Important Questions (1 mark): Forces & Laws of Motion for Class 9 - Free

Weight of the body is the force by which it is attracted by earth.
The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position. So, the spring force does not always pushes a body.
Frictional force always acts opposite to the surface of contact.

If a branch of a tree is vigorously shaken, some of the leaves may get detached from tree due to inertia of rest. The leaves tend to remain in rest but branches come in motion.

At equilibrium, the acceleration is equal to zero. So, there is no net change in acceleration of the moving body.

Inflated balloon lying on the surface of a floor moves forward when pierced with a pin due to Newton’s third law of motion. The air coming out of the balloon forces the balloon in opposite direction.

The SI unit of force is Newton, which is equal to the mass/ acceleration. 
Newton =Kg/ms²= Kgms⁻².

Inertia is the property of a body by virtue of which the body unable to change the state of rest or state of motion and direction as inertia is the tendency to oppose the change in state or direction.

To every action, there is an equal and opposite reaction. This is the newton’s third law of motion.

The force acting on a body is equal to rate of change of momentum. Change of momentum = 10 Kg/s, time taken = 0.01 second. Force = 10/0.01 = 1000N.

One Newton (1N) is equal to 10⁵ dyne. Both dyne and Newton is unit of force.

Inertia of the body is proportional to the mass of the body. 5 Rs coin is heavier than 5 Rs note and 1 Rs coins. So, inertia of 5 Rs coin more than others.

The force always occurs in pairs. According to third law of motion, there is equal and opposite reaction. So two force, action and reaction acts in pair always.

Rocket moves forward according to newton’s third law of motion. The gas and other substance coming out from the tail push the rocket in forward direction. Newton’s laws are universal.

A force has both magnitude as well as direction. Force is a vector quantity. Force is the product of mass and acceleration.

A high jumper runs for a while before taking high jump so that the inertia of motion help him in taking a long jump. Inertia of motion tends to remain in state of motion.

Frictional force always opposes relative motion between the surfaces of contact. The direction of force is always in opposite direction.

When body is dropped from height, the potential energy deceases and kinetic energy increases. A ball dropped from height will conserve only kinetic energy.

Gravitational force is very weak force as it depends upon mass of the body and distance between two objects. Electrostatic force is non-contact force.

The inertia of the body is proportional to mass of the body. So, body P with masses 10 kg will have more inertia in comparison to body Q of mass 5 kg.

The two forces acting on same body, which are equal and opposite direction forms action and reaction. This is the newton’s thirds law of motion.

The velocity of the body falling towards depends upon gravitational force acting on it. So, both the body of masses 1 kg and 5 kg will have same velocity at particular height and fall at same time at the ground.

The ball moving on level ground comes to rest after covering a short distance due to frictional force acting on it. So, ball slows to a stop because there is a force on the ball opposing the motion.

Inertia is directly proportional to the mass of the body. Higher the mass of the body, the higher will be the inertia.

Action and reaction are equal and opposite but even they do not cancel each other because they acts in opposite direction. Hence the statement is correct.

The branch of mathematics, called Calculus was stated by Newton. Force is the product of mass and acceleration. So, acceleration= force /mass. Doubling of force will increase the acceleration if mass is kept same.

As described by the third of Newton's laws of motion of classical mechanics, all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first.
Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
Momentum is not a scalar quantity. Momentum is a vector, which means it has a magnitude and a direction. Linear momentum is the product of an object's.
When two forces acting on an object are not equal in size, we say that they are unbalanced forces. The overall force acting on the object is called the resultant force . If the forces are balanced, the resultant force is zero. An object having balanced forces definitely cannot be accelerating. This means that it could be at rest and staying at rest or could be in motion at constant velocity.