Physics 1 Dynamics: Contact and Non-Contact Forces

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Contact and Non-Contact Forces in Equilibrium

Explore types of contact and non-contact forces and forces in equilibrium. Create a free body diagram and study the physics if a situation.

Contact and Non-Contact Forces

Examples of contact and non-contact forces

  • Contact Forces (involve touching)
    • Applied force: The push or pull from another object
    • Normal force: a push perpendicular to the surface reacting to weight
    • Tension: the pull in a rope
  • Field Forces (non-contact)
    • Weight: The force created by the attraction due to gravity
    • Electrostatic Force: The force caused by charge interactions
How many contact and non-contact forces are acting on box A

In a free body diagram

  • The object is represented as a dot
  • All forces are drawn as arrows
  • All arrow tails must originate touching the object the objects the force is acting
  • Only real forces not components should be included when asked to draw a free body diagram.
Close up free body diagram of object A

Q: How Many Contact and Non-Contact Forces are acting on box A?

  • List the forces and if they are contact or non-contact

Contact

  • Normal (FN)
  • Tension (FT) 

Non-contact

  • Weight (Fg)

Weight (Fg)

Forces have the MKS unit Newton derived from mass times acceleration (kg x m/s2).

One Newton of force is the force required to accelerate a 1 kg mass by 1 m/s2.

Objects are pulled down by the attraction of the earth by a force equal to their weight (Fg).

Fg = mg

  • Mass (m) in the unit kg
  • Acceleration due to gravity at the surface of a planet (g) in the unit m/s2.

Acceleration due to gravity on earth is around 9.8 m/s2.  In physics problems, including ones provided by college board this is often rounded to 10 m/s2.

FgA = (5)(10) = 50 N

FgB = (15)(10) = 150 N

The 5kg box would have a weight of 50 N down and the 15kg box would have 150 N down when using 10 m/s2 as g.

***Note: Subscripts (gA) attached to Force are descriptive and may change from book to book or site to sight FgA : the force of gravity on box A

Mass and Weight of Box A and Box B

Many problems in AP physics stick to variables so it is important to be able to determine the mathematical expression or equation for a situation without numbers.

For box A, its weight would be mAg

For box B, its weight would be mBg

mAg and mBg

 

A box in the air with only its weight (Fg) force would be in free fall accelerating at 10 m/s2

A box on the ground would have a perpendicular to the surface reaction force to weight (Fg) called normal force (FN)

When you have a simple situation with only a box on a horizontal surface, the normal force is directed straight up and is equal to an object weight.

Free body diagram of A alone
Weight and Normal Force on Box A

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