Students can rely on these Class 8 Science Notes Chapter 5 Exploring Forces Class 8 Notes for complete exam preparation.
Class 8 Science Chapter 5 Exploring Forces Notes
Class 8 Science Curiosity Chapter 5 Notes
Class 8 Science Chapter 5 Notes – Exploring Forces Notes Class 8
→ Force is a push or a pull on an object resulting from the object’s interaction with another object. Its standard unit is newton(N)
→ Frictional force comes into play in the opposite direction when an object is sliding or trying to slide over another object.
→ Magnetic force acts between two magnetic substances when placed near to each other.
→ Electrostatic force is exerted by one charged body on another charged or uncharged body when placed near to it.
→ Gravitational force is the force with which the earth or any other planet or heavenly body, attracts any object towards itself.
→ Weight of a body is the measure of force acting on the body due to the gravitational pull of the earth.
→ Mass of an object remains unchanged where as the weight can vary from place to place.
Upthrust or buoyant force is the upward force which acts on an object when it is placed in a liquid.
→ If a person moves a heavy box lying on the floor in the forward direction, then what will the direction of force on the box?
→ In which direction does an object move, when two equal forces act on an object in the opposite directions?
→ Give some examples of the cases in which the application of force only changes the shape of the object on which the force is applied.
→ Which force acts on all the objects in the universe, whether big or small?
→ Why do you think that a ball rolling on the ground, gradually slows down and finally comes to a stop?
→ An inflated balloon is pressed against a wall after it was rubbed by a synthetic cloth. The balloon sticks to the wall. Why?
What is Force?
In order to move an object, it has either to be pushed or pulled. A push or a pull acting on an object is known as force. Thus, any object is in motion due to the application of force.
The direction in which body is pushed or pulled is called direction of force.
Actions like kicking, opening, shutting, lifting, hitting, etc. are examples of push or pull. These actions usually result in some kind of change in the state of motion of an object.
Two friends, one is pushing and other is pulling the box with rope (A push-pull force)
What Can a Force Do to the Bodies on Which it is Applied?
The strength of the force is expressed by its magnitude and direction. So, the change in the magnitude or direction of applied force always result into the change in effect produced by applied force.
- If forces are applied on an object in the same direction, then they add to one another, e.g. In the fig. (a), two men apply pushing force together in the same direction on the heavy box, the two forces add upto provide a much bigger force.
- If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces, e.g. In the Fig. (b), the box will move in that direction where stronger push is applied.
Are Forces an Interaction Between Two or More Objects?
- If we try to push a table, we have to push it with our hands. This means our hands and the table are two objects interacting with each other.
- So, at least two objects must interact for a force to come into play.
- The magnitude of force can be mathematically measured by the standard unit known as newton and its symbol is given as N.
- Whenever two objects interact, each object experiences a force from the other in the mutually opposite direction. As soon as the interaction ceases, the two objects no longer experience the force.
What Are the Different Types of Forces?
Contact forces
In many situations, we find that to apply a force on a body, physical contact is necessary. Forces of this type which acts only when there is a physical contact between the objects is known as contact force. Some of them are
- Muscular Force : The force resulting due to the action of muscles in our body is known as muscular force. Muscular force comes into play when our muscles contract or elongate while doing an activity. Animals, birds, fishes, insects etc. use muscular forces for their movement and survival.
- Friction : Friction is the force that opposes the motion of an object moving or trying to move over another surface. It always acts in the opposite direction of motion and arises due to irregularities on the surfaces in contact, even if they appear smooth. These tiny irregularities interlock and resist movement.
Frictional force not only acts between solid objects but also act in fluids as well as gases. This is the reason why aeroplanes, ships and high speed trains are designed with a specific shape to reduce this friction.
Non-Contact Forces
These forces came into action without any contact with one another, e.g. Magnetic force, electrostatic force and gravitational force.
• Magnetic force : The force exerted by magnet is called the magnetic force. Like poles of two magnets repel each other and unlike poles attract each other. A magnet can attract iron nails without touching them or a magnet exerts a force even without touching an object. So, it is considered as a non-contact force.
• Electrostatic force : The force exerted by a charged body on another charged body is known as electrostatic force, e.g. When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction and if this electrically charged comb is brought near the small pieces of paper, then it exerts an electrostatic force of attraction on them (pieces of paper) even if they are not in touch with each other.
• Gravitational force : The force due to which every body of the universe attracts another body, whether the object is small or large is known as gravitational force. Objects or things fall towards the earth because it pulls them. This force is called the force of gravity or gravity and this is an attractive force.
Weight and Its Measurement
- The weight of an object is the force with which the Earth pulls it downward, measured in newtons (N). It varies with location.
- A spring balance is used to measure weight.
- On the other hand, the mass of an object is the amount of matter it contains. It remains constant everywhere and is measured in kilograms (kg).
Floating and Sinking
- A liquid exerts an upward force called buoyant force or upthrust on objects placed in it. Gravity pulls the object down, while buoyant force pushes it up.
- If gravity is stronger, the object sinks and if buoyant force is equal to or greater, it floats. Buoyant force depends on the density of the liquid.
- According to Archimedes’ Principle, an object immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces. If this force is less than the object’s weight, it sinks and if equal, it floats.
Some rocks can also float on water. The reason is that when they are formed, such as during volcanic eruptions, they cool quickly and gases and air gets trapped inside them making them porous and hence decreasing their overall density. An example of such kind of a stone is a Pumice stone.