PHYSICS FORM ONE TOPIC 4:FORCE

PHYSICS FORM ONE TOPIC 4:FORCE

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PHYSICS FORM ONE TOPIC 4:FORCE

 

FORCE

A force is an interaction that causes a change. In mechanics, a force is an interaction that causes a change in velocity (an interaction that causes acceleration).
Explain the concept of force
For better understanding of this concept, let’s ask ourselves the following question:
  1. What causes/makes a body at rest to move?
  2. What causes the same body in motion to stop?
The answers to these questions, is of course that a force is required to do so. Here, an applied force to a body can cause the body at rest to move or if already moving a body application of force can do the following when applied to a body.
  1. Force can cause a change in the way the object moves
  2. Change its size or shape
  3. Change the direction in which an object is moving
A force is defined as a push or pulls experienced by an object. The force is usually described in terms of its size (magnitude) and direction.

The S.I Unit of Force
State the S.I unit of force
The SI unit of force is a Newton (N).
A Newton (N) can be defined as the amount of force required to give a mass of one kilogram (1kg) an acceleration of 1m/s2
Measuring Instrument (Device)
The instrument that is used to measure the amount of force that is exerted on an object is called spring balance.
A spring balance can be used to measure small forces. It consists of a coiled spring fixed to the other end with a hook at the other end. The body upon which the force acts is attached to the hook. The distance through which the spring is stretched is directly proportional to the force applied by the balance.
A pointer is attached to the spring and the force is indicated on a calibrated scale.
PHYSICS FORM ONE TOPIC 4:FORCE
A spring balance

PHYSICS FORM ONE TOPIC 4:FORCE

Identify effects of forces
A force acting on an object may cause the object to change shape, to start moving, to stop moving, to accelerate or decelerate.
When two objects interact with each other they exert a force on each other, the forces are equal in size but opposite in direction.
Resultant force
The forces acting on an object can be replaced with a single force that causes the object to behave in the same way as all the separate forces acting together did, this one overall force is called the resultant force.
If the resultant force acting on an object is ZERO then:
  • The object will remain stationary if it was stationary when the resultant force became zero.
  • Move at a constant (steady) speed in a straight line if it was moving when the resultant force became zero.
If the resultant force acting on an object is NOT ZERO then:
  • The object will accelerate or decelerate (speed up or slow down).
The Effects of Forces on Materials
Justify the effects of forces on materials
Example 3
A rectangular bar having a cross-sectional area of 80 mm2has a tensile force of 20 kN appliedto it. Determine the stress in the bar.
PHYSICS FORM ONE TOPIC 4:FORCE

Example 4
A circular section cable has a tensile force of 1 kN applied to it and the force produces a stress of7.8 MPa in the cable. Calculate the diameter of the cable.
Example 5
A split pin requires a force of 400 N to shear it. The maximum shear stress before shear occurs is120 MPa. Determine the minimum diameter of the pin.

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