FORM 2-PHYSICS-TOPIC 5-SIMPLE MACHINES
A simple machine is a non-powered mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force. A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force.
The Structure of a Screw Jack
Describe the structure of a Screw Jack
Structure of Screw Jack
A screw Jack consist of a cylinder with a spiral ridge runs round it. The spiral is called the thread (T) and the distance between two adjacent threads is called the pitch (p) of the screw.
Consider the following diagram.
The bolt and screw
The Screw Jack
The Mechanichal Advantage, Velocity Ratio and Efficiency of a Screw Jack
Determine the mechanical advantage, velocity ratio and efficiency of a Screw Jack
Mechanical advantage of the screw jack
M.A = Load/Effort
Velocity ratio (V.R)
V.R = Circumference of circle of radius (R)/Pitch of screw
V.R = 2pR/P
Example 7
Example 1
A screw jack with a pitch of 0.2cm and a handle of length 50cm is used to lift a car of weight 1.2 x 104N. If the efficiency of the screw is 30%, find:
- The velocity ratio and mechanical Advantage of the machine
- The effort required to raise the car
Solution;
Date given
Pitch = 0.2cm
Radius= 50cm
Load = 1.2 x104N
Efficiency of jack =30%
V.R=?
M.A=?
Effort+?
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The Screw Jack in Daily Life
Use the Screw Jack in daily life
Screw jacks are used to lift heavy loads despite the large friction they produce. The heavier the load the higher the friction force. They are self-locking, meaning that when the applied force is removed, they do not rotate backwards. They are used in adjusting workplace chairs and tables. They also help in pulling and pushing machine equipment as well as tightening mechanical parts.
