CBSE Questions for Class 9 Physics Force And Laws Of Motion Quiz 17 - MCQExams.com

Friction opposes the relative motion between two surfaces in contact. It acts on both surfaces. When some children try to push a box on a rough floor box with a small force, the box does not move because of friction acting in a direction opposite to the push. This friction force arises between two surfaces in contact. It balances the pushing force and therefore the box does not move.

If the block is pulled from both the sides with equal forces, the block will not move because they cancel out the effect of each other. Such forces are called balanced forces and do not change the state of rest or of motion of an object, hence X & Y would be balanced if they are equal, therefore options $$B&C$$ are correct.

 An object moves with a uniform velocity when the forces (pushing force and frictional force) acting on the object are balanced and there is no net external force on it. If an unbalanced force is applied to the object, there will be a change either in its speed or in the direction of its motion. Thus, to accelerate the motion of an object, an unbalanced force is required and the change in its speed (or in the direction of motion would continue as long as this unbalanced force is applied. However, if this force is removed completely, the object would continue to move with the velocity it has acquired till then. Hence, the given statement is false.

In the air-filled balloon, balloon, and air inside form a kind of system. Initially when the system is at rest, so the momentum of the system is also zero. As soon as the balloon is set free air escapes out with some great velocity, balloon moves in the direction opposite to air rushing out.So that their momentum are in opposite direction and cancel out to be zero, which is same as initial momentum.

In the air-filled balloon, balloon, and air inside form a kind of system. Initially when the system is at rest, so the momentum of the system is also zero. As soon as the balloon is set free air escapes out with some great velocity, balloon moves forward in the direction opposite to air rushing out, before firing both gun and bullet are at rest, therefore momentum of the system is zero. When the bullet is fired the bullet moves forward and gun recoils back. Rockets and jet engines also work on the law of conservation of momentum. In these hot gases produced by burning of fuel rush out with large momentum, Due to this, these machines gain an equal and opposite momentum. This momentum enables the rockets and jet engines with very high velocities, hence option $$D$$ is correct.

Rockets and jet engines work on the law of conservation of momentum. In these hot gases produced by burning of fuel rush out with large momentum, Due to this, these machines gain an equal and opposite momentum,in order to conserve total momentum.This momentum enables the rockets and jet engines with very high velocities, hence the given statement is true.

Action-reaction forces may not produce accelerations of equal magnitudes.
This is because each force acts on a different object that may have a different mass, hence the given statement is false.

Motorboats work on the principle of conservation of momentum, it pushes the water backward and gets pushed forwards in reaction to conserve momentum. Hence, the given statement is correct.

When you walk, you push the road below backwards. The road exerts an equal and opposite force on your feet to make you move forward, hence option $$C$$ is correct.

$$u_x=10\sqrt{2}\,\cos 45^0=10\,m/s$$