Force and Pressure – MCQs with Answers and Explanations
CBSE Class 8 Science – Chapter 11: Force and Pressure
Class: 8 | Subject: Science | Chapter: 11 – Force and Pressure
Board: Central Board of Secondary Education (CBSE) – NCERT Based
Topic 1: Basic Ideas of Force
Correct Answer: (B) A push or pull acting on an object
Explanation: In science, force is defined as a push or pull on an object that arises due to the interaction between objects. It can change the state of rest or motion of an object.
Correct Answer: (B) One object interacts with another object
Explanation: Force is the result of an interaction between two objects. Without interaction, no force can act (e.g., hand and ball, magnet and nail).
Correct Answer: (D) Changing the mass of an object
Explanation: A force can change the state of motion, speed, direction, or shape of an object. It does not change the mass of the object.
Correct Answer: (C) Magnitude and direction
Explanation: Force is a vector quantity. It has both magnitude (how strong) and direction (which way it acts). Both are needed to describe it fully.
Correct Answer: (B) Newton
Explanation: Force is measured in newton (N) in the SI system. Joule is for energy, pascal for pressure and watt for power.
Correct Answer: (C) A ball changing direction after being hit by a bat
Explanation: When the bat hits the ball, force is applied which changes the direction and often the speed of the ball. This is clear evidence of force acting.
Topic 2: Effects of Force and Balanced / Unbalanced Forces
Correct Answer: (B) Changed the state of rest of the ball
Explanation: The ball was initially at rest. The kick applied a force that changed its state from rest to motion. This is one of the main effects of force.
Correct Answer: (B) Equal and opposite forces whose net effect is zero
Explanation: Balanced forces are equal in magnitude and opposite in direction. Their effects cancel out, so they do not change the state of motion of the object.
Correct Answer: (A) Balanced forces
Explanation: The book experiences its weight (downward force) and an equal upward force from the table (normal reaction). These are balanced forces and the book remains at rest.
Correct Answer: (B) Change the state of motion of the object
Explanation: Unbalanced forces have a non-zero net effect. They can start or stop motion, or change speed or direction of an object.
Correct Answer: (C) Remains stationary due to balanced forces
Explanation: Equal forces in opposite directions balance each other. The net force becomes zero, so there is no change in motion and the rope stays still.
Correct Answer: (C) A trolley being pushed and starting to move
Explanation: When the trolley starts moving, the push force is greater than the opposing friction. The forces become unbalanced, causing motion.
Topic 3: Contact Forces – Muscular Force and Friction
Correct Answer: (B) Exerted due to the action of muscles
Explanation: Muscular force is produced by the muscles of humans and animals. It always involves contact, such as lifting, pushing or pulling objects.
Correct Answer: (C) A magnet attracting an iron nail
Explanation: The magnet attracting an iron nail involves magnetic force, which is a non-contact force. The others involve muscles and hence muscular force.
Correct Answer: (B) Always opposes the relative motion
Explanation: Friction acts between two surfaces in contact, and it always acts in the direction opposite to the motion or the attempted motion.
Correct Answer: (B) Acts only when surfaces are in contact
Explanation: Friction arises due to the interlocking of irregularities of two surfaces. Therefore, it acts only when the surfaces are in contact, so it is a contact force.
Correct Answer: (C) Walking on a rough road without slipping
Explanation: We are able to walk because friction between our feet and the ground provides grip. Without this friction, we would slip. Here, friction is helpful.
Correct Answer: (B) Applying lubricating oil or grease
Explanation: Lubricants like oil and grease fill the gaps between rough surfaces and make them smooth, thus reducing friction and wear and tear in machines.
Correct Answer: (D) It is both useful and harmful
Explanation: Friction helps in walking, writing, and gripping objects (useful), but it also causes wear, tear and energy loss as heat (harmful). So, it is called a necessary evil.
Correct Answer: (C) Making the surfaces rougher
Explanation: Rough surfaces have more irregularities that interlock, increasing friction. Polishing, lubrication and ball bearings reduce friction, not increase it.
Topic 4: Non-contact Forces – Magnetic, Electrostatic and Gravitational
Correct Answer: (D) Frictional force
Explanation: Friction acts only when two surfaces touch each other, so it is a contact force. The other three can act from a distance and are non-contact forces.
Correct Answer: (B) Magnetic force
Explanation: Magnets attract magnetic materials like iron, nickel and cobalt using magnetic force, which can act from a distance.
Correct Answer: (B) Electrostatic force
Explanation: Rubbing charges the comb. The charged comb attracts small neutral paper pieces by electrostatic force, a non-contact force between charges.
Correct Answer: (C) It is the force with which Earth attracts all objects towards itself
Explanation: Gravitational force is the attractive force by which the Earth pulls all objects toward its center, causing objects to fall when dropped.
Correct Answer: (B) Magnetic and gravitational forces
Explanation: Both magnetic and gravitational forces can act without direct contact between bodies, so they are non-contact forces.
Correct Answer: (B) Air friction is absent and gravity acts equally on both
Explanation: In vacuum, there is no air resistance. Both objects fall only under gravity and have the same acceleration, so they reach together, irrespective of mass.
Correct Answer: (B) Always attract
Explanation: Unlike poles (N and S) of magnets attract each other, while like poles (N–N or S–S) repel. This is a characteristic property of magnetic force.
Correct Answer: (C) Gravitational force
Explanation: The Moon is held in orbit around the Earth by the gravitational attraction between the Earth and the Moon.
Topic 5: Pressure – Basics and Pressure due to Solids
Correct Answer: (B) Force ÷ Area
Explanation: Pressure = Force / Area. It tells us how much force is acting on unit area of a surface.
Correct Answer: (C) Pascal
Explanation: The SI unit of pressure is pascal (Pa), which is equal to 1 N/m². Newton is unit of force; joule of energy; watt of power.
Correct Answer: (B) Decreasing the area of contact
Explanation: From P = F/A, when force is constant, lower area gives higher pressure. That’s why sharp knives (small area) cut better.
Correct Answer: (B) To decrease pressure on the head
Explanation: The cloth pad increases the area over which the load acts. For the same weight, larger area means lower pressure, making it more comfortable.
Correct Answer: (C) Sharp knife has smaller area of contact
Explanation: A sharp knife’s thin edge reduces area of contact. For the same cutting force, pressure becomes higher and cutting becomes easier.
Correct Answer: (D) High-heeled shoes, because area is less
Explanation: High heels have small area of contact. For the same weight, pressure on sand is greater, so the heels sink more into soft sand.
Correct Answer: (B) Half
Explanation: Using P = F/A, if A becomes 2A while F is constant, P becomes F/2A, i.e., half of its original value. So pressure reduces by half.
Correct Answer: (D) 6000 Pa
Explanation: P = F/A = 300 ÷ 0.05 = 300 ÷ (5/100) = 300 × (100/5) = 300 × 20 = 6000 Pa. This shows how area affects the pressure on the head.
Topic 6: Pressure in Fluids – Liquids and Gases
Correct Answer: (C) Can flow, like liquids and gases
Explanation: Substances that can flow are called fluids. Both liquids (like water) and gases (like air) are fluids and exert pressure.
Correct Answer: (C) On the walls and the bottom
Explanation: Liquids exert pressure on all surfaces in contact—bottom as well as side walls. This is seen when water flows out from holes made on the sides of a container.
Correct Answer: (C) It increases with depth
Explanation: The deeper you go into a liquid, the greater is the weight of the liquid column above that point. Hence, pressure increases with depth.
Correct Answer: (B) Water from a lower hole in a bottle goes farther than from an upper hole
Explanation: In the classic bottle experiment, water jet from the lowest hole travels farthest because pressure at greater depth is higher, pushing water out more strongly.
Correct Answer: (C) Because water pressure is more at bottom
Explanation: Water pressure increases with depth. The bottom of a dam faces the highest pressure, so the wall is made thicker there to withstand this pressure safely.
Correct Answer: (B) Air molecules keep colliding with the container walls
Explanation: Gas molecules move randomly and collide with the walls of the container. These collisions exert force on the walls, producing gas pressure.
Topic 7: Atmospheric Pressure and Applications
Correct Answer: (C) Atmosphere
Explanation: The blanket of air around the Earth is called the atmosphere. It exerts pressure on the Earth’s surface called atmospheric pressure.
Correct Answer: (B) The air column of the atmosphere
Explanation: The entire column of air above a surface has weight and exerts force on that surface. This force per unit area is called atmospheric pressure.
Correct Answer: (C) Pressure inside our body balances outside atmospheric pressure
Explanation: Fluids (blood and other body fluids) inside our body exert pressure outward. This internal pressure balances the atmospheric pressure, so we do not get crushed.
Correct Answer: (C) Atmospheric pressure pushes the liquid into the region of low pressure inside the straw
Explanation: When we suck, we reduce the air pressure inside the straw. The atmospheric pressure on the liquid surface is now greater and pushes the liquid up into the straw.
Correct Answer: (C) Air pressure inside the cup is much less than outside atmospheric pressure
Explanation: When pressed, air under the cup is pushed out creating low pressure. Higher atmospheric pressure outside then pushes the cup against the wall so it sticks.
Correct Answer: (B) Pressure inside the syringe decreases and liquid is pushed in by higher external pressure
Explanation: Pulling the plunger back creates low pressure inside. The liquid outside is at higher pressure (atmospheric or blood pressure) and is pushed into the syringe through the needle.
Correct Answer: (B) Internal air pressure becomes greater than the strength of the balloon material
Explanation: As more air is added, pressure increases. When internal pressure exceeds what the balloon material can bear, it tears and bursts.
Correct Answer: (C) Water pressure increases with depth and can harm the body
Explanation: At great depths, water pressure is very high. Special suits help divers withstand this pressure and protect them from being crushed by it.