Question

1. which statement best describes the pe-ke transformations for a javelin, starting from the instant the javelin leaves the thrower’s hand until it hits the ground.

a. initial pe is transformed to ke until the javelin reaches the high point of its arc. on the way back down, ke is transformed into pe. at every point in the flight, mechanical energy is being transformed into heat energy.
b. initial ke is transformed to pe until the javelin reaches the high point of its arc. on the way back down, pe is transformed into ke. at every point in the flight, mechanical energy is being transformed into heat energy.
c. initial pe is transformed to ke until the javelin reaches the high point of its arc. on the way back down, there is no transformation of mechanical energy. at every point in the flight, mechanical energy is being transformed into heat energy.
d. initial ke is transformed to pe until the javelin reaches the high point of its arc. on the way back down, there is no transformation of mechanical energy. at every point in the flight, mechanical energy is being transformed into heat energy.

Explanation:

To solve this, we analyze the energy transformations of a javelin in flight:

1. Initial Energy: When the javelin leaves the hand, it has kinetic energy (KE) from the throw.
2. Ascent (to high point): As the javelin rises, its speed decreases (KE decreases) and height increases (potential energy, PE, increases). Thus, initial KE is transformed into PE.
3. Descent (from high point to ground): As the javelin falls, height decreases (PE decreases) and speed increases (KE increases). Thus, PE is transformed into KE.
4. Mechanical Energy Loss: Due to air resistance, mechanical energy (PE + KE) is continuously converted to heat energy (thermal energy) throughout the flight.

Now, evaluate the options:

• Option a: Incorrect (claims initial PE → KE; javelin starts with KE).
• Option b: Correct (KE→PE on ascent, PE→KE on descent, and mechanical energy→heat energy throughout).
• Option c: Incorrect (initial PE→KE is wrong, and claims no transformation on descent).
• Option d: Incorrect (claims no transformation on descent).

Answer:

b. Initial KE is transformed to PE until the javelin reaches the high point of its arc. On the way back down, PE is transformed into KE. At every point in the flight, mechanical energy is being transformed into heat energy.