All 10 short questions of 9th Class Physics Chapter 3, Dynamics, with exam-ready answers in the book's words. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
A force can start a body moving or stop it, speed it up or slow it down, and change its direction of motion. Because velocity is a vector, changing either its size or its direction is a change in motion.
Forever, as long as no force acts on it. By Newton's first law a body keeps moving with uniform velocity unless a resultant force acts on it, and in free space there is no friction or air resistance to slow it.
The product of a force and the (short) time for which it acts: impulse = F × t. It equals the change in momentum of the body; SI unit N s.
On the Earth, friction and air resistance always act on moving bodies and slow them down. So a body never keeps moving forever on its own, as the first law describes; we only see it approximately, for example on smooth ice.
Because of inertia. The passengers' bodies were at rest and tend to stay at rest. The car and seat move forward suddenly, so the seat presses into their backs and they seem to be pushed back.
A body's acceleration depends on all the forces acting on it together. Forces in opposite directions cancel partly, so only the resultant (net) force produces the acceleration: F = ma uses the net force.
Pull a heavy box across the floor with a spring balance and note the reading. Then put the box on rollers (round pencils or a trolley) and pull again. The reading is much smaller, so rolling friction is much less than sliding friction.
The constant velocity a falling body reaches when the upward air resistance becomes equal to its weight, so the net force and the acceleration become zero.
He should fire the rocket away from the spaceship. The exhaust gases are pushed away from the ship (action), and by Newton's third law they push the astronaut towards the ship (reaction).
