Forces
What's covered
Key facts
A car accelerating from 0 to 20 m/s in 5 s has a = 20 ÷ 5 = 4 m/s².
A 200 m journey in 50 s gives an average speed of 4 m/s. [Institute of Physics (IOPSpark), Time, distance and speed] speed of 16.7 m/s (90 000 m ÷ 5400 s).
Beyond the elastic limit, a spring is permanently (plastically) deformed and does not return to its original length.
A longer spanner gives a bigger moment for the same force because d is larger. [Institute of Physics (IOPSpark), Episode 203: Turning effects] a moment of 15 × 0.4 = 6 N·m.
A 0.5 kg ball travelling at 8 m/s has momentum p = mv = 0.5 × 8 = 4 kg m/s.
A curved line on a distance-time graph shows changing speed (acceleration).
Newton's first law: an object remains at rest, or moves with constant velocity, unless a resultant (non-zero) force acts on it.
Atmospheric pressure decreases as altitude increases because there is less air above pressing down.
An object is in equilibrium when the resultant force on it is zero — it is either stationary or moving with constant velocity.
A typical alert driver's reaction (thinking) time is around 0.7 s.
Sample questions
A taste of the 181 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
A car increases its speed from 5 m/s to 25 m/s in 4 seconds. What is its acceleration?
- •100 m/s²
- •20 m/s²
- ✓5 m/s²
- •7.5 m/s²
What is the difference between speed and velocity?
- •Speed includes direction but velocity does not — speed is the vector
- •Speed uses m/s and velocity uses km/h — the same quantity, different units
- ✓Velocity is a vector (speed in a direction); speed is a scalar
- •Velocity is always numerically greater than speed for the same object
What is the limit of proportionality for a spring?
- •The force at which the spring constant suddenly doubles in value
- ✓The force beyond which extension is no longer proportional to force
- •The minimum force needed to start a spring stretching when load is applied
- •The point at which a spring first begins to store elastic potential energy
What is the equation for a moment?
- •moment = force + distance
- •moment = force ÷ distance
- ✓moment = force × distance
- •moment = mass × distance
A 1200 kg car at 15 m/s hits a stationary 800 kg car and they couple. Combined speed?
- •12 m/s
- •15 m/s
- •7.5 m/s
- ✓9 m/s
What does the gradient of a distance-time graph represent?
- •Force
- •Mass
- ✓Speed
- •Time
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