.01 0.01 * These values are approximate and intended only for comparison.