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All 12 Flashcards — Newton's laws of motion (F = ma)
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Question
State Newton's first law.
Answer
With **zero net force**, an object stays at rest or keeps moving at **constant velocity**. (Motion needs no force — only a change in motion does.)
Question
State Newton's second law.
Answer
The **net force** equals mass × acceleration: **F = ma**, with the acceleration in the same direction as the net force.
Question
State Newton's third law.
Answer
If A exerts a force on B, then **B exerts an equal and opposite force on A**. The pair acts on **different objects**.
Question
What is the unit of force?
Answer
The **newton (N)**. 1 N = 1 kg m s⁻² (the force that gives a 1 kg mass an acceleration of 1 m s⁻²).
Question
Which force do you put into F = ma?
Answer
The **net (resultant)** force — every force on the object added together, with direction.
Question
Why don't Newton's third-law pairs cancel out?
Answer
Because they act on **different objects**. Two forces only cancel when they act on the **same** object.
Question
Two objects joined by a string — what do they have in common?
Answer
The **same acceleration** — connected bodies move together.
Question
How do you find the tension in a string joining two masses?
Answer
Apply **F = ma** to **one** of the masses on its own: tension = that mass × the shared acceleration.
Question
An elevator accelerates upward. Is the cable tension bigger or smaller than the weight?
Answer
**Bigger** — the cable must support the weight **and** provide the extra net force to accelerate it up (T − mg = ma).
Question
Formula linking net force and acceleration?
Answer
$F = ma = \dfrac{\Delta p}{\Delta t}$ — net force = mass × acceleration = rate of change of momentum.
Question
A 5.0 kg mass feels a 20 N net force. Acceleration?
Answer
a = F ÷ m = 20 ÷ 5.0 = **4.0 m s⁻²**.
Question
Net force vs single force?
Answer
A **single** force is just one push/pull; the **net** force is all of them combined. Only the net force goes into F = ma.
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Full study notes for Newton's laws of motion (F = ma)
Topic 1.2 hub
Forces and momentum
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