Peter's Physics Pages

Bridging Course - Lecture 8 (Dynamics - example problems)

Example problems. Try these for practice.

34. A 2 kg mass and a 1 kg mass lie in contact on a smooth horizontal plane. A constant horizontal force of 6 N presses on the 2 kg mass in the direction of the 1 kg mass. Find
(a) the values of all the forces. ans F12 = F21 = 2 N
(b) the common acceleration of the blocks.

35. A 6 kg mass and a 4 kg mass are tied together with a light inexstensible string and suspended around a fixed pulley. The system is at rest at the moment it is released. Find
(a) the acceleration of the masses after release. ans 1.96 m.s-2
(b) the tension in the string after release. ans 47 N

36. A 7 kg block lies on a smooth horizontal table. A light inextensible string attached to the block passes over a light pulley at the edge of the table and a 3 kg mass is hung on the other end of the string. Assume that the system is at rest at the moment it is released. Find
(a) the tension in the string after release.
(b) how fast the system is moving 5 s after release.

37. An 18 kg block lies on a smooth plane which is inclined upwards at an angle of 30° to the horizontal. A light inextensible string attached to the block passes up over a light pulley at the top of the plane and then down to a 7 kg mass at the other end. Assume that the system is at rest at the moment it is released. Find
(a) the acceleration of the system after release.
(b) the tension in the string after release.

38. A 6 kg block lies on a smooth plane which is inclined upwards at an angle of 30° to the horizontal. A light inextensible string attached to the block passes up over a light pulley at the top of the plane and then down to a 4 kg mass at the other end. Assume that the smaller mass is initially moving upwards at 2 m.s-1. Find the time for the 4 kg mass to come (momentarily) to rest. ans 2.04 s

 E1. A 16 kg block lies on a smooth plane which is inclined upwards at an angle of 30° to the horizontal. A light inextensible string attached to the 16 kg block passes up over a light pulley at the top of the plane and then down to a 9 kg mass which hangs on the other end of the string. Assume that the system is at rest at the moment it is released to move. (a) draw a diagram showing all the forces acting on each of the two blocks. (b) write equations which apply Newton’s law of motion to each block. (c) find the acceleration of the system after release (with direction specified). (d) find the tension in the string after release. E2. A human femur will fracture if the compressional force exceeds 2x105 N. A person of mass 60 kg falls off a ladder lands on one leg. The person is travelling at 4.5 m s-1 just as they hit the ground and they keep the leg rigid so that they stop in 1.25 ms.
Find the average force on their femur. Comment on the situation (one relevant line).

E3. A space shuttle out past the Moon is travelling at 50 m.s-1. It fires its retro-rockets to slow it down and the shuttle decelerates at 2m.s-1. Find the apparent weight of an 80 kg astronaut during the deceleration.