Q4. An astronaut on a space station, where everything floats, wants to push a 100 kg box so that it speeds up at 0.5 m s. What force does she need? Would the force be different on the Earth, for the same acceleration on a smooth horizontal floor?
F = ma = 100 × 0.5, which is 50 N. It is the same on the Earth’s smooth floor, because the box’s mass, and so its inertia, does not change. Floating only means it does not press on a floor; it is not massless.
Q5. The inertia of a body depends on (a) its speed, (b) its mass, (c) the net force on it, (d) the value of g where it is. Choose the correct option.
(b). Inertia depends only on the mass of the body. Speed, the force acting and the local value of g do not change it.
Q6. A ball is released from rest at a height of 1.25 m on a smooth plane. Treating it as sliding without friction, find its speed at the bottom, and how far it travels up a second smooth plane inclined at 20°. Take g = 9.8 m s−2.
Up the second plane it regains 1.25 m of height, so
Q7. A passenger standing in a bus is thrown backward when the bus starts suddenly, and forward when it stops suddenly. Name the kind of inertia in each case and explain.
Starting: inertia of rest. Friction moves the feet with the bus, but the upper body tends to stay at rest, so it falls backward relative to the bus. Stopping: inertia of motion. The feet stop with the bus, but the upper body tends to keep moving, so it lurches forward.