Exam study guide
IB MYP Physics Forces and Motion MCQ Practice
Welcome to your focused practice session on IB MYP Physics: Forces and Motion! This section is designed to help you solidify your understanding of fundamental concepts through original multiple-choice questions (MCQs), calculations, and conceptual challenges. Mastering the principles of motion, different types of forces, and Newton's Laws is crucial for success in your MYP science assessments. Dive in to test your knowledge, identify areas for improvement, and build confidence.
Quick revision summary
- Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force (inertia).
- Newton's Second Law: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (F_net = ma).
- Newton's Third Law: For every action, there is an equal and opposite reaction.
- Force (N): A push or pull that can cause a change in an object's motion. Vector quantity.
- Mass (kg): A measure of the amount of matter in an object; resistance to acceleration. Scalar quantity.
- Weight (N): The force of gravity acting on an object (W = mg, where g ≈ 9.8 m/s² on Earth).
- Displacement (m): Change in position; vector quantity.
- Velocity (m/s): Rate of change of displacement; vector quantity.
- Acceleration (m/s²): Rate of change of velocity; vector quantity.
- Distance-time graphs: Slope = speed/velocity. Velocity-time graphs: Slope = acceleration, Area = displacement.
- Balanced forces result in zero net force, leading to constant velocity (or rest). Unbalanced forces result in acceleration.
Understanding Forces
Forces are fundamental to understanding how objects interact and move. They can be contact forces, like friction and normal force, or non-contact forces, such as gravity. The concept of net force is crucial: it's the vector sum of all individual forces acting on an object, and it determines the object's acceleration.
Principles of Motion
Motion is described using terms like displacement, velocity, and acceleration. These are vector quantities, meaning they have both magnitude and direction. Understanding how these quantities relate to each other and how they are represented graphically (e.g., on distance-time or velocity-time graphs) is essential for solving problems in kinematics.
Practice questions
Question 1 (2 marks)
An astronaut pushes a 25 kg box with a force of 10 N in the absence of gravity and air resistance. What is the acceleration of the box?
Answer: 0.4 m/s²
Explanation: According to Newton's Second Law, F = ma. We are given F = 10 N and m = 25 kg. Rearranging the formula to solve for acceleration, a = F/m. So, a = 10 N / 25 kg = 0.4 m/s². The absence of gravity and air resistance means the applied force is the net force.
Question 2 (2 marks)
Which statement best describes a consequence of Newton's First Law of Motion?
- A car accelerates when the driver steps on the gas pedal.
- A satellite orbits Earth at a constant speed because gravity provides the necessary centripetal force.
- A book resting on a table remains at rest unless someone pushes or pulls it.
- When you push a wall, the wall pushes back on you with an equal and opposite force.
Answer: C
Explanation: Newton's First Law, also known as the Law of Inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion with constant velocity, unless acted upon by an unbalanced force. Option C directly illustrates an object at rest remaining at rest in the absence of an unbalanced force. Option A describes acceleration (Newton's Second Law), Option B describes circular motion (related to unbalanced force), and Option D describes action-reaction pairs (Newton's Third Law).
Question 3 (1 marks)
A car travels 300 meters in 20 seconds. What is its average speed?
Answer: 15 m/s
Explanation: Average speed is calculated by dividing the total distance traveled by the total time taken. Average speed = Distance / Time = 300 m / 20 s = 15 m/s.
Question 4 (2 marks)
When a tennis racket hits a tennis ball, which of the following statements represents an action-reaction pair as described by Newton's Third Law?
- The force of the racket on the ball and the air resistance on the ball.
- The force of the racket on the ball and the force of the ball on the racket.
- The force of gravity on the ball and the normal force on the ball.
- The force of the player on the racket and the force of the racket on the ball.
Answer: B
Explanation: Newton's Third Law states that for every action, there is an equal and opposite reaction. An action-reaction pair involves two objects interacting, with each exerting a force on the other. In this scenario, the racket exerts a force on the ball, and simultaneously, the ball exerts an equal and opposite force on the racket. The forces in an action-reaction pair always act on different objects.
Question 5 (2 marks)
An object's velocity-time graph shows a horizontal line above the time axis. What does this indicate about the object's motion?
Answer: The object is moving at a constant positive velocity (constant speed in a constant direction).
Explanation: A horizontal line on a velocity-time graph indicates that the velocity is not changing over time. If the line is above the time axis, it signifies a positive velocity. Therefore, the object is moving with a constant positive velocity, meaning its speed and direction are unchanging, and its acceleration is zero.
Question 6 (2 marks)
Which factor does NOT affect the magnitude of kinetic friction between two surfaces?
- The normal force pressing the surfaces together.
- The coefficient of kinetic friction between the surfaces.
- The area of contact between the surfaces.
- The nature of the surfaces (e.g., rough or smooth).
Answer: C
Explanation: The formula for kinetic friction is f_k = μ_k * N, where μ_k is the coefficient of kinetic friction and N is the normal force. This shows that kinetic friction depends on the normal force and the nature of the surfaces (represented by μ_k). Surprisingly, the area of contact does not significantly affect the magnitude of kinetic friction, as long as the normal force remains the same. The speed of relative motion also generally has little effect for typical speeds.
Question 7 (3 marks)
An elevator is accelerating upwards at 2 m/s². A passenger inside the elevator has a mass of 60 kg. What is the apparent weight of the passenger during this acceleration? (Take g = 10 m/s² for simplicity)
Answer: 720 N
Explanation: When the elevator accelerates upwards, the normal force (apparent weight) exerted by the floor on the passenger must be greater than their actual weight. Using Newton's Second Law, F_net = ma. The forces acting on the passenger are the normal force (N) upwards and their weight (W = mg) downwards. So, N - mg = ma. Rearranging for N: N = mg + ma = m(g + a). Substituting the values: N = 60 kg * (10 m/s² + 2 m/s²) = 60 kg * 12 m/s² = 720 N.
Common mistakes
- Confusing mass and weight: Mass is an intrinsic property of an object (amount of matter, measured in kg), while weight is the force of gravity acting on that mass (measured in N, and changes with gravitational field strength). Always remember W = mg.
- Misidentifying action-reaction pairs: Newton's Third Law pairs always act on *different* objects. For example, the weight of a book (Earth pulling on book) and the normal force on the book (table pushing on book) are NOT an action-reaction pair. The action-reaction pair for the book's weight is the book pulling on the Earth.
- Incorrectly interpreting slopes and areas on motion graphs: On a distance-time graph, the slope is velocity. On a velocity-time graph, the slope is acceleration, and the area under the curve is displacement. Students often mix these up.
- Forgetting vector nature of displacement, velocity, acceleration, and force: Direction matters! Forgetting to account for positive and negative directions can lead to incorrect calculations, especially when dealing with changes in direction or opposing forces.
Exam tips
- Always draw free-body diagrams for force problems: This helps visualize all forces acting on an object and their directions, making it easier to apply Newton's Laws and resolve forces into components.
- Pay attention to units and significant figures: Ensure all quantities are in consistent SI units (e.g., kg for mass, m/s for velocity, N for force). Rounding too early or providing too many significant figures can cost marks.
- Break down complex problems: For multi-step problems, identify knowns and unknowns, then break the problem into smaller, manageable steps. Often, solving for an intermediate value is necessary before reaching the final answer.
- Practice interpreting graphs extensively: Velocity-time and displacement-time graphs are common in exams. Understand what the slope and area represent for each type of graph, and practice sketching graphs based on given motion descriptions.
Why this matters for exams
The principles of motion and forces form the cornerstone of physics and are a core component of the IB MYP Physics curriculum. Mastery of these concepts is essential not only for achieving high scores in assessments but also for understanding more complex topics later on, such as energy, momentum, and waves. Expect questions that test your conceptual understanding, problem-solving skills, and ability to interpret graphical representations of motion.
Frequently asked questions
What is the difference between speed and velocity?
Speed is a scalar quantity that measures how fast an object is moving (magnitude only), while velocity is a vector quantity that measures both the speed and the direction of an object's motion.
How do I know if the forces acting on an object are balanced or unbalanced?
Forces are balanced if the net (resultant) force acting on the object is zero. This means the object will either remain at rest or move at a constant velocity (zero acceleration). Forces are unbalanced if the net force is non-zero, which will cause the object to accelerate (change its velocity).
What is inertia, and how is it related to mass?
Inertia is the property of an object to resist changes in its state of motion. An object with greater mass has greater inertia, meaning it is harder to start moving if it's at rest, or harder to stop/change its direction if it's already in motion.
Does friction always oppose motion?
Friction generally opposes the relative motion or tendency of motion between surfaces in contact. Static friction prevents motion, while kinetic friction opposes existing motion. However, friction can also be enabling, like when you walk (friction between your shoes and the ground pushes you forward) or a car accelerates (friction between tires and road provides the forward force).