Quick answer: A force is a push or a pull, measured in newtons with a spring scale. Also, forces act in equal and opposite pairs on two different objects. When the forces on an object balance, the net force is zero and its motion does not change. However, unbalanced forces make it speed up, slow down or change direction.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS2-1, MS-PS2-2
What is a force?
Whenever you open a door, kick a ball or hold a bag, you are using forces. Moreover, forces explain why things start moving, stop, speed up, change direction or change shape.
A force is a push or a pull that one object exerts on another. In addition, we measure force in newtons (N).
Forces always come in pairs. When object A pushes on object B, B pushes back on A with a force of equal size in the opposite direction. Moreover, the two forces act on different objects.
If you push a wall with 40 N, the wall pushes back on your hand with 40 N. On a skateboard you would roll away, because the force of the wall acts on you.
What are contact and non-contact forces?
Also, some forces need the two objects to touch. Furthermore, others act across a gap.
| Type | How it acts | Examples |
|---|---|---|
| Contact force | The objects touch | Push, pull, friction, tension in a rope, air resistance, support force |
| Non-contact force | Acts across a gap | Gravity, magnetic force, electrostatic force |
How do you draw and measure a force?
Forces have a size and a direction. In addition, we draw each one as an arrow. Also, the arrow starts on the object, points in the direction of the force, and its length is proportional to the size. If 1 cm stands for 10 N, then you draw a 30 N force 3 cm long.
A spring scale, also called a newton meter, measures force. The harder you pull, the more the spring stretches, and the pointer moves along the scale. In addition, always read the scale at eye level and check that it starts at zero.
A spring scale has a pointer exactly one mark below 6 N and the marks are 0.5 N apart. What does it read?
The mark below 6 N is 6.0 − 0.5 = 5.5 N.
What are balanced and unbalanced forces?
Moreover, several forces can act on one object at the same time. Furthermore, to find their combined effect, add the forces that point the same way and subtract those that point the opposite way. Furthermore, we call the result the net force.
Net force and balanced forces
The net force is the overall force on an object after you add all the forces, taking direction into account. Forces balance when the net force is zero.
| Situation | Net force | Effect on motion |
|---|---|---|
| Balanced forces | 0 N | Object at rest stays at rest; moving object keeps the same speed in a straight line |
| Unbalanced forces | More than 0 N | Object speeds up, slows down or changes direction, in the direction of the net force |
In a tug of war in Karachi, team A pulls with 450 N to the left and team B pulls with 380 N to the right. Moreover, find the net force.
net force = 450 N − 380 N = 70 N to the left, so the rope accelerates towards team A.
Ngozi pushes a trolley along a smooth corridor in Enugu at a steady speed. She pushes with 25 N. What is the friction force?
A steady speed means the forces balance, so friction is 25 N in the opposite direction.
The size of the net force also matters. A larger net force produces a bigger change in motion. The same net force changes the motion of a heavy object less than that of a light one.
Common mistake about forces
In addition, many people think that an object needs a force to keep moving. Also, it is not. A moving object keeps going at constant speed with zero net force. Moreover, you push a trolley only to cancel friction.
Draw a quick free-body sketch for every problem: one dot for the object and one arrow for each force. Label every arrow with its size, then add left against right and up against down.
- A force is a push or a pull; the unit is the newton (N), measured with a spring scale.
- Furthermore, contact forces need touching (friction, tension); non-contact forces act across a gap (gravity, magnetism).
- Net force: add forces in the same direction and subtract forces in opposite directions.
- In addition, balanced forces (net force 0 N) mean no change in motion; unbalanced forces change speed or direction.
Pushes, pulls and balanced forces: frequently asked questions
What is a force in simple words?
A force is a push or a pull. Also, it can start an object moving, stop it, change its speed or direction, or change its shape.
What is the difference between contact and non-contact forces?
Contact forces need the objects to touch, like friction and tension. In addition, non-contact forces act across a gap, like gravity and magnetism.
What is a newton?
Furthermore, the newton (N) is the SI unit of force. A small apple pulls down on your hand with a force of about 1 N.
What happens when forces on an object are balanced?
Moreover, the net force is zero. An object at rest stays at rest, and a moving object keeps moving at the same speed in a straight line.
Why do forces come in pairs?
Every interaction involves two objects. When one pushes on the other, the second pushes back with an equal force in the opposite direction.
