Grade 11 Physics Unit 4 : Dynamics

This unit takes you from kinematics (how things move) to dynamics (why they move). You’ll discover the laws that govern every push, pull, and collision — from a bajaj accelerating through Addis Ababa traffic to the Grand Ethiopian Renaissance Dam converting water’s energy into electricity. Dynamics is the #1 most tested Physics unit on the ESSLCE, so mastering it is essential.

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Chapter 4.1 – The Concept of Force and Newton’s Laws of Motion

This chapter introduces force as the cause of motion and lays down the three fundamental laws.

• Define force as a push or pull that can change an object’s state of rest or motion, and distinguish between contact forces and field forces.
• State Newton’s three laws of motion: the law of inertia, F = ma, and action-reaction pairs.
• Apply Newton’s second law to calculate acceleration, weight (W = mg), and normal force.
• Analyze motion using free-body diagrams and identify inertial vs non-inertial frames of reference.

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Chapter 4.2 – Frictional Force

• Explore the force that resists motion between surfaces in contact.
• Distinguish between static friction (prevents motion) and kinetic friction (opposes ongoing motion), and use f = μN to calculate each.
• Draw free-body diagrams for objects on inclined planes, resolving weight into components along and perpendicular to the slope.
• Solve Atwood’s machine problems to find acceleration and tension in connected-mass systems.

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Chapter 4.3 – The First Condition of Equilibrium

• Learn what it means for forces to be perfectly balanced.
• State the first condition of equilibrium: the vector sum of all forces on an object is zero (ΣF = 0).
• Resolve forces into x and y components and solve equilibrium problems using simultaneous equations.

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Chapter 4.4 – Work, Energy and Power

• Connect force and motion to the concept of energy.
• Define work as W = Fd cos θ and identify when work is zero, positive, or negative.
• Calculate kinetic energy (KE = ½mv²), gravitational potential energy (PE = mgh), and elastic potential energy (PE = ½kx²).
• Distinguish conservative forces (gravity, springs) from non-conservative forces (friction) and apply the work-energy theorem (W_net = ΔKE).
• Calculate average power (P = W/t) and instantaneous power.

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Chapter 4.5 – Conservation of Energy

• Apply one of the most powerful principles in all of physics.
• State the law of conservation of mechanical energy: in the absence of non-conservative forces, KE + PE remains constant.
• Solve problems where energy transforms between kinetic and potential forms — such as a block sliding down a frictionless incline.

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Chapter 4.6 – Impulse and Linear Momentum

• Discover why collisions behave the way they do.
• Define linear momentum (p = mv) as a vector quantity and relate it to Newton’s second law.
• Calculate impulse (J = FΔt = Δp) and explain how airbags and seatbelts reduce force by increasing collision time.
• Apply conservation of momentum to solve elastic, inelastic, and perfectly inelastic collision problems in isolated systems.
• Use the center of mass concept to describe the motion of a system of particles.

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Learning Outcomes

By the end of this unit, you’ll be able to:

• Analyze the relationship between net force and acceleration using Newton’s laws and free-body diagrams.
• Calculate friction forces on flat and inclined surfaces and solve equilibrium problems.
• Apply the concepts of work, energy, and power to real-world situations, including energy transformations.
• Solve collision and impulse problems using conservation of momentum in isolated systems.