An object at rest or in motion has different forces acting on it. A book on a table exerts a force on the table.
Similarly, external forces are acting on the book. The external forces acting on the book are the force of gravity and an upward force exerted by the table called the normal force.
If the forces acting on an object in opposite directions are equal, the book remains at rest. Forces acting on a body at rest are balanced.
If one of the forces acting on a body at rest is greater than the other, the body starts moving. The forces are unbalanced.
The net force is the sum of all external forces acting on an object—the forces can act downwards, upwards, or horizontally.
When a horizontal force is applied to a book lying on a table, the net force acting on the book is the sum of all the external forces.
Where Fg is the force of gravity
FN is the normal force
Fa is the force applied
Ff is the frictional force
If the force applied is higher than the sum of the force of gravity, frictional force, and normal force, the book starts moving.
Net forces can do the following;
- Increase the acceleration of a body in motion.
- Bring a body in motion to rest
- Change the direction of a body in motion
Acceleration on a moving object is caused by net force. The magnitude of net force acting on an object is obtained as a product of object mass and acceleration.
Where Fnet = Net force
M = Mass
Here are ten examples of net force in real life.
1. Kicking a ball
When you kick a ball at rest, you exert action force on the ball. The ball exerts reaction force on your foot.
Other external forces acting on the ball include the force of gravity and frictional force from air.
The frictional force acts against action force, while the force of gravity acts against normal force.
Fnet = Fa + FN – (Fr + Fg).
2. Tug of war
If ten men are in a tug of war, five on each side are pulling a rope, the force acting on the rope is a sum of action force by the mean on each side.
If the action force of the men on each side is equal, their net force is zero.
When the action force of the men on one side exceeds the other, the net force causes the rope to move to the side with greater action force.
3. A moving car at a road bend
When a moving car is passing a road bend, forces of friction and gravity acts against the action force, preventing accidents.
A moving vehicle is likely to follow a straight line but frictional force from the air and on the road acts against its force resulting in deceleration.
4. The force of current against the wind on a sailing boat
The main forces acting on a sailing boat are gravity and the reaction force of the wind.
Frictional force and reaction force from water acts against the direction of the moving boat.
The net force can be obtained as: (Fa + Fg) – (Ff + FN)
5. An object floating on water
A wooden chip in a glass of water floats on water. The forces acting on the chip are gravity acting downwards and buoyancy acting upwards.
The net force is given by; Fg – Fb since the two forces are acting in different directions.
6. Opening and closing a door.
When you open a door by pushing or pulling, the action force acts against frictional force hence opening the door.
The net force acting on the door is given as;
Fnet = Fg +Fa – Fr
7. Pushing a trolley
When you push a trolley, you exert action force on it. The net force on the trolley is the sum of action force, the force of gravity, and frictional force.
Fnet = Fa + Fg – Ff
8. A moving car coming to a halt
A moving car decelerates before coming to a halt. There are forces acting on the moving resistance force from air.
For the car to stop, the net force should be zero (the opposing forces are balanced)
9. Net force on a see-saw
When you put an object on one end of a see-saw, the end drops. The force of gravity acts on the object pulling it down with the beam.
The net force is zero until force you apply force on one end.
10. Force acting on a person seated on a chair
When you sit on a chair, you exert force on the chair until you attain stability when the net force acting on your chair is zero.
The net force is zero when the sum force of gravity and action force equals the normal force. (upward force)
Net forces control the state of an object. The acceleration and direction of motion are dependent on the net force acting on an object.