Forces, Newton's laws and moments
Pushes and pulls
- A force is a push or a pull.
- A force can change an object's shape, its speed, or its direction.
- This lesson covers stretching, Newton's laws, friction, turning effects, and balance.
Stretching: Hooke's law
- Hang a load on a spring and it stretches. The stretch is the extension.
- On a load–extension graph the line is straight at first: extension is proportional to load.
- The point where it stops being straight is the limit of proportionality.
- The spring constant is the force per unit extension:
- A large $k$ means a stiff spring.
A force of $8.0\ \text{N}$ stretches a spring by $0.040\ \text{m}$ (within the limit of proportionality). What is the spring constant, in N/m?
$k = \dfrac{F}{x} = \dfrac{8.0}{0.040} = 200\ \text{N/m}$.
Resultant force and Newton's laws
- Add forces along a line to get the resultant force (one way positive, the other negative).
- If the resultant force is zero: the object stays at rest, or keeps moving at constant velocity (Newton's first law).
- If the resultant force is not zero: the object accelerates in the direction of the force:
- Every force comes in a pair: if A pushes B, then B pushes back on A with an equal, opposite force (Newton's third law).
The resultant force on a moving car is zero. What does the car do?
Newton's first law: with zero resultant force, an object keeps moving at constant velocity (or stays at rest).
A resultant force of $6.0\ \text{N}$ acts on a $2.0\ \text{kg}$ trolley. What is its acceleration, in m/s²?
$F = ma$, so $a = \dfrac{F}{m} = \dfrac{6.0}{2.0} = 3.0\ \text{m/s}^2$.
Friction
- Friction is the force between two surfaces that touch.
- It always acts against the motion, and it makes surfaces heat up.
- Drag (friction in a liquid or gas, like air resistance) slows objects down in the same way.
Moments — the turning effect
- The moment of a force is its turning effect about a pivot:
- The unit is the newton metre (N m).
- Principle of moments: when something is balanced, total clockwise moment = total anticlockwise moment.
- An object is in equilibrium when there is no resultant force and no resultant moment.
A spanner is pushed with $20\ \text{N}$ at a perpendicular distance of $0.50\ \text{m}$ from the bolt. What is the moment, in N m?
moment = force × perpendicular distance = $20 \times 0.50 = 10\ \text{N m}$.
A see-saw balances. On the left, a $30\ \text{N}$ weight sits $0.20\ \text{m}$ from the pivot. On the right, a weight sits $0.60\ \text{m}$ from the pivot. What is the right-hand weight, in N?
Balanced ⇒ clockwise = anticlockwise: $30 \times 0.20 = F \times 0.60$, so $F = \dfrac{6.0}{0.60} = 10\ \text{N}$.
Centre of gravity and stability
- The centre of gravity is the single point where all the weight seems to act.
- An object is more stable when its centre of gravity is low and its base is wide.
- It tips over when the centre of gravity passes outside the base.
Which object is the most stable (hardest to tip over)?
A low centre of gravity and a wide base make an object stable. The tall, thin shapes tip over easily.
You've got it
- spring constant $k = \dfrac{F}{x}$, straight up to the limit of proportionality
- resultant zero → constant velocity; resultant force $F = ma$
- moment = force × perpendicular distance; balanced ⇒ clockwise = anticlockwise moments
- stable = low centre of gravity + wide base