Unit 5 Dynamics II: Constant Force

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This is where we make the connection between forces acting on an object and its motion. If the forces acting on an object are unbalanced, acceleration will occur. What variables affect an object's acceleration?

Common sense would tell us that more force produces a larger acceleration. Anyone who has ever watched baseball knows that a bunt exerts a small force on a baseball and produces a smaller acceleration. If a player wants to "knock it out of the park", more force will produce a larger acceleration.

  • Acceleration seems to be proportional to unbalanced force.

Consider a racing bicycle Vs. an old clunker. The racing bike is less massive and responds better to the applied force you exert on it than the heavy old clunker. More mass might act to decrease acceleration.

  • Acceleration seems to be inversely proportional to mass.

The equation that relates force, mass and acceleration is Newton's second law:

SF = ma

In order to solve quantitative problems with Newton's second law, we need to be able to draw force diagrams, analyze information and apply the Newton's second law equation. At this point, we will only concentrate on motion in one direction. The unbalanced force will act in only one dimension; either the x or y direction.

Mass Vs Weight

Mass and weight are often mistakenly thought to be the same thing.

Mass is the quantity of matter, whereas weight refers to the force of gravity exerted by the earth on a mass. Mass does NOT depend on the gravitational pull and cannot be represented by a force diagram.

A 1 kg object will have a mass of 1 kg anywhere; earth, moon or space. The weight of a 1 kg mass will vary from 9.8 N on the surface of the earth to 1.6 N on the surface of the moon to 0 N in space far away from any planet or star. The force of gravity is the weight of an object and is represented in the force diagram of the object.

Friction

Friction (Ff) is a force that opposes motion when two objects are in contact. It is an electromagnetic force that is caused by the interaction of the electron clouds of the atoms of each object in contact.

There are essentially two kinds of friction:

1) Static Friction- The friction that occurs between stationary objects in contact.

2) Kinetic Friction - The friction that occurs between two objects sliding against one another.

Two things determine the amount of friction between two surfaces:

  • Coefficient of Friction (m) 
    •  - this is a way of describing the amount of "stickiness" between surfaces. The coefficient of friction between dry pavement and the rubber of your tires is a high number. The coefficient of friction between rubber tires and an icy road is nearly zero.The coefficient of static friction (ms) between two surfaces is always greater than the coefficient of kinetic friction (mk).
  • Normal Force (FN) 
    • - the force that a surface pushes up on an object that is on it. Though sometimes, the normal force is equal in magnitude to the weight of the object, it is not always the case. Consider the diagram below. Only in the one case is the normal force equal in magnitude to the weight.

The frictional force Ff can be calculated by using the following formula:

Ff = mFN

Friction always acts to oppose motion. If an object is moving east, friction acts towards the west. If an object is being pushed north, friction opposes the push by acting towards the south.

Some Interesting Links About:

Friction:           http://chemweb.richmond.edu/~rubin/pedagogy/131/131notes/131notes_61.html
Coefficient Of Friction: http://chemweb.richmond.edu/~rubin/pedagogy/131/131notes/131notes_62.html
 

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