Machines and Movement Part 2


Gravity and Center of Gravity
Gravity is the  force of attraction between objects and the earth. Gravity gives objects there weigh. Like other kind of force, it is measured in newtons (N). The earth gravity is 10Nkg-1 . The weight (W )of a mass (m )is given by W=mg.
The centre of Gravity of a body is the point at which all the weight of a body may be considered to act. A low centre of gravity gives greater stability.
A pulley is a machine made up of a wheel and rope/chain/or belt. Examples of pulleys are flag poles, sail boats, and cranes.  Pulleys allow you to move objects up and down. Effort occurs at one end (pull) while the load moves at the other end (push).


The number of wheels on the pulley determines the effort force that is needed:

1 wheel = 1/1 effort  2 wheels = ½  effort  3 wheels = 1/3 effort

Fixed pulley – effort force equals the load force; the fulcrum does not move

example: clothesline

The load is still easier to move because pulling down and inward is easier than pushing up against the force of gravity.

Moveable pulley – the pulley moves with the load

example: block and tackle

Complex machines are made by combining simple machines:

examples: engine, ratchet, mountain bike


A screw  is an inclined plane wrapped around a shaft or cylinder. The inclined plane allows the screw to move itself when rotated. Examples of a screw are clamps, spiral stair cases, and wrenches.  Screws hold things together.
The special groove on the screw is called the thread.


A wedge is a double-inclined plane . it pushes two objects (or portions of a single object) apart. Example of a wedge are knives, forks and axe.
A wedge changes the direction of force.
Inclined Plane
An inclined plane is a plane surface set at an angle to another surface. Inclined planes help us move things to low or higher places. it requires less force to move up a ramp to a higher elevation than to climb to that height vertically. The wedge is often considered a specific type of inclined plane.
incline plane 2 incline plane
Wheel and Axle
Wheel and axle: A wheel is a circular device that is attached to a rigid bar in its centre (axle). The axle will turn when there is a force on the wheel. A car, wagon, and roller skates are just few examples of wheel and axle.  A wheel and axle enables us to move things from place to place.
wheel and axle
  1. Name four types of simple machines.
  2. Define the following terms; pulley, inclined plane and wheel & axle.
  3. What is gravity ?
  4. Write the formula for calculating weigh.

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