1.4.1 Newton’s Laws and Balanced Forces

1.4.1 Newton's Laws and Balanced Forces: Explain the effect of balanced forces using Newton's first law, find resultant forces along a line and describe objects at rest or moving at constant velocity.

Almost every real object has several forces acting on it, and what matters is their combined effect.

Force

A force is a push or a pull measured in newtons (N); it is a vector, so its direction always matters.

Resultant force

The resultant is the single force that has the same effect as all the forces acting on an object combined.

Balanced forces

Forces are balanced when they are equal in size and opposite in direction, so the resultant force is zero.

Unbalanced forces

If the forces do not cancel, the resultant is not zero, and the motion of the object will change.

Newton's First Law

Statement

An object remains at rest or continues to move with constant velocity unless acted on by an unbalanced force.

At rest

A book on a table stays still because its weight downwards is balanced by the reaction force of the table upwards.

Constant velocity

A car at a steady 20 m/s in a straight line has driving force equal to the friction and air resistance.

Inertia

Inertia is the tendency of an object to resist any change in its motion, and a larger mass has greater inertia.

Situation	Forces	Motion
Parked car	Weight = reaction	At rest
Car cruising	Engine force = resistance	Constant velocity
Car braking	Resistance > engine force	Slowing down

A passenger in a car braking suddenly lurches forward because the body tends to keep moving at the original velocity.

A seat belt provides the unbalanced force needed to slow the passenger together with the car.

A crate pushed at constant speed along a floor shows that the pushing force is balanced by friction.

A space probe far from any planet keeps travelling at constant velocity without using any fuel.

Adding or removing force from one side of a balanced system immediately makes the resultant non-zero.

Question: A crate is pushed to the right with a force of 120 N and friction acts to the left. The crate moves at constant velocity. State the friction force. Then 30 N acts to the right and 20 N to the left on another box; find the resultant.

Constant velocity means the forces are balanced, so friction = 120 N to the left.
Take right as positive: resultant = 30 − 20 = +10 N.
The resultant is 10 N to the right, so the second box accelerates.

Answer: Friction 120 N to the left; second box resultant 10 N to the right.

1.4.1 – Key Points for the Exam

Resultant force: The single force with the same effect as all the forces acting together.
Balanced forces: Forces that cancel so that the resultant is zero.
Inertia: The resistance of an object to any change in its state of motion.
Newton: The SI unit of force.

Common Mistakes

Thinking a force is needed to keep an object moving at constant velocity; only a change of motion needs a resultant force.
Saying balanced forces mean the object is at rest, when it may also be moving at constant velocity.
Adding forces without regard to direction.
Forgetting the unit newton or the direction when quoting a resultant.