In the wake of contemplating this unit, the understudies will have the capacity to:
• Define energy, power, dormancy, erosion and centripetal power.
• Solve issues utilizing the mathematical statement Force = change in energy/change in time.
• Explain the idea of power by functional illustrations of every day life.
• State Newton’s laws of movement.
• Distinguish in the middle of mass and weight and take care of issues utilizing F= ma, and w = mg.
• Calculate strain and quickening in a string amid movement of bodies associated by the string and ignoring frictionless pulley utilizing second law of movement.
• State the law of discussion of energy.
• Use the guideline of protection of force in the impact of two articles.
• Determine the speed after impact of two articles utilizing the law of preservation of energy.
• Explain the impact of grinding on the movement of a vehicle with regards to tire surface, street conditions including sliding, braking power.
• Demonstrate that moving contact is much lesser than sliding grating.
• List different strategies to lessen contact.
3.1 FORCE, INERTIA AND MOMENTUM
Newton’s laws of movement are of key significance in comprehension the reasons for movement of a body. Before we examine these laws, it is fitting to comprehend different terms, for example, power, idleness and energy.
We can open an entryway either by pushing or pulling it. Figure 3.1 demonstrates a man pushing a truck. The push may move the truck or alter the course of its movement or may stop the moving truck. A batsman in figure 3.2 is altering the course of a moving ball by pushing it with his bat.
A force moves or tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body.
Galileo watched that it is anything but difficult to move or to stop light questions than heavier ones. Heavier articles are hard to move or if moving then hard to stop. Newton inferred that everyone opposes to the adjustment in its condition of rest or of uniform movement in a straight line. He called this property of matter as latency. He related the dormancy of a body with its mass; more prominent is the mass of a body more prominent is its latency.
Inertia of a body is its property due to which it resists any change in its state of rest or motion.
A projectile has a little latency because of its little mass. Yet, why does its effect is so solid when it is shot from the weapon?
Then again, the effect of a stacked truck on a body coming its direction is vast regardless of the fact that the truck is moving gradually. To clarify such circumstance, we characterize another physical amount called force.
Momentum of a body is the quantity of motion it possesses due to its mass and velocity.
3.2 NEWTON’S LAWS OF MOTION
Newton was the first to formulate the laws of motion known as Newton’s laws of motion.
NEWTON’S FIRST LAW OF MOTION
First law of motion deals with bodies which are either at rest or moving with uniform speed in a straight line. According to Newton’s first law of motion, a body at rest remains at rest provided no net force acts on it. This part of the law is true as we observe that objects do not move by themselves unless someone moves them. For example, a book lying on a table remains at rest as long as no net force acts on it.
Similarly, a moving object does not stop moving by itself. A ball rolled on a rough ground stops earlier than that rolled on a smooth ground. It is because rough surfaces offer greater friction. If there would be no force to oppose the motion of a body then the moving body would never stop. Thus Newton’s first law of motion states that:
A body continues its state of rest or of uniform motion in a straight line provided no net force acts on it.
NEWTON’S SECOND LAW OF MOTION
Newton’s second law of motion deals with situations when a net force is acting on a body. It states that:
When a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.
If a force produces an acceleration a in a body of mass m, then we can state mathematically that
NEWTON’S THIRD LAW OF MOTION
Newton’s third law of motion deals with the reaction of a body when a force acts on it. Let a body A exerts a force on another body B, the body B reacts against this force and exerts a force on body A. The force exerted by body A on B is the action force whereas the force exerted by body B on A is called the reaction force. Newton’s third law of motion states that:
To every action there is always an equal but opposite reaction.
According to this law, action is always accompanied by a reaction force and the two forces must always be equal and opposite. Note that action and reaction forces act on different bodies.
Consider a book lying on a table as shown in figure 3.8. The weight of the book is acting on the table in the downward direction. This is the action. The reaction of the table acts on the book in the upward direction. Consider another example. Take an air-filled balloon as shown in figure 3.9. When the balloon is set free, the air inside it rushes out and the balloon moves forward. In this example, the action is by the balloon that pushes the air out of it when set free. The reaction of the air which escapes out from the balloon acts on the balloon. It is due to this reaction of the escaping air that moves the balloon forward.
TENSION AND ACCELERATION IN A STRING
Consider a piece bolstered by a string. The upper end of the string is altered on a stand as appeared in figure 3.11. Give w a chance to be the heaviness of the square. The square pulls the string descending by its weight. This causes a pressure T in the string. The strain T in the string is acting upwards at the square. As the piece is very still, in this way, the heaviness of the square acting downwards should be adjusted by the upward pressure T in the string. Hence the strain T in the string must be equivalent and inverse to the weight w of the square.
LAW OF CONSERVATION OF MOMENTUM
Momentum of a system depends on its mass and velocity. A system is a group of bodies within certain boundaries. An isolated system is a group of interacting bodies on which no external force is acting. If no unbalanced or net force acts on a system, then according to equation 3.14 its momentum remains constant. Thus the momentum of an isolated system is always conserved. This is the Law of Conservation of Momentum. It states that:
The momentum of an isolated system of two or more than two interacting bodies remains constant.
Consider the example of an air-filled balloon as described under the third law of motion. In this case, balloon and the air inside it form a system. Before releasing the balloon, the system was at rest and hence the initial momentum of the system was zero. As soon as the balloon is set free, air escapes out of it with some velocity. The air coming out of it possesses momentum. To conserve momentum, the balloon moves in a direction opposite to that of air rushing out.
Have you noticed why a moving ball stops? Why bicycle stops when the cyclist stops pedalling?
Naturally there must be some force that stops moving objects. Since a force not only moves an object but also stops moving object.
The force that opposes the motion of moving objects is called friction.
Friction is a force that comes into action as soon as a body is pushed or pulled over a surface. In case of solids, the force of friction between two bodies depends upon many factors such as nature of the two surfaces in contact and the pressing force between them. Rub your palm over different surfaces such as table, carpet, polished marble surface, brick, etc. You will find smoother is the surface, easier it is to move over the surface. Moreover, harder you press your palm over the surface, more difficult would it be to move.
Why friction opposes motion? No surface is perfectly smooth. A surface that appears smooth has pits and bumps that can be seen under a microscope. Figure 3.17 shows two wooden blocks with their polished surfaces in contact. A magnified view of two smooth surfaces in contact shows the gaps and contacts between them.
Wheel is one of the most important inventions in the history of mankind. The first thing about a wheel is that it rolls as it moves rather than to slide. This greatly reduces friction. Why?
When the axle of a wheel is pushed, the force of friction between the wheel and the ground at the point of contact provides the reaction force. The reaction force acts at the contact points of the wheel in a direction opposite to the applied force. The wheel rolls without rupturing the coldwelds. That is why the rolling friction is extremely small than sliding friction. The fact that rolling friction is less than sliding friction is applied in ball bearings or roller bearings to reduce losses due to friction.
The wheel would not roll on pushing it if there would be no friction between the wheel and the ground. Thus, friction is desirable for wheels to roll over a surface. It is dangerous to drive on a wet road because the friction between the road and the tyres is very small. This increases the chance of slipping the tyres from the road. The threading on tyres is designed to increase friction. Thus, threading improves road grip and make it safer to drive even on wet road.
A cyclist applies brakes to stop his/her bicycle. As soon as brakes are applied, the wheels stop rolling and begin to slide over the road. Since sliding friction is much greater than rolling friction.
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