Effects of Force
Section: Motion, Forces & Energy | Syllabus: Cambridge IGCSE Physics (0625)
What Forces Do
- A force is a push or pull acting on an object
- Forces can:
- change the size or shape of an object (e.g. stretching a spring, squashing a ball)
- change the speed of an object (make it speed up or slow down)
- change the direction of motion of an object
- An object either remains at rest, or continues moving in a straight line at constant speed, unless a resultant force acts on it
- A resultant force may change the velocity of an object by changing its speed, its direction, or both
A force causing an object to change shape, a force changing the speed of a car, and a force changing the direction of a satelite
Resultant Force
- Resultant force is the single force that has the same effect as all the individual forces acting on an object combined
- To find the resultant of forces acting along the same straight line:
- forces in the same direction - add them together
- forces in opposite directions - subtract the smaller from the larger; the resultant acts in the direction of the larger force
Resultant force in three scenarios: same direction, opposite direction, and equilibrium
- If two opposing forces are equal in size, the resultant is zero and the object is in equilibrium
Worked Example
- Mweembe pushes a box with 20 N to the right. Friction acts with 8 N to the left.
What is the resultant force?A pushed box with friction: 20 N push, 8 N friction, 12 N resultant
- Resultant = 20 - 8 = 12 N to the right
Effect of Resultant Force on Motion
- If resultant force = 0: the object remains at rest, or continues moving at constant speed in a straight line
- If resultant force ≠ 0: the object accelerates - its velocity changes (in speed, direction, or both)
Balanced driving force and friction giving zero resultant and constant speed
Load-Extension Graphs and Hooke's Law
- A force can change the size or shape of an object
- a force can make an object stretch, compress, bend, or change shape
- for example, when you pull on a spring, the spring becomes longer
- the amount by which the spring changes length depends on the force applied
- Extension is the increase in length of an object when a force is applied
- extension is calculated by comparing the spring's new length with its original length
- extension = new length - original length
- extension is measured in metres (m)
- Picture what happens to the spring:
- with no load, the spring has its original length
- when a load is added, the spring stretches
- as more load is added, the extension generally increases
A spring before and after a load is added, showing its original length, extended length and extension
- The relationship between load and extension can be investigated experimentally:
- hang a spring from a fixed support and measure its original length before adding any load
- add a known load to the spring and measure its new length
- calculate the extension: extension = new length − original length
- repeat for a range of different loads, recording load (N) and extension (m) each time
- plot a load-extension graph, with load on the y-axis and extension on the x-axis
Load-extension graph for a spring
- Interpreting a load-extension graph:
- a straight line through the origin shows that load and extension are directly proportional
- a curve shows that load and extension are no longer proportional
Limit of Proportionality
Extended Only
- The limit of proportionality is the point beyond which load and extension are no longer proportional
- beyond this point, the graph is no longer a straight line
- This point can be identified on a load-extension graph as the point where the line stops being straight and starts to curve
- Note: this is different from the "elastic limit" - you only need to know about the limit of proportionality, not the elastic limit
Spring Constant
Extended Only
- Spring constant (k) is defined as the force per unit extension
- it is a measure of the stiffness of a spring - a larger k means a stiffer spring
- Formula:
- k = (F/x)
- Where:
- k = spring constant (N/m)
- F = force/load (N)
- x = extension (m)
- Rearranged:
- F = kx
Worked Example: Spring Constant
- A load of 6 N causes a spring to extend by 0.03 m. Calculate the spring constant.
- k = (F/x) = (6/0.03) = 200 N/m
Worked Example: Extension
- A spring with k = 200 N/m has a load of 10 N applied. Calculate the extension.
- x = (F/k) = (10/200) = 0.05 m
Force, Mass and Acceleration (F = ma)
Extended Only
- Formula:
- F = ma
- Where:
- F = resultant force (N)
- m = mass (kg)
- a = acceleration (m/s²)
- The resultant force and the acceleration are always in the same direction
- Rearrangements:
- a = (F/m) - a greater force or a smaller mass gives a greater acceleration
- m = (F/a)
Worked Example: F = ma
- A resultant force of 2400 N acts on a car of mass 1200 kg. Calculate the acceleration.
- a = (F/m) = (2400/1200) = 2 m/s^2 in the direction of the force
Friction
- Solid friction is the force between two solid surfaces in contact that may impede (resist) motion
- friction also produces heating at the surfaces in contact
- it converts kinetic energy into thermal energy (heat)
- Drag in a liquid: friction acts on any object moving through a liquid, opposing its motion
- Drag in a gas (air resistance): friction acts on any object moving through a gas, opposing its motion
Friction on a solid surface, drag in a liquid, and air resistance in a gas
Circular Motion
Extended Only
- When a force acts perpendicular to the motion of an object (at right angles to its velocity), the object moves in a circular path
- the force is always directed toward the centre of the circle
Force acting perpendicular to the motion of an object, resulting in circular motion with the force directed toward the centre
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