ENERGY “Energy is the ability to do work or to produce a change.”
Forms of energy Potential energy – stored energy *Elastic potential energy (rubber bands, basketball, etc.) Gravitational potential energy *Chemical potential energy (food, fuels, etc.) *Electrical potential energy (covered in Chap. 5 : Electricity) Kinetic energy – movement energy *Heat energy (covered in Secondary 1) *Light energy (covered in Chap. 2 and 3 : Light and Colour) *Sound energy Those marked with * are not very important.
Gravitational Potential energy Gravitational energy is the energy an object has was it is raised above the ground. The amount of gravitational energy depends on mass and height. The larger the mass, the greater the gravitational energy. The higher the object, the greater the gravitational energy. The formula for finding Gravitational P.E. (in J) = mgh (Mass in kg x 10 N (on Earth) x Height above ground)
Kinetic Potential energy Kinetic potential energy is the energy found in a moving body. The amount of kinetic energy depends on mass and speed. The larger the mass, the greater the kinetic energy. The faster the object moves, the greater the kinetic energy. The formula for finding Kinetic P.E. (in J) = 1/2mv2 (1/2x Mass in kg x Speed (m/s) x Speed)
Principle of Conservation of Energy Energy cannot be created or destroyed, but only converted from one form to another. Examples of Energy changes: Burning pile of wood Chemical Potential energy -> Heat energy + Light energy Basketball drops from a height Gravitational Potential energy -> Kinetic energy
Explaining why a basketball's rebound height is less than the initial dropped height When the ball rebounds, air resistance is present. Kinetic energy is converted to sound and heat energy when the ball hits the ground. Hence the rebound height is less than the initial dropped height.
Turning Effect and Moment The tendency for a force to produce rotation. A force is a push or a pull that tends to produce motion, stop motion or change an object’s shape or size. This turning effect is called moments. Moment is the product of the force applied and the perpendicular distance from the pivot to the line of action of force.
Principle of Moment The sum of anticlockwise moments about a pivot is equal to the sum of clockwise moments about the same pivot when an object is in equilibrium. Clockwise moments = Anti-clockwise moments For the principle to work: Object must be pivoted at one point. A force must act on the object, but not at the pivot.
Formula of Moment Moment (in N m) = Force (N) x Perpendicular Distance (m)