Effects Of Forces

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Effects Of Forces (Co-ordinated Sciences (Double Award) [CIE], Physics) — use them in your lesson, or run the topic as a live class game.

Notes

Resultant Forces

A **force** is a push or pull that acts on an object due to interaction with another object.
Forces can change an object's size, shape, or motion (speed or direction).
**Resultant force** is the single force that describes all forces acting on a body.
**Balanced forces** are equal in magnitude and opposite in direction; they cancel out $(resultant = 0)$ .
**Unbalanced forces** produce a non-zero resultant force, causing a change in motion.
To calculate resultant force, assign positive/negative signs to directions and sum the forces.

Newton's First Law

Newton's first law: Objects remain at rest or move with constant velocity unless acted on by a resultant force.
Constant velocity means no change in speed or direction.
If resultant force is zero, the object is either stationary or moving at constant velocity.
Example: A mug on a table stays at rest unless a force acts on it.
Example: A car moving at constant speed has balanced driving and frictional forces.

Newton's Second Law (Extended)

Newton's second law: Acceleration is proportional to resultant force and inversely proportional to mass.
Equation: **F = ma**, where $F =$ resultant force $(N), m =$ mass $(kg), a =$ acceleration $(\frac{m}{s}^{2})$ .
Acceleration occurs in the same direction as the resultant force.
For a given force, larger mass gives smaller acceleration.

Investigating Force & Extension (Extended)

Aim: Investigate relationship between force and extension of a spring.
Independent variable: force (F); dependent variable: extension (x).
Method: Hang masses on a spring, measure extension with a ruler, repeat for different masses.
Calculate force as weight $(W = mg)$ and extension as final length minus original length.
Plot force against extension; a straight line through origin shows direct proportionality (Hooke's law).

Hooke's Law (Extended)

Hooke's law: Extension of an elastic object is directly proportional to force applied, up to the **limit of proportionality**.
Equation: **F = kx**, where $k =$ spring constant $(\frac{N}{m}), x =$ extension (m).
Spring constant measures stiffness: high $k =$ stiff spring, low $k =$ stretchy spring.
Force-extension graph: linear region obeys Hooke's law; gradient = spring constant.
Beyond limit of proportionality, graph curves and Hooke's law no longer applies.

Friction

**Friction** opposes motion and causes heating (energy transfer).
Solid friction arises from surface imperfections; can be reduced by lubrication or smoothing.
Fluid friction (drag) occurs when objects move through gases or liquids; particles collide with object.
Air resistance is a type of drag that slows motion and heats the object and air.
Streamlining reduces fluid friction.

Moments

A **moment** is the turning effect of a force about a pivot.
Moment = force × perpendicular distance from pivot (unit: N m).
Increasing distance from pivot reduces the force needed for the same moment.
**Principle of moments** (extended): For a balanced object, total clockwise moment = total anticlockwise moment.

Equilibrium & Centre of Gravity

**Equilibrium**: forces are balanced (no resultant force) and moments are balanced (no resultant moment).
**Centre of gravity** is the point where the weight of an object acts.
For symmetrical objects, centre of gravity is at the geometric centre.
Stability depends on centre of gravity position: stable if it lies above the base; topples if outside base.
Wide base and low centre of gravity increase stability.

Investigating Centre of Gravity

Aim: Find centre of gravity of an irregularly shaped plane lamina using suspension method.
Method: Suspend lamina from a point, use plumb line to mark vertical line; repeat from two other points.
Intersection of the three lines gives the centre of gravity.
Allow lamina to settle before marking; punch holes before suspending to avoid shifting centre of gravity.

Reflection and refraction of light (not directly related to forces, but illustrates vector direction)

Practice questions

Free preview — 8 of 12 questions. Sign up to see them all.

1.What is a force?
Easy
- AA push or a pull that acts on an object due to interaction with another object
- BA measure of the mass of an object
- CThe speed of an object in a given direction
- DThe energy stored in an object
2.State two effects that a force can have on an object.
Easy
3.A car of mass 900 kg accelerates from 0 to 27 m/s in 3 seconds. Calculate the force required to produce this acceleration.
Medium
- A8910
- B7290
- C8100
- D9720
4.Newton's first law states that an object will remain at rest or move with constant velocity unless acted on by a resultant force.
Easy
True or false?
5.Complete the sentence using the correct word.
Medium
Hooke's law states that the extension of an elastic object is directly proportional to the force applied, up to $the ____$ .
6.Match each term to its correct description.
Medium
- Moment
- Centre of gravity
- Equilibrium
- The turning effect of a force
- The point through which weight acts
- State where forces and moments are balanced
7.Arrange the steps to find the centre of gravity of an irregular lamina in the correct order.
Medium
- Hang the lamina from a clamp
- Draw a line along the plumb line
- Repeat with two other holes
- Punch three holes near the edges
- The intersection of lines is the centre of gravity
8.A spring has a spring constant of 4900 N/m. A weight compresses it from 40 cm to 33 cm. What is the weight?
Hard
- A343 N
- B3430 N
- C34.3 N
- D0.343 N

Slides

Effects of Forces

A force is a push or a pull that acts on an object due to interaction with another object.
Forces can change an object's size, shape, or motion (speed or direction).
Examples: thrust speeds up a car, friction slows it down, gravity changes a comet's direction, and compression changes a spring's shape.

Which of the following is NOT an effect of a force?

Changing an object's shape
Changing an object's colour
Changing an object's speed
Changing an object's direction

Resultant Forces

A resultant force is a single force that describes all forces acting on an object.
Forces are balanced if they cancel out $(resultant =$ 0); unbalanced if they $don't (resultant \ne 0)$ .
To calculate resultant force, assign positive/negative directions and add forces.

Two forces act on an object: 5 N to the right and 3 N to the left. What is the resultant force?

2 N to the right
2 N to the left
8 N to the right
8 N to the left

Newton's First Law

Newton's first law: Objects remain at rest or move with constant velocity unless acted on by a resultant force.
Constant velocity means no change in speed or direction.
If resultant force is zero, the object is either stationary or moving at constant velocity.

Which of these are examples of Newton's first law?

A book resting on a table stays at rest until someone picks it up.
A car speeding up when the driver presses the accelerator.
A space rock drifting at constant speed in a straight line in deep space.

Newton's Second Law

Newton's second law: acceleration is proportional to resultant force and inversely proportional to mass.
Equation: $F = ma (force$ in N, mass in kg, acceleration in $\frac{m}{s}^{2})$ .
A larger resultant force gives a larger acceleration; a larger mass gives a smaller acceleration for the same force.

A 500 kg car accelerates at $2 \frac{m}{s}^{2}$ . What is the resultant force?

250 N
1000 N
500 N
2000 N

Hooke's Law

Hooke's law: extension of an elastic object is directly proportional to the applied force, up to the limit of proportionality.
Equation: $F = kx$ , where k is the spring constant (N/m) and x is extension (m).
The spring constant measures stiffness: high k means stiff spring.

Hooke's law states that extension is directly proportional to the applied force up to $the ____ of$ proportionality.

Investigating Force & Extension

Experiment: hang masses on a spring and measure extension.
Force = weight $= mg (g = 9.8 N/kg)$ .
Plot force vs extension; a straight line through origin shows Hooke's law.

In a force-extension experiment, what does a straight line through the origin indicate?

The spring has reached its limit of proportionality
Force is directly proportional to extension
The spring constant is zero
The spring is not obeying Hooke's law

Friction

Friction opposes motion and causes heating.
In solids, friction is due to surface imperfections; can be reduced by lubrication or smoothing.
In fluids, friction (drag) is due to particle collisions; can be reduced by streamlining.

Which of these are ways to reduce friction?

Adding lubricant between surfaces
Making surfaces rougher
Streamlining the shape of an object moving through air

Moments

A moment is the turning effect of a force about a pivot.
Moment = force × perpendicular distance from pivot (unit: N m).
Increasing the distance from the pivot reduces the force needed to produce the same moment.

A force of 10 N is applied 0.5 m from a pivot. What is the moment?

5 N m
20 N m
0.5 N m
10 N m

Principle of Moments & Equilibrium

Principle of moments: for a balanced object, total clockwise moment = total anticlockwise moment.
Equilibrium requires both zero resultant force and zero resultant moment.
Example: a see-saw balanced when the child's moment equals the adult's moment.

For an object to be in equilibrium, the sum of clockwise moments must equal the sum $of ____$ moments.

Centre of Gravity & Stability

Centre of gravity is the point where the weight of an object acts.
For symmetrical objects, it lies at the centre of symmetry.
Stability: an object is stable if its centre of gravity is above its base; a low centre of gravity and wide base increase stability.

Which of these increase stability?

Low centre of gravity
Narrow base
Wide base
High centre of gravity

Unlock all 12 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon