1.4.1Dynamics
Newton’s Laws and Balanced Forces
5 min read 5 parts Lesson 11 of 22
Section 1 of 5Forces and Resultants
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.
Section 2 of 5Newton's First Law
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 |
Section 3 of 5Everyday Applications
1
A passenger in a car braking suddenly lurches forward because the body tends to keep moving at the original velocity.
2
A seat belt provides the unbalanced force needed to slow the passenger together with the car.
3
A crate pushed at constant speed along a floor shows that the pushing force is balanced by friction.
4
A space probe far from any planet keeps travelling at constant velocity without using any fuel.
5
Adding or removing force from one side of a balanced system immediately makes the resultant non-zero.
Section 4 of 5Worked Example
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 positiveresultant = 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.
Section 5 of 5Exam Checklist & Tips
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.