Motion and Force

Solution: Forces can be categorized as contact and non-contact forces.
The examples of non- contact forces are Gravitational, magnetic, electrostatic, etc.
The examples of contact forces are friction, chemical, electrical, etc.

The centre of mass of a rigid body is a point whose position is fixed with respect to the body as a whole. The point may or may not lie in the body. The position of the centre of mass of a rigid body depends on the following:

(1) Shape of the body
(2) Distribution of mass in the body

Statement 1 is incorrect. The weight of the body would decrease. Weight is the function of gravity.
As moon's gravity is much lesser than Earth. So, the weight of the body on the moon would decrease.
Statement 2 is correct. An athlete often runs before taking a long jump because by running first he brings himself in the state of motion and then it becomes easier for him to take a long jump.

The stone will fly in the direction of the applied force and tangentially to the path.

Solution: A body can have a constant speed but a changing velocity because the direction can change while the speed is constant.

Statement 1 is correct. Oil and grease help to reduce friction and increase the movability.
Statement 2 is also correct. A lubricant is a substance introduced to reduce friction between moving surfaces.
Statement 3 is incorrect. Shoes and tyres are formed to increase the friction.

When the car moves towards North- East direction, the outer wheel cover the large radius as compared to the inner wheel.

In case 1, work is not done because the force applied and displacement makes the angle of 90⁰.
In case 2, the man is doing work because he is applying force result in respective motion between boat and stream.
In case 3, the direction of the motion of body moving along the circular track is always at right angles to centripetal force.
In case 4, weight of car and its displacement are at right angles.

Statement 1 is incorrect. The action being you jumping imparts an equal force on the boat pushing it backwards with only drag to resist its motion.
Statement 2 is also incorrect. Weight of a body is greater in plains and lesser on hilltops.

When a jet plane flies, air passing through its wings and angle or wings is such as the air pressure below the wings is more then upper side of wings so due to this aerodynamic effect plane can fly against gravity.

Solution 1 is correct. When a road goes straight up, the slope is large and hence the frictional force is small. Now due to insufficient friction, the wheels slip on the road. Moreover for a large slope, more power will have to be used.
Solution 2 is also correct. An inflated balloon will rise to a definite height in atmosphere and then it will come to rest. It will come at rest when the atmospheric pressure is equal to the air pressure inside the balloon.

Statement 1 is incorrect. When a bomb is thrown from an aeroplane, it will follow a hyperbolic path.
Statement 2 is correct. Work done by a body in a circular motion is zero, because the force applied and displacement makes the angle of 90⁰.

Statement 1 is incorrect. In circular motion, the speed of the body always remains the same.
Statement 2 is incorrect. The earth's orbit around the sun is elliptical. So, the earth moves at a constantly varying velocity as it moves closer to the sun.

Centripetal force is a force that makes a body to follow a curved path. E.g.

1. Roller coaster
2. Turning of wheels in circular track
3. Whirling of a stone tied to a string
   Centrifugal force is the apparent outward force that draws a rotating body away from the center of rotation.
   Rotation of tubes containing a mixture at high speed for precipitation is called centrifugation.

Statement 1 is correct. The weight of an object would be minimum, when it is placed at the centre of earth because it experiences gravitational pull from all the sides. Thus, the net weight of an object remains to be zero.
Statement 2 is also correct. The horizontal force will drive away the stone and the gravitational pull will bring it down. Due to strong breeze, it drags in straight path.