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Multiple Masses
Tension in
Ropes and Cables

When a crane exerts a force on one end of a
cable, each particle in the cable, exerts an equal
force on the next particle in the cable. This
creates tension throughout the cable

Tension – magnitude of the force exerted on or
by a cable, rope, or string
Assumptions

1. mass of the rope or cable is very small and therefore
we ignore compared to the load and does not affect
motion

2. tension is the same at every point in a rope or cable

3. If the rope or cable passes over a pulley, the direction
of the tension changes but the magnitude stays the
same. (pulley is frictionless and its mass is negligible)
Connected Objects

Elevators are not simply suspended from cables.
Instead the supporting cable passes over a pulley
and then back down to a heavy, moveable
counter-weight. Gravitational forces act
downward on the counter-weight creating
tension in the cable. The cable then exerts an
upward force on the elevator
Atwood Machines

George Atwood (1745-1807) built a machine he
used to determine the value of g
Assigning Direction to the Motion of
Connected Objects

With the Atwood machine the masses are moving in
different direction but connected objects move as a unit

The connecting coble or rope changes only the
direction of the forces acting on the objects and has no
effect on the magnitude of the forces. You can assign
the direction of the motion as being from one end of
the cable or rope to the other
Se figure 10.12 page 482


When working with a group of connected
objects we call it a system. The forces exerted
through a rope or cable are called internal
forces

The forces that affect the motion of a system are
known as external forces (gravity, friction)
Objects Connected at an Angle


In the lab, a falling weight is often used to
provide a constant force to accelerate dynamic
carts. Gravitational forces act downward and the
string exerts a horizontal force on the cart
Steps for Problem Solving





Analyze the forces on each individual object
(FBD)
Assign a direction to the motion
Draw the connecting string or rope as though it
was a straight line
Draw a FBD for the resulting system
Use Newton’s 2nd Law to calculate acceleration