Forces

Dynamics

Why do objects move like they do?

Weight

This force is the result of the gravitational attraction between the mass in question and the mass of the earth. The weight of a body is the gravitational force experienced by that body, which on earth is given by the formula

W = mg

TensionA string that is taut is said to be under tension. The force that arises in any body when it is stretched is called tension. A tension force in a string is created when two forces are applied in opposite directions at the ends of the string.

To say that there is tension in a string means that an arbitrary point on the string is acted upon by two forces (the tension T).

Normal reaction forces

If a body touches another body, there is a force of reaction or contact force between the two bodies. This force is perpendicular to the body exerting the force.

Drag forces

Drag forces are forces that oppose the motion of a body through a fluid (a gas or a liquid).

Upthrust

Any object placed in a fluid experiences an upward force called upthrust. If the upthrust force equals the weight of the body, the body will float in the fluid. If the upthrust is less than the weight, the body will sink.

FrictionFrictional forces oppose the motion of a body. Friction arises whenever one body slides over another. In this case we speak of kinetic friction. Friction also arises whenever there is just a tendency for motion, not necessarily motion itself. In this case, we speak of static friction.

Hooke´s law

If we try to extend a spring, a force pulls the spring back to its original length; if we try to compress a spring, again a force tries to pull the spring back to its original length. The force in the spring has a simple relationship to the amount by which the spring is extended or compressed.

T = k

The extension or compression of the spring must not be too large, otherwise Hooke’s law isn’t applicable. That range is called as the elastic limit.

Free-body diagrams

A free-body diagram is a diagram showing the magnitude and direction of all the forces acting on a chosen body. The body is shown on its own, free of its surroundings and of any other bodies it may be in contact with.

Questions

Newton’s first law

In ancient times, Aristotle had mantained that a force is what is required in order to keep a body in motion. The higher the speed, the larger the force needed.

Galileo, 2000 years after Aristotle, was the first to realize that the state of no motion and the state of motion with constant speed in a straight line are indistinguishable from each other.

Newton’s first law

When no forces act on a body, that body will either remain at rest or continue to move along a straight line with constant speed.

Newton’s first law

Newton’s first law is also called the law of inertia. Inertia is the reluctance of a body to change its state of motion. Inertia keeps the body in the same state of motion when no forces act on the body.

Inertial frames of reference

A system on which no forces act is called an inertial frame of reference. Observers belonging to different inertial frames will come up with the same laws of physics.

Equilibrium

When the net force on a body is zero, the body is said to be in equilibrium.

If a body is displaced slightly from its equilibrium position, the net force on the body may or may not be zero. If it is still zero, the position of equilibrium is called a neutral equilibrium position.

Equilibrium

If the net force in the displaced position tends to move the body back towards the initial equlibrium position, then we speak of stable equilibrium. If, on the other hand, the net force on the body tends to make it move even further from the initial position, we speak of unstable equilibrium.

Equilibrium

Equilibrium of a point particle means that the net force on the point is zero.

Example questions

More questions…

Newton’s second lawThe net force on a body is proportional to that body’s acceleration and is in the same direction as the acceleration.

F = ma

Newton’s sencond law

The equation F = ma defines the unit of force.

One newton is the force required to accelerate a mass of 1 kg by 1 ms-2 in the direction of the force.

Example questions

Terminal velocityWhen a body moves through a fluid (a gas or liquid), it experiences an opposing force that depends on the speed of the body. If the speed is small, the opposing force is proportional to the speed, whereas for larger speeds the force becomes proportional to the square of the speed.

As the speed increases, so does the opposing force and hence after a while, it will become equal to the weight.

Terminal velocity

The inclined plane

Newton’s third law

If body A exerts a force F on body B, then body B exerts an equal but opposite force on body A.

Questions