lecture 6: forces and torques of the body

force

• how objects interact

• Push (compression) or pull (tension)

• Produces changes in some physical qualities

• SI unit = N

• 1 N = 1kg m/s^2

• Is a vector

• Forces add as a vector

• Free body diagram (forces are all pulling)

• Force can cause an acceleration of an object

force of gravity

• on me by Earth

mass (kg)

• mg

• g is in 9.8 N.kg

weight (N)

• the size (magnitude) of the gravitational force

balanced forces

• net force on water is zero, thus the water is stationary

natural state of an object

• in motion with constant velocity and direction

newtons first law

1. Every object in a state of uniform motion teds to remain in that state of motion unless acted on by an external force

• 'law of inertia'

• An object stays in its state of rest (or of uniform motion in a straight line) as long as no net force acts on it

  • Net (or resultant) force means sum of all forces acting on the object. We always consider the direction of the forces

  • E.g. Book (object) on table

  • Gravitational force on book by Earth

  • Contact or Normal force (N) on book by table

  • Gravitational force actually acts on all parts of the object, but shown as if it acts at one point, the 'centre of mass'

newtons second law

2. The relationship between an object's mass (m), its acceleration (a), and applied (net) force (F) is F=ma

• Acceleration (a) of an object is

  • Directly proportional to the net force on the object

  • Inversely proportional to the object's mass

  • Direction of acceleration = direction of net force

  • Mathematically;

  

newtons third law

1. For every action there is an equal and opposite reaction

net force on an object

• Newton's 3rd Law states that for every action there is an equal and opposite reaction

• In order to determine the net force on an object, you need to consider the object alone (consider a person falling towards earth)

newtons third law; normal force and tension

• Forces (push or pull)

  • Only acts perpendicular (normal) to surfaces

• Forces (tension)

• The same everywhere along the rope (doesn't matter where you hold the rope)

equilibrium and balance

• For an object to be in equilibrium, the net force must be zero

• In equilibrium an object must also have zero net torque

torque (moment of force)

• Torque: an application of a force that causes rotation

• Unit: Nm

• Torque (moment of a force) around a given axis is

 

τ=LF

 

• L= perpendicular distance from the line of action of the force to the axis or pivot point (L is also called the lever arm)

 

• Rotational equilibrium requires zero net torque, Στ=0

rotational equilibrium

• requires zero net torque, Στ=0

• requires no angular acceleration

centre of mass

• Many objects exhibit translational and rotational motion

• Even if the object is rotating, there appears to be a point that follows the path of the translational motional

• That point is the centre of mass (axis of rotation)

example; muscle acting on an angle

“magnifying” force or distance moved

• In common cases of levers, a small force moved through a large distance is changed to a large force moved through a small distance

• R/r = "the mechanical advantage"

• (mg)r = (Fp)R

• Muscles only constact small distances

• Levers in the body allow fingers and feet etc., to move further and faster

classes of levers