Effects of Force
Section: Motion, Forces & Energy | Syllabus: Cambridge AS Level Physics 9702
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 Key Point A resultant force may change the velocity of an object by changing its speed, its direction, or both.
Resultant Force Resultant Force 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 - Same Direction A box with two arrows both pointing right: one labelled "8 N →" and another labelled "4 N →".
Resultant = 8 + 4 = 12 N to the right. A single larger arrow below the box labelled "Resultant = 12 N →". Resultant Force - Opposite Directions A box with "15 N →" pointing right and "5 N ←" pointing left.
Resultant = 15 − 5 = 10 N to the right. A single arrow below labelled "Resultant = 10 N →". Note: if forces were equal (e.g. 10 N each, opposite), resultant = 0 and the object is in equilibrium. Worked Example: Finding the Resultant A box is pushed with 20 N to the right.
Friction acts with 8 N to the left. What is the resultant force? 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 (speed, direction, or both). Figure: Balanced vs Unbalanced Forces Two scenarios. Left: A car with equal arrows "Driving force = 500 N →" and "Friction = 500 N ←".
Text below: "Resultant = 0. Constant speed." Right: A car with "Driving force = 800 N →" and "Friction = 300 N ←". Text below: "Resultant = 500 N →. Car accelerates forward." Load-Extension Graphs and Hooke’s Law When a load (force) is applied to a spring, the spring extends.
The relationship between load and extension can be investigated: Hang a spring from a fixed support. Measure its natural length with a ruler. Add a known load (weight) and measure the new length. Extension = new length − natural length.
Repeat for increasing loads, recording load (N) and extension (m) each time. Plot a load-extension graph (load on y-axis, extension on x-axis). Figure: Load-Extension Graph A graph with "Extension (m)" on the x-axis and "Load (N)" on the y-axis.
From the origin, a straight line rises with constant gradient - this is the proportional region where Hooke’s Law holds. At a clearly marked point labelled "Limit of proportionality (P)", the line begins to curve upward more steeply, showing the load-extension relationship is no longer proportional.
The curved region continues beyond P. Limit of Proportionality The point beyond which load and extension are no longer proportional - the graph is no longer a straight line. Hooke’s Law no longer applies beyond this point.
Below the limit of proportionality, Hooke’s Law applies: Extension is directly proportional to the applied load. The load-extension graph is a straight line through the origin. Spring Constant Spring Constant (k) Force per unit extension.
A measure of the stiffness of a spring - a larger k means a stiffer spring. k = F / x 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) 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 - greater force or smaller mass gives 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/s² in the direction of the force Friction Solid Friction The force between two solid surfaces in contact that may impede (resist) motion. Friction also produces heating at the surfaces.
Solid friction acts between surfaces in contact and opposes motion. It converts kinetic energy into thermal energy (heat) at the surfaces. 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. Figure: Friction Between Surfaces A wooden block sliding to the right across a rough surface. Two arrows: "Motion →" above the block, "Friction ←" below the block pointing opposite to motion.
A note: "The surfaces heat up as kinetic…
Interactive revision notes, videos and practice questions load below.