91影视

Under what conditions is the momentum of a system constant? Can the $\mathbf{x}$ component of momentum be constant even if the$\mathbf{y}$ component is changing? In what circumstances? Give an example of such behavior.

In order to close a door, you throw an object at the door. Which would be more effective in closing the door, a 50g tennis ball or a 50g lump of sticky clay? Explain clearly what physics principles you used to draw your conclusion.

Consider a head-on collision between two objects. Object 1, which has mass ${\mathit{m}}_{\mathbf{1}}$, is initially in motion, and collides head-on with object 2, which has mass${\mathit{m}}_{\mathbf{2}}$and is initially at rest. Which of the following statements about the collision are true?

(1)${\stackrel{\mathbf{鈫�}}{\mathbf{p}}}_{\mathbf{1}\mathbf{\text{,initial}}}\mathbf{=}{\stackrel{\mathbf{鈫�}}{\mathbf{p}}}_{\mathbf{1}\mathbf{,}\mathbf{\text{final}}}\mathbf{+}{\stackrel{\mathbf{鈫�}}{\mathbf{p}}}_{\mathbf{2}\mathbf{,}\mathbf{\text{final}}}$.

(2)$\left|{\stackrel{鈫�}{p}}_{1,\text{final}}\right|\mathbf{<}\left|{\stackrel{鈫�}{p}}_{1\text{, initial}}\right|$.

(3) If${\mathit{m}}_{\mathbf{2}}\mathbf{鈮�}{\mathit{m}}_{\mathbf{1}}$, then$\left|螖{\stackrel{鈫�}{p}}_1\right|\mathbf{>}\left|螖{\stackrel{鈫�}{p}}_2\right|$.

(4) If${\mathit{m}}_{\mathbf{1}}\mathbf{鈮�}{\mathit{m}}_{\mathbf{2}}$, then the final speed of object 2 is less than the initial speed of object 1.

(5) If${\mathit{m}}_{\mathbf{2}}\mathbf{鈮�}{\mathit{m}}_{\mathbf{1}}$, then the final speed of object 1 is greater than the final speed of object 2.

A ball whose mass is 0.2kg hits the 铿俹or with a speed of 8 m/s and rebounds upward with a speed of 7m/s. The ball was in contact with the 铿俹or for$\mathbf{0}.\mathbf{5}\mathbf{ms}\left(\mathbf{0}.\mathbf{5}脳\mathbf{10}-\mathbf{3}\mathbf{s}\right)$.

(a) What was the average magnitude of the force exerted on the ball by the 铿俹or? (b) Calculate the magnitude of the gravitational force that the Earth exerts on the ball.

(c) In a collision, for a brief time there are forces between the colliding objects that are much greater than external forces. Compare the magnitudes of the forces found in parts (a) and (b).

In outer space a rock whose mass is 3kg and whose velocity was$\mathbf{(}\mathbf{3900}\mathbf{,}\mathbf{-}\mathbf{2900}\mathbf{,}\mathbf{3000}\mathbf{)}\mathbf{m}\mathbf{/}\mathbf{s}$struck a rock with mass 13kg and velocity$\mathbf{(}\mathbf{220}\mathbf{,}\mathbf{-}\mathbf{260}\mathbf{,}\mathbf{300}\mathbf{)}\mathbf{m}\mathbf{/}\mathbf{s}$. After the collision, the 3kg rock鈥檚 velocity is$\mathbf{(}\mathbf{3500}\mathbf{,}\mathbf{-}\mathbf{2300}\mathbf{,}\mathbf{3500}\mathbf{)}\mathbf{m}\mathbf{/}\mathbf{s}$. (a) What is the 铿乶al velocity of the 13kg rock? (b) What is the change in the internal energy of the rocks? (c) Which of the following statements about Q (transfer of energy into the system because of a temperature difference between system and surroundings) are correct? (1)$\mathbf{Q}\mathbf{鈮�}\mathbf{0}$ because the duration of the collision was very short. (2)$\mathbf{Q}\mathbf{=}{\mathbf{E}}_{\mathbf{thermal}}$ of the rocks. (3)$\mathbf{Q}\mathbf{鈮�}\mathbf{0}$ because there are no signi铿乧ant objects in the surroundings. (4)$\mathbf{Q}\mathbf{=}\mathbf{鈭�}\mathbf{k}$ of the rocks.

A spring has an unstretched length of 0.32 m. a block with mass 0.2 kg is hung at rest from the spring, and the spring becomes 0.4 mlong.Next the spring is stretched to a length of 0.43 mand the block is released from rest. Air resistance is negligible.

(a) How long does it take for the block to return to where it was released? (b) Next the block is again positioned at rest, hanging from the spring (0.4 m long) as shown in Figure 10.43. A bullet of mass 0.003 kg traveling at a speed of 200 m/s straight upward buries itself in the block, which then reaches a maximum height above its original position. What is the speed of the block immediately after the bullet hits? (c) Now write an equation that could be used to determine how high the block goes after being hit by the bullet (a height h), but you need not actually solve for h.

A 6kg mass traveling at speed 10m/s strikes a stationarymass head-on, and the two masses stick together.

(a) What was the initial total kinetic energy?

(b) What is the final speed?

(c) What is the final total kinetic energy?

(d) What was the increase in internal energy of the two masses?

An alpha particle (a helium nucleus, containing 2 protons and 2 neutrons) starts out with kinetic energy of 10 MeV (10 脳 106 eV), and heads in the +x direction straight toward a gold nucleus (containing 79 protons and 118 neutrons). The particles are initially far apart, and the gold nucleus is initially at rest. Assuming that all speeds are small compared to the speed of light, answer the following questions about the collision. (a) What is the final momentum of the alpha particle, long after it interacts with the gold nucleus? (b) What is the final momentum of the gold nucleus, long after it interacts with the alpha particle? (c) What is the final kinetic energy of the alpha particle? (d) What is the final kinetic energy of the gold nucleus? (e) Assuming that the movement of the gold nucleus is negligible, calculate how close the alpha particle will get to the gold nucleus in this head-on collision.

A Fe-57 nucleus is at rest and in its first excited state, 14.4 keV above the ground state (14.4 脳 103 eV, where 1 eV = 1.6脳10鈭�19 J). The nucleus then decays to the ground state with the emission of a gamma ray (a high-energy photon). (a) Wthe recoil speed of the nucleus? (b) Calculate the slight difference in eV between the gamma-ray energy and the 14.4 keV difference between the initial and final nuclear states. (c) The 鈥淢枚ssbauer effect鈥� is the name given to a related phenomenon discovered by Rudolf M枚ssbauer in 1957, for which he received the 1961 Nobel Prize for physics. If the Fe-57 nucleus is in a solid block of iron, occasionally when the nucleus emits a gamma ray the entire solid recoils as one object. This can happen due to the fact that neighbouring atoms and nuclei are connected by the electric interatomic force. In this case, repeat the calculation of part (a) and compare with your previous result. Explain briefly

A particle of mass m, moving at speed $\mathit{v}\mathbf{=}\frac{\mathbf{4}}{\mathbf{5}}\mathit{c}$, collides with an identical particle that is at rest. The two particles react to produce a new particle of mass M and nothing else. (a) What is the speed V of the composite particle? (b) What is its mass M?