Physics Questions

Scarbo

2010-05-20 12:42 am

1. A boy kicked a ball of 1.2 kg on level grassland. The ball rolled for 20m before it stooped. What was the resistance? Express it in terms of the initial speed of the ball u.
ANS: 0.03u^2
2. At lift-off, the Saturn V rocket has an initial acceleration of 12ms^-2. The mass of the rocket is 3x10^5 kg. What is the thrust of the rocket? ANS: 6.6x10^6 N

3. In a life, a 2kg mass hangs at the end of a spring balance of negligible weight.
a) Name the force(s) acting on the 2 kg mass.
b) Find the tension, T in the string if the lift
i/ moves steadily upwards at 2ms^-1, and ANS: 20N
ii/ moves steadily downwards at 2ms^-1. ANS: 20N
c) Find the tension, T in the string if the lift
i/ accelerates upwards at 2ms^-2, and ANS: 24N
ii/ decelerates downwards at 2ms^-2. ANS: 16N
iii/ accelerates downwards at 2ms^-2, and ANS: 16N
iv/ decelerates upwards at 2ms^-2. ANS: 24N

4. A 3 kg mass placed on a smooth horizontal table is connected to a 2 kg mass.
Assuming that the string is light and inextensible, and the pulley is frictionless. Find
a) The acceleration of the system, and ANS: 4ms^-2
b) The tension in the string. ANS: 12N

回答 (1)

天同

2010-05-20 4:19 am

✔ 最佳答案

1. Use work-done = change of kinetic energy
Fr x 20 = (1/2)(1.2)u^2
where Fr is the resistance force.
hence, Fr = 0.03u^2

2. Usw net-force = mass x acceleration
Ft - 3x10^5g = (3x10^5) x 12
where Ft is the rocket thrust, and g is the acceleration due to gravity, taken to be 10 m/s2
hence, Ft = 6.6 x 10^6 N

3. (a) Spring tension, and weight of mass
(b) (i) T - 2g = 0, since uniform velocity indicates zero acceleration, net force = 0.
hence T = 20 N
(ii) same as in (i)
(c) Taking upward direction nas +ve,
(i) T + (-2g) = 2 x 2, i.e. T = 24 N
(ii) T + (-2g) = 2x2 , i.e. T = 24 N
(iii) T + (-2g) = 2 x(-2) , i.e. T = 16 N
(iv) T + (-2g) = 2 x (-2), i.e. T = 16 N
[Note: some of your given answers are wrong]

3. (a) Use net-force = mass x acceleration
2g = (2+3)a, where a is the acceleration of the system
hence, a = 4 m/s2
(b) Consider the 3 kg mass,
Tension = 3 x 4 N = 12 N

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