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Physics - Forces

Acceleration requires force. The only way to accelerate a body is to apply force. If a body has no force acting on it, then its velocity will never change. If a body is moving at 200 kilometers per hour in the westward direction, it will continue moving at that velocity forever unless some force acts on it. If a body is stationary, it will continue to be stationary forever unless some force acts on it. Force has direction. It is a vector quantity. Force is measured in newtons.

Rule number 5:
When a force acts on a body, it produces an acceleration equal to the force divided by the mass of the body. Remember this equation in any of these 3 forms: A=F/M, F=MA, M=F/A.

When we say an object weighs 400 pounds, what we are really saying is that the earth is pulling on that object with a force of 400 pounds. The object’s mass is about 182 kilograms and its weight is 182 kilograms times 9.8 m/sec2 or approximately 1780 newtons.

Force and Weight

Rules 4 and 5 combined mean that every object on or near the earth experiences a downward force equal to its own mass times the acceleration produced by the earth’s gravity which is 9.8 m/sec2. The force that an object experiences due to gravity is called the object’s weight.

Weight is important in 2 ways.
  1. If an object weighs 500 newtons, that means that gravity is causing that object itself to experience a downward force of 500 newtons.
  2. It also means that when that object rests or hangs on some other body or surface, the object imposes a force of 500 newtons on that body or surface.

Since weight is a type of force, it is measured in newtons. The point at which all the weight of an object is being concentrated is called the object’s center of gravity. For objects that are of uniform mass throughout the center of gravity is at the geographic center of the object.

Bodies can experience more than one force at a time. If a body experiences more than one force at a time, it might or might not accelerate. Whether a body will actually accelerate depends on the sum total of all the forces acting on it. That sum is called net force.

Rule number 6:
For bodies at rest: When one body exerts a force on another, the other body exerts an equal and opposite force back on the first.

Rotational force (Torque)

If two kids are sitting on a seesaw, they are exerting on the seesa6w a force about an axis. A force that is on an object about an axis is called rotational force or torque. Torque can be clockwise or counterclockwise. When force is exerted about an axis, the distance between the force and the axis is called the lever arm.

Rule number 7:
Torque equals force times lever arm. To calculate the net torque imposed on the seesaw, all you have to do is this. For each child, take the force exerted and multiply it by the child’s distance from the axis then subtract these two forces from each other and you will get the net force exerted on the axis of the seesaw.

Understand the Force of Friction

Think about a box sitting on a ground and imagine you are going to push it to the right. It turns out at a very instant, you begin to push, a force starts working against you and that force is the force of friction between the box and the ground it is sitting on. When an object is resting on a surface and we just begin to slide it along the surface, the force of friction in the opposite direction is equal to the product of the following:

  1. The force that the object is exerting directly perpendicular to the surface.
  2. The coefficient of friction between the two surfaces.

The first quantity is equal to the weight of the object if the surface is horizontal. If the surface is not horizontal, then the first quantity is equal to the component of the object’s weight that is perpendicular to the surface. This is called the normal force.

Rule number 8:
Force of friction equals normal force times coefficient of friction. Different materials have different coefficients of friction. It turns out that for any two surfaces there are usually two coefficients of friction. One applies at the instant before motion begins and the other applies after the motion is already in progress. The first is called the coefficient of static friction and the second is called the coefficient of kinetic friction.