NCERT MCQ Solutions for Class 8 Science Curiosity Chapter 5 Exploring Forces

Force as a push or pull acting on an object due to interaction with another object. This interaction causes effects like movement, change in speed or direction or shape. Force always involves two objects in contact or at a distance.

The SI unit of force is newton, represented by the symbol N. It measures the amount of force applied on an object and is named after Sir Isaac Newton, who formulated the laws of motion.

Contact forces require physical contact between objects. Muscular force is produced by muscles during activities like walking, lifting or pushing. It involves physical interaction and is thus a contact force unlike gravitational or electrostatic forces which act without contact.

Electrostatic force as a non-contact force. It occurs when a charged body exerts a force on another object, even without direct contact, as shown in the activity with rubbed balloons or a charged plastic scale.

Gravity is the force with which the Earth attracts objects towards itself. It is a non-contact and always attractive force, responsible for pulling all thrown or dropped objects back to the ground.

When two similarly charged objects like rubbed balloons are brought near, they repel each other. This activity to illustrate that like charges repel and unlike charges attract, a key property of electrostatic force.

A force can change an object’s speed, direction or shape. It may also bring an object to rest or make it move. Multiple changes can happen at once depending on the nature and direction of the force.

Friction acts in a direction opposite to that of motion. It resists the movement of one surface over another. Rougher surfaces have more friction and can bring moving objects to a stop more quickly.

Friction depends on surface roughness. Rough surfaces like sandpaper have more irregularities which lock into those of another surface, creating more resistance to movement compared to smoother surfaces like glass or tile.

Buoyant force or upthrust as the upward force exerted by liquids on objects placed in them. This force acts opposite to gravity and can cause objects to float if it equals or exceeds their weight.

If the buoyant force exerted by water is equal to or greater than the weight of the object, it floats. Otherwise, it sinks.
This principle is described and demonstrated using wooden blocks and metal objects in water.

Weight is the force with which the Earth pulls an object towards itself. It is dependent on gravity and varies from place to place. Weight (a force) from mass (a constant property).

Since weight is a force, it is also measured in newtons (N), like all other forces. The spring balance used in the chapter is marked in newtons to measure the weight of different objects under Earth’s gravity.

The Moon’s gravitational pull is weaker than Earth’s. As a result, the weight of an object becomes one-sixth of its weight on Earth, while its mass remains unchanged since mass is constant everywhere.

A spring balance measures force or weight. It stretches when a weight is hung from it, and the scale on the balance indicates the force of gravity acting on the object.

Gravity as the force that Earth exerts on all objects, pulling them toward its center.
This force always acts downward, and it’s the reason why objects fall to the ground when released. Gravity operates uniformly across the Earth’s surface in a downward direction.

Gravitational force is the invisible pull that Earth exerts on all objects. It is responsible for keeping us grounded and for bringing back objects that are thrown upwards.
Without gravity, we and all things would float away from the Earth’s surface.

Rough surfaces have tiny projections and pits which interlock with those of another surface.
These interlocking irregularities create greater resistance to motion, increasing friction compared to smoother surfaces where such interlocking is minimal or absent.

Polishing makes a surface smoother by reducing the number of irregularities. Smoother surfaces experience less resistance when moving against each other, thereby reducing friction.
This principle is used in machines and vehicles to improve efficiency and motion.

Applying force to a rubber band changes its shape temporarily. When released, it regains its original form, showing that forces can deform and restore elastic materials.
This example illustrates the reversible change in shape due to applied force.

Rolling objects slow down and stop due to friction between their surface and the ground.
This frictional force opposes motion and gradually reduces the ball’s speed until it comes to rest, even on seemingly smooth surfaces.

Force can change an object’s motion, direction or shape but cannot change its mass. Mass is a fixed quantity of matter in an object.
Only external processes like cutting or burning can reduce mass, not mere application of force.

Air exerts resistance on falling objects. A feather, being lighter and broader, experiences more air resistance than a stone.
This slows its fall. Without air (like in a vacuum), both would fall at the same rate due to gravity alone.

If equal and opposite forces are applied on an object, they cancel each other out, resulting in no net movement.
Tug-of-war where the rope does not move until one side applies a greater force than the other. 

Kicking a football involves muscular force. The muscles in the leg apply a push on the ball, changing its speed and direction. Muscular force as a contact force, since it involves direct interaction with the object being moved.

Rubbing a plastic scale transfers electrons and charges it electrostatically.
This charged scale attracts small bits of paper due to electrostatic force, which is a non-contact force acting at a distance.

Whether an object sinks or floats depends on the balance between the downward gravitational force (weight) and the upward buoyant force (upthrust). If weight exceeds upthrust, the object sinks. If upthrust is equal to or more, it floats.

Although primarily covered in the electricity chapter, the concept is briefly touched on here. When current passes through a wire, it produces a magnetic field.
A nearby magnet responds to this field and changes its direction, proving the interaction between electricity and magnetism.

Water exerts an upward buoyant force on the boat. If this force equals or exceeds the boat’s weight, it floats. This principle helps large ships and small boats remain on the water surface despite their heavy mass.

Force to do mechanical work, it must cause displacement of the object in the same direction.
If no movement occurs or the force is perpendicular, no work is done. Pushing a wall without movement is an example of zero work.