Exam study guide
Cambridge IGCSE Physics Forces Practice Questions
Mastering the fundamental concepts of forces is crucial for excelling in Cambridge IGCSE Physics. This comprehensive set of practice questions, complete with detailed explanations and expert tips, is designed to strengthen your understanding of key principles such as Newton's Laws of Motion, various types of forces, and their impact on objects and their motion.
Quick revision summary
- A force is a push or pull that can change an object's motion, measured in Newtons (N) and is a vector quantity.
- Newton's First Law (Inertia): An object maintains its state of rest or uniform motion unless acted upon by a non-zero resultant force.
- Newton's Second Law: The resultant force on an object is directly proportional to its acceleration and its mass (F = ma).
- Newton's Third Law: For every action, there is an equal and opposite reaction force acting on different objects.
- Common forces include weight (gravitational), normal force, friction, air resistance, tension, and thrust.
The Nature of Forces
Forces are fundamental to understanding how objects interact and move in the universe. They are vector quantities, meaning they have both magnitude and direction. Understanding the different types of forces, such as gravity, friction, tension, and normal force, is the first step towards analyzing motion. The SI unit for force is the Newton (N).
Newton's Laws of Motion
Sir Isaac Newton's three laws provide the foundation for classical mechanics. His First Law describes inertia, stating that an object's velocity remains constant unless acted upon by an external force. The Second Law quantifies the relationship between force, mass, and acceleration (F=ma), allowing us to calculate how objects respond to forces. His Third Law explains action-reaction pairs, highlighting that forces always occur in pairs acting on different objects.
Calculating and Analyzing Forces
Many problems in IGCSE Physics require calculating resultant forces and applying them to determine acceleration, or vice-versa. It's essential to identify all forces acting on an object, resolve them into components if necessary, and then sum them vectorially to find the net (resultant) force. This resultant force is then used with Newton's Second Law to predict or explain the object's motion.
Practice questions
Question 1 (4 marks)
A parachutist jumps out of an aircraft. Describe the forces acting on the parachutist immediately after jumping and as they approach terminal velocity.
Answer: Immediately after jumping, the main force acting on the parachutist is their weight (due to gravity) acting downwards, with very little or negligible air resistance acting upwards. As they approach terminal velocity, their weight remains constant downwards, and the air resistance acting upwards increases until it is equal in magnitude to their weight.
Explanation: Initially, with low speed, air resistance is negligible compared to weight. As speed increases, air resistance grows. Terminal velocity is achieved when the upward air resistance balances the downward weight, resulting in zero resultant force and thus constant velocity.
Question 2 (3 marks)
A car of mass 1200 kg accelerates uniformly from rest to 20 m/s in 8 seconds. Calculate the resultant force acting on the car during this acceleration.
Answer: First, calculate acceleration: a = (v - u) / t = (20 m/s - 0 m/s) / 8 s = 2.5 m/s². Then, resultant force F = ma = 1200 kg × 2.5 m/s² = 3000 N.
Explanation: The problem requires two steps: first finding the acceleration using a kinematic equation (change in velocity over time), and then applying Newton's Second Law (F=ma) to determine the net force responsible for that acceleration.
Question 3 (1 marks)
Which statement best describes Newton's First Law of Motion?
- Every action has an equal and opposite reaction.
- Force is equal to mass times acceleration.
- An object will remain at rest or in uniform motion unless acted upon by a resultant force.
- The rate of change of momentum is directly proportional to the applied force.
Answer: An object will remain at rest or in uniform motion unless acted upon by a resultant force.
Explanation: Newton's First Law, often called the Law of Inertia, describes that an object's state of motion (either at rest or moving at a constant velocity) will not change without a net external force. The other options describe Newton's Third Law, Second Law, and an alternative form of the Second Law, respectively.
Question 4 (2 marks)
A book is resting on a table. Identify the action-reaction pair of forces involved between the book and the table according to Newton's Third Law.
Answer: The book exerts a downward force on the table (action). The table exerts an upward force on the book (reaction).
Explanation: Newton's Third Law states that forces occur in pairs. These forces are equal in magnitude, opposite in direction, and act on different objects. It's crucial not to confuse this with balanced forces acting on a single object, where the forces would cancel out.
Question 5 (3 marks)
A worker pushes a crate of mass 50 kg along a horizontal floor with a force of 150 N. If the frictional force opposing the motion is 30 N, calculate the acceleration of the crate.
Answer: Resultant force = Applied force - Frictional force = 150 N - 30 N = 120 N. Acceleration a = F_resultant / m = 120 N / 50 kg = 2.4 m/s².
Explanation: To find the acceleration, first calculate the resultant force by subtracting the opposing frictional force from the applied pushing force. Then, use Newton's Second Law (F=ma) to find the acceleration, dividing the resultant force by the mass.
Question 6 (2 marks)
Explain why astronauts on the International Space Station appear weightless, even though Earth's gravitational pull is still significant at that altitude.
Answer: Astronauts on the ISS appear weightless because they are in a continuous state of freefall around the Earth. Both the ISS and the astronauts inside it are constantly falling towards Earth under the influence of gravity, but their high orbital velocity means they continuously fall 'around' the Earth, creating the sensation of weightlessness (microgravity).
Explanation: Weightlessness is not due to a lack of gravity, but rather due to being in constant freefall. In this state, there is no normal contact force to oppose the gravitational force, leading to the sensation of zero weight. The gravitational field strength at ISS altitude is still of that on Earth's surface.
Question 7 (1 marks)
An object is moving at a constant velocity. Which statement correctly describes the forces acting on it?
- There is a resultant force acting in the direction of motion.
- There is no resultant force acting on the object.
- The forces acting on the object are unbalanced.
- The object is accelerating.
Answer: There is no resultant force acting on the object.
Explanation: According to Newton's First Law, an object moving at a constant velocity implies zero acceleration. Zero acceleration means that the resultant (net) force acting on the object must be zero, indicating that all forces are balanced.
Question 8 (4 marks)
A rocket has a mass of 5000 kg. When its engines are fired, they produce an upward thrust of 70,000 N. Calculate the initial upward acceleration of the rocket. (Take g = 10 N/kg).
Answer: Weight of rocket = mass × g = 5000 kg × 10 N/kg = 50,000 N (acting downwards). Resultant upward force = Thrust - Weight = 70,000 N - 50,000 N = 20,000 N. Initial upward acceleration a = F_resultant / m = 20,000 N / 5000 kg = 4 m/s².
Explanation: First, calculate the rocket's weight, which acts downwards. Then, determine the resultant force by subtracting the weight from the upward thrust. Finally, apply Newton's Second Law (F=ma) to find the initial acceleration using the calculated resultant force and the rocket's mass.
Common mistakes
- Confusing action-reaction pairs with balanced forces: Action-reaction forces act on *different* objects, while balanced forces act on the *same* object and sum to zero.
- Failing to calculate the resultant force: Students often use an individual force (e.g., applied force) in F=ma instead of the net force acting on the object.
- Incorrectly stating or mixing units: Always ensure mass is in kilograms (kg), acceleration in m/s², and force in Newtons (N) when using F=ma.
Exam tips
- Always draw free-body diagrams: For any problem involving multiple forces, drawing a clear diagram with labeled forces and their directions can help visualize the problem and correctly sum forces.
- Show all working steps and units: Even if your final answer is incorrect, clear working allows for method marks may still be awarded for correct working. Ensure units are consistent throughout your calculations.
- Pay attention to keywords: Terms like 'constant velocity,' 'at rest,' 'terminal velocity,' or 'uniform acceleration' indicate specific conditions regarding resultant forces (e.g., zero resultant force for constant velocity).
Why this matters for exams
Understanding forces is a cornerstone of IGCSE Physics. Questions on Newton's Laws, calculating resultant forces, and describing different types of forces frequently appear in both Paper 2 (Multiple Choice) and Paper 4 (Theory), forming a significant portion of the mechanics section. A solid grasp of these principles is essential for tackling more complex topics and securing higher marks.
Frequently asked questions
What is the difference between mass and weight?
Mass is a measure of the amount of matter in an object, a scalar quantity measured in kilograms (kg), and remains constant regardless of location. Weight, on the other hand, is the gravitational force exerted on an object, a vector quantity measured in Newtons (N), and varies depending on the gravitational field strength (g) of the location (Weight = mass × g).
How does friction affect motion?
Friction is a force that opposes motion or the tendency of motion between surfaces in contact. It can slow down moving objects, convert kinetic energy into heat, and is essential for activities like walking or driving. Without friction, objects would struggle to start moving or stop.
Can an object move if there is no resultant force acting on it?
Yes, according to Newton's First Law, an object can move if there is no resultant force acting on it, provided it was already in motion. In this case, it will continue to move at a constant velocity (constant speed in a straight line). If it was initially at rest, it would remain at rest.