End of Chapter Exercises: Gravity and Mechanical Energy
E - 1. Give one word/term for the following descriptions.

(a) The force with which the Earth attracts a body.
(b) The unit for energy.
(c) The movement of a body in the Earth’s gravitational field when no other forces act on it.
(d) The sum of the potential and kinetic energy of a body.

(e) The amount of matter an object is made up of.

E - 2. Consider the situation where an apple falls from a tree. Indicate whether the following statements regarding this situation are TRUE or FALSE. Write only ’true’ or ’false’. If the statement is false, write down the correct statement.
(a) The potential energy of the apple is a maximum when the apple lands on the ground.
(b) The kinetic energy remains constant throughout the motion.
(c) To calculate the potential energy of the apple we need the mass of the apple and the
height of the tree.
(d) The mechanical energy is a maximum only at the beginning of the motion.
(e) The apple falls at an acceleration of 9,8 m·s−2.

E - 3. Consider a ball dropped from a height of 1 m on Earth and an identical ball dropped from 1 m on the Moon. Assume both balls fall freely. The acceleration due to gravity on the Moon is one sixth that on Earth. In what way do the following compare when the ball is dropped on Earth and on the Moon.


E - 4. A man fires a rock out of a slingshot directly upward. The rock has an initial velocity of 15 m·s−1.
(a) How long will it take for the rock to reach its highest point?
(b) What is the maximum height that the rock will reach?
(c) Draw graphs to show how the potential energy, kinetic energy and mechanical energy of the rock changes as it moves to its highest point.

E - 5. A metal ball of mass 200 g is tied to a light string to make a pendulum. The ball is pulled to the side to a height (A), 10 cm above the lowest point of the swing (B). Air friction and the mass of the string can be ignored. The ball is let go to swing freely.
(a) Calculate the potential energy of the ball at point A.
(b) Calculate the kinetic energy of the ball at point B.
(c) What is the maximum velocity that the ball will reach during its motion?

E - 6. A truck of mass 1,2 tons is parked at the top of a hill, 150 m high. The truck driver lets the truck run freely down the hill to the bottom.
(a) What is the maximum velocity that the truck can achieve at the bottom of the hill?
(b) Will the truck achieve this velocity? Why/why not?

E - 7. A stone is dropped from a window, 3 metres above the ground. The mass of the stone is 25 grams.
(a) Use the Equations of Motion to calculate the velocity of the stone as it reaches the ground.
(b) Use the Principle of Conservation of Energy to prove that your answer in (a) is correct.

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Physics 10.III Gravity and Mechanical Energy

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