Force and Equilibrium of Objects
Senior 1 · Physics
Force and Equilibrium of Objects
Lesson 2 of 5
22 min
Think of a football match on a school field in Nyanza. A player kicks the ball from rest: it starts to move. Another player heads it: it changes direction. The goalkeeper catches it: it stops. After the match someone sits on the ball: it is squashed out of shape, then springs back. The same kind of thing, a force, did all of these. In this lesson you will list what forces can do, and learn to draw the forces acting on an object clearly enough to predict what it will do next.
Set up five stations around the classroom. At each one, apply a force and record what changes.
| Station | What you do | What the force changes |
|---|---|---|
| 1 | push a toy car that is standing still | it starts to move |
| 2 | push a slowly moving trolley from behind | it speeds up |
| 3 | stop a rolling ball with your hand | it slows down and stops |
| 4 | tap a rolling ball from the side | its direction changes |
| 5 | stretch a rubber band; squeeze a lump of plasticine | the shape changes: the band springs back, the plasticine stays squashed |
Important
Safety. Roll balls gently and along the floor, never throw them. Do not stretch rubber bands towards anyone's face. Wash your hands after using plasticine.
Definition
A force can: start an object moving, stop it, change its speed, change its direction, or change its shape or size.
A change of shape is elastic deformation if the object returns to its original shape when the force is removed (a rubber band, a spring), and plastic deformation if it stays changed (plasticine, wet clay, a dented car door).
Definition
A free-body diagram shows ONE object on its own, with every force acting ON it drawn as an arrow. Each arrow starts on the object, points in the direction of the force, and is labelled with the force's name or size.
The resultant force is the single force that has the same effect as all the forces together. If the forces are balanced, the resultant is zero.
Figure 2.1: Free-body diagrams. (a) The table's push R balances the book's weight W. (b) The tension T in the cord balances the lamp's weight. (c) At steady speed, the forward driving force balances the resistive forces (friction and air resistance), and R balances W.
In all three diagrams the forces are balanced, so the resultant force is zero. The book and the lamp stay at rest, and the cyclist keeps moving at a steady speed in a straight line. An unbalanced force would change the motion: start it, stop it, speed it up, slow it down or turn it.
Worked example 1: a book on a table
A book of mass 0.50 kg lies on a table. Find the two forces on it and the resultant force.
The book is at rest, so the table's normal reaction must balance its weight: upwards. Resultant force .
Worked example 2: pushing a box
Keza pushes a box across the floor with a force of 30 N. Friction on the box is 12 N. Find the resultant force and describe its effect.
Take Keza's push as positive. Friction acts the opposite way:
The forces are unbalanced, so the box speeds up.
Worked example 3: tension in a rope
A lamp of mass 2.0 kg hangs at rest from a cord. Find the tension in the cord.
The lamp is at rest, so the tension balances the weight:
Warning
Common mistakes in free-body diagrams. (1) Drawing forces that the object exerts on OTHER things (such as the book pushing down on the table): draw only forces acting ON your object. (2) Adding a "force of motion" in the direction the object moves: a moving object does not need a forward force to keep going at a steady speed; it needs one only to overcome friction. (3) Arrows that do not start on the object.
Tip
Weight, then touches. Every free-body diagram starts with the weight. Then run your finger around the object: every place it touches something gives one more force.
Rwandan potters shape wet clay by pressing and pinching it: a plastic deformation, which is why the pot keeps its shape. A bamboo pole or a sisal rope carrying a heavy load bends or stretches and then returns to its shape when the load is removed: an elastic deformation. Builders must never load a beam beyond its elastic limit, the point past which it stays bent and may break.
Types of forces and free body diagrams (Khan Academy). Watch how each force on an object is drawn as a separate arrow starting on the object.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the force-effects activity, the free-body diagrams, the worked examples and the practice exercises.
