GCSE · Physics · Edexcel · Spec 1PH0

Newton's first law

A car cruising in a straight line at a steady speed has zero overall force on it. Sounds wrong? It isn't.

Physics · Forces

Newton's first law: is the resultant force zero?

If it is, nothing about the motion changes: a body at rest stays at rest, and a moving body keeps the same speed in the same direction. Pick a situation, find the resultant, and see.

Resultant force · F = ma

Net force0 N

A car parked on a flat road. Its weight pulls down with 10 000 N. The road pushes up with 10 000 N (the arrow labelled Normal).

Selected scenario

Parked car

Net force

0 N

Motion

Stays at rest

Stays at rest. 10 000 N − 10 000 N = 0. Two forces act, but they cancel, so nothing changes: the car stays exactly where it is.

Watch out: Balanced forces do not mean 'not moving'. The parked car and the cruising car have exactly the same resultant force: zero.

Be honest

The puck on perfect ice

An ice-hockey puck is hit and slides off across the ice. Imagine the ice is perfectly smooth and there is no air, so once the puck leaves the stick there is no friction and no air resistance. The resultant force on it is zero.

What happens to the puck after it leaves the stick?

It stops almost straight away. Once the stick stops pushing, there's nothing to keep it moving.It keeps going for a while, then gradually slows down as the push from the stick wears off.It keeps sliding at the same speed, in the same straight line.It keeps getting faster, because there's nothing holding it back.

How sure are you?

I'm sureI think soI'm guessing

It stops almost straight away. Once the stick stops pushing, there's nothing to keep it moving.

This is a common misconception

This is the big one, and it feels right, because everything you push in real life stops when you stop pushing. But real things stop because friction and air resistance push against them, not because the push has gone. Take those away and the resultant force is zero, so nothing changes: the puck keeps going. A moving object doesn't need a force to keep moving. It only needs one to change how it moves.

It keeps going for a while, then gradually slows down as the push from the stick wears off.

This is a common misconception

Real pucks do slow down, so this sounds sensible. But the stick's push isn't stored inside the puck and slowly used up. The push stops the instant the puck leaves the stick. A real puck slows because friction and air resistance give it a resultant force backwards. Here there are none, so there is no slowing at all.

It keeps sliding at the same speed, in the same straight line.

This matches the evidence

Yes. Zero resultant force means no change in velocity: same speed, same direction, for as long as the resultant stays zero. That's Newton's first law for a moving body.

It keeps getting faster, because there's nothing holding it back.

This is a common misconception

Nothing holds it back, but nothing pushes it forwards either. To speed up, it would need a resultant force in its direction of motion. With a zero resultant, its speed stays exactly the same.

A force isn't needed to keep something moving. A resultant force is needed to change its motion: to speed it up, slow it down or turn it.

Finding the resultant along a line

Problem

A cyclist rides along a straight, flat road. Pedalling gives a forward force of 150 N. Air resistance is 90 N and friction is 60 N, both acting backwards. What is the resultant force, and what happens to the cyclist's motion?

When the resultant isn't zero: which way does it point?

The same car from the top of the page is moving to the right, engine off, braking. Tap each force.

Tap a force to see what it does.

Read the motion, find the force

Zero or not zero?

This time you can't see any forces, only the motion. For each one, decide: is the resultant force zero, or not?

Still to sort

Resultant force is zero (0)

At rest, or moving at a steady speed in a straight line.

Where the line is: A steady speed is not enough on its own: the direction must stay the same too.

Resultant force is not zero (0)

Speeding up, slowing down, or changing direction.

Where the line is: A change of direction counts, even if the speed never changes.

7 of 7 still to sort.

Watch out: 'Steady speed' and 'constant velocity' are not the same thing. Velocity includes direction.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

No resultant force? Then nothing about the motion changes.

What you need to know

Newton's first law: if the resultant force on a body is zero, a body at rest stays at rest, and a moving body keeps moving at the same speed in the same direction (constant velocity).
The resultant force is the overall effect of all the forces on a body. Along a line, add the forces acting one way and subtract the forces acting the opposite way.
If the resultant force is not zero, the body accelerates: its speed changes, its direction changes, or both. A resultant in the direction of motion speeds it up; one opposite to the motion slows it down.
Velocity includes direction, so a body moving at a steady speed while changing direction has a changing velocity, and that needs a non-zero resultant force.

The big picture

Newton's first law says what happens when the resultant force on a body is zero: if it is at rest it stays at rest, and if it is moving it keeps moving at the same speed in the same direction. The resultant force is the overall effect of all the forces, found along a line by adding the forces one way and subtracting the ones pointing the opposite way. If the resultant is not zero, the velocity changes: the body speeds up, slows down or changes direction. Because velocity includes direction, even a body moving at a steady speed round a bend must have a resultant force on it.

Key points

1Zero resultant force means no change in motion, not no motion.

2At rest or at constant velocity? Then the resultant force is zero, however many forces act.

3Resultant force with the motion: speeds up. Against the motion: slows down.

4Steady speed round a bend is not constant velocity, so there must be a resultant force.

5Nothing needs a force to keep moving. A resultant force is needed to change how it moves.

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Worked example

Problem

A plane flies level in a straight line at a constant velocity. Its engines give a forward thrust of 120 kN. What is the air resistance (drag) acting on the plane?

⚠ Watch out

Thinking a moving object needs a resultant force to keep it moving. A car at a steady speed on a straight road has zero resultant force on it. A resultant force is only needed to change the motion.

🧠

Memory hook

No resultant, no change. Objects are creatures of habit: whatever they are doing, sitting still or cruising in a straight line, they keep doing it until a resultant force makes them change.

✓

Check yourself

A ball thrown straight up slows down on the way up. Is the resultant force on it zero? If not, which way does it point, and how can you tell from the motion alone?

Flashcards

(11)

What does Newton's first law say happens when the resultant force on a body is zero?

A body at rest stays at rest. A moving body keeps moving at the same speed in the same direction (constant velocity).

What is the resultant force?

The overall effect of all the forces acting on a body.

How do you find the resultant of forces acting along a line?

Add the forces acting one way, then subtract the forces acting the opposite way.

A body is moving at constant velocity with several forces acting on it. What is the resultant force?

Zero. The forces are balanced, even though it is moving.

What does 'velocity' include that 'speed' does not?

Direction. Two cars at the same speed heading different ways have different velocities.

Is the resultant force zero on a car going round a bend at a steady speed?

No. Its direction is changing, so its velocity is changing, and that needs a non-zero resultant force.

What does it mean when we say a body accelerates?

Its velocity changes: its speed changes, its direction changes, or both.

The resultant force on a moving body points the same way as its motion. What happens?

It speeds up.

The resultant force on a moving body points opposite to its motion. What happens?

It slows down (decelerates), still moving the same way while it slows.

Does a moving object need a resultant force to keep it moving?

No. A resultant force is only needed to change its speed or direction.

Why does a sliding object slow down in real life?

Friction and air resistance act against its motion, so the resultant force points backwards.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Newton's first law: is the resultant force zero?

The puck on perfect ice

Zero or not zero?

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