Chapter 9-Force and Laws of Motion

Balanced and Unbalanced Forces

• What happens when some children try to push a box on a rough floor? If they push the box with a small force, the box does not move because of friction acting in a direction opposite to the push . This friction force arises between two surfaces in contact; in this case, between the bottom of the box and floor’s rough surface. It balances the pushing force and therefore the box does not move.

• If the children push the box harder still, the pushing force becomes bigger than the friction force. There is an unbalanced force. So the box starts moving.

• When all of the forces acting on an object are equal and there is no net external force acting on it, the object moves with a constant velocity.

• The object will change in speed or direction depending on whether an unbalanced force is applied to it. So, an unbalanced force is needed to accelerate an object's motion.

• As long as this imbalanced force is applied, the object's speed (or direction of motion) would change.

• Even if all forces are eliminated, the object would keep moving at the velocity it had attained up to that point.

First Law of Motion

• Galileo deduced that objects move with a constant speed when no force acts on them.
• An unbalanced (external) force is required to change the motion of objet but no net force is needed to sustain the uniform motion of the object.

Newton’s First Law of Motion

• A body remains in a state of rest or of uniform motion in a straight line unless compelled to change that state of rest or of the uniform motion by an applied force.
• In other words, all objects resist a change in their state of motion
• ==Inertia is the qualitative term for an object's tendency to remain at rest or to keep moving at a constant speed in the absence of an external force. The first law of motion is frequently referred to as the law of inertia because of this.==

Applications

• When a motorcar makes a sharp turn at a high speed, we tend to get thrown to one side. This can again be explained on the basis of the law of inertia. We tend to continue in our straight-line motion. When an unbalanced force is applied by the engine to change the direction of motion of the motorcar, we slip to one side of the seat due to the inertia of our body.
• The law of inertia can be used to explain some of the phenomena we encounter while driving a car. Until the driver applies braking force to bring the motorcar to a stop, we typically stay at rest in relation to the seat. When the brakes are applied, the car slows down, but due of our inertia, we often stay in the same state of motion.
• When a bus abruptly starts moving while we are standing in it, we often tumble backwards. This because both the bus and our feet, which are in contact with the bus floor, are in motion as a result of the bus' abrupt start. But because of its inertia, the rest of our body resists this motion.

Inertia and Mass

• Heavier or more massive objects offer larger inertia.
• Quantitatively, the inertia of an object is measured by its mass.
• We may thus relate inertia and mass as follows: ==Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.==

Newton’s Second Law of Motion

==The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.==

• The impact produced by the objects depends on their mass and velocity.
• We are aware that a stronger force is needed to produce a faster velocity when an item is being accelerated. In other words, it looks that there is some significant quantity that combines the object's mass and velocity. Newton first introduced one such property, called momentum. An object's mass, m, and velocity, v, are multiplied to create its momentum, or p. that is p = mv
• Momentum has both direction and magnitude. Its direction is the same as that of velocity, v. The SI unit of momentum is kilogram-metre per second (kg m s-1). Since the application of an unbalanced force brings a change in the velocity of the object, it is therefore clear that a force also produces a change of momentum.
• The change of momentum of a body is not only determined by the magnitude of the force but also by the time during which the force is exerted on the body.Momentum has both direction and magnitude. Its direction is the same as that of velocity, v. The SI unit of momentum is kilogram-metre per second (kgm/s). Since the application of an unbalanced force brings a change in the velocity of the object, it is therefore clear that a force also produces a change of momentum.
• The force required to change an object's momentum may then be deduced to rely on the rate of change in momentum over time.

Applications

• A fielder in the ground slowly pushes his hands backwards while catching a quick-moving cricket ball. The fielder lengthens the period of time during which the ball's high velocity lowers to zero by doing so. As a result, the ball's acceleration is lowered, which also lessens the impact of catching a ball that is travelling quickly.
• Either a cushioned bed or a bed of sand is used to catch the athletes when they fall. By doing this, the athlete's fall will take longer to come to a stop after performing the jump. As a result,the rate of change of momentum is reduced,thus the force is also reduced.

Newton’s Third Law of Motion

==Newton's third law of motion states that to every action, there is an equal and opposite reaction.==

• The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
• These two forces are always equal in magnitude but opposite in direction.
• These forces act on different objects and never on the same object.
• The two opposing forces are also known as action and reaction forces
• The action and reaction always act on two different objects, simultaneously.
• The action and reaction forces are always equal in magnitude, but this does not mean that they will always result in accelerations that are equal in magnitude.This is thus because every force acts on a separate object, whose masses may differ.

Applications

• Let's say you are at rest and plan to begin walking on a road. The second law of motion states that you must accelerate, which calls for a force.Which force is this? Is it the physical effort you do when walking? Is it moving in the direction we want it to? No, you move the ground beneath you backward. Your feet experience an equal and opposing force from the road, which pushes you forward.
• When a gun is fired, the bullet experiences a forward force. The gun is subjected to an equal and opposing force from the bullet. The gun will then recoil as a result. The acceleration of the gun is substantially lower than the acceleration of the bullet because the gun has a much larger mass than the bullet.
• When a sailor jumps out of a rowing boat. As the sailor jumps forward, the force on the boat moves it backwards due to newton’s third law of motion.