Force

• Definition of Force

  • A force is a physical cause that changes or tends to change the state of rest or uniform motion of a body in a straight line.

  • Introduced by Sir Isaac Newton in his three laws of motion.

• Characteristics of Force

  • Described in everyday terms: push, pull, stretch, squeeze.

  • Causes displacement (e.g., pushing a door opens it, squeezing a cloth changes its shape).

Force as a Push

• Definition: A body is pushed if its motion is away from the source of the applied force.

• Examples:

  • Pushing a chair or table to move it.

  • Pushing a bus or car that has problems.

  • Birds flapping wings to push against air.

• Effects of Push:

  1. Change in state of rest or motion.

  2. Change in shape and size.

Force as a Pull

• Definition: A body is pulled if its motion is toward the source of the applied force.

• Examples:

  • Pulling a door to open it.

  • Pulling a person from danger.

  • A horse pulling a cart.

• Characteristics:

  • Forces exist in pairs and oppose each other.

  • Changes depend on the magnitude of forces.

  • Changes occur in the direction of the resultant force.

Force as a Stretch

• Definition: A body is stretched if a force increases its length in the direction of the force.

• Examples:

  • Stretching a wire with a weight.

  • Pulling a bowstring.

  • Stretching a spring.

• Characteristics:

  • On release, it tends to return to original shape.

  • Greater force leads to greater change in shape.

Force as a Compress

• Definition: A body is compressed if a force decreases its volume.

• Examples:

  • Compressing cotton using a hydraulic machine.

  • Compressing gas in a container.

• Characteristics:

  • Changes occur within elastic limit (doesn't break).

  • More force leads to greater compression.

Force as a Squeeze

• Definition: A body is squeezed if a force changes its shape.

• Examples:

  • Squeezing a toothpaste tube.

  • Squeezing washed clothes.

  • Squeezing a gum tube.

Effects of a Force

1. Changes dimensions of non-rigid bodies (e.g., dough, plaster).

2. Makes an object move or stops a moving object (e.g., kicking a ball).

3. Changes speed or direction or both (e.g., cyclist pedaling faster).

4. Force required to keep a body in motion comes from external application (Newton's laws).

Units of Force

• In CGS:

  • Unit: dyne (dyn)

  • 1 dynam = 1 g × 1 cm/s²

• In SI:

  • Unit: newton (N)

  • 1 N = 1 kg × 1 m/s²

• Relationship:

  • 1 N = 10^5 dyn

Translational and Rotational Motion

• Translational Motion:

  • Motion in a straight line due to force.

• Rotational Motion:

  • Motion around an axis when force is applied.

Turning Force (Torque)

• Definition: The moment of a force which causes rotation.

• Formula:

  • Torque (τ) = Force (F) × Distance from pivot (d)

• Units:

  • CGS: dyn cm, SI: Nm

Equilibrium of Bodies

• Definition: A body is in equilibrium when forces acting on it produce no change in motion.

• Conditions for Equilibrium:

  1. Algebraic sum of all forces = 0.

  2. Algebraic sum of all moments about a point = 0.

Centre of Gravity

• Definition: Point where the resultant force due to gravity acts.

• Characteristics:

  • Dependent on shape and mass distribution.

  • For symmetrical objects, the center of gravity is located at geometric center.

  • Stability increases with a lower center of gravity and larger base area.

Circular Motion

• Uniform Circular Motion: Constant speed along a circular path.

• Centripetal Force: Force directed towards the center to maintain circular motion.

  • Formula: F = (m v²)/r, where m = mass, v = velocity, r = radius.

• Centrifugal Force: Apparent force acting away from the center, caused by inertia.

Summary of Key Terms

• Force: Physical cause changing state of motion.

• Equilibrium: No net change in motion for multiple forces.

• Torque: turning effect of force.

• Moment of Couple: Parallel forces causing rotation.

• Centre of Gravity: Point of resultant gravitational force.