Unit 2 of Physics: Forces and Newton's Laws.
This unit covers Newton's three laws, free-body diagrams and friction — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers Newton's three laws, free-body diagrams and friction — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Newton's Three Laws
Newton's three laws describe how forces affect the motion of objects. Students must be able to identify which law applies in a given scenario and use F = ma to solve for unknown quantities. Understanding that the net force determines acceleration — not individual forces — is critical.
Key Points
- First Law (Inertia): An object at rest stays at rest, and an object in motion stays in motion, unless acted on by a net external force.
- Second Law: Net force equals mass times acceleration (F_net = ma); doubling force doubles acceleration if mass is constant.
- Third Law: For every action force, there is an equal and opposite reaction force acting on a DIFFERENT object.
- A net force of zero means zero acceleration — the object may still be moving, just at constant velocity.
A 10 kg box is pushed with a 30 N applied force on a frictionless surface. What is the acceleration?
Using Newton's Second Law: F_net = ma, so a = F_net / m = 30 N / 10 kg = 3 m/s². Since the surface is frictionless, the applied force is the only horizontal force, making it the net force. The answer is 3 m/s² in the direction of the push.
2 Free-Body Diagrams
A free-body diagram (FBD) shows all forces acting on a single object as arrows pointing away from a dot representing the object. Students must be able to draw and interpret FBDs to set up Newton's Second Law equations correctly. Every force must have both a correct direction and a labeled magnitude.
Key Points
- Include only forces acting ON the object — never forces the object exerts on others.
- Common forces: weight (mg, downward), normal force (perpendicular to surface), tension (along rope, away from object), friction (opposing motion or tendency to move).
- On an incline, weight must be resolved into components parallel and perpendicular to the surface.
- If the object is in equilibrium, all force vectors in the FBD must sum to zero in both x and y directions.
A 5 kg block sits on a flat table. Draw and label all forces, then find the normal force.
The FBD shows two forces: weight (W = mg = 5 × 10 = 50 N) pointing downward, and normal force (N) pointing upward. Since the block is not accelerating vertically, the net vertical force is zero: N − W = 0, so N = 50 N. The normal force equals the weight only because the surface is horizontal and there is no vertical applied force.
3 Friction
Friction is a force that opposes the relative motion (or tendency of motion) between two surfaces in contact. Students must distinguish between static and kinetic friction and apply the formulas f_s ≤ μ_s·N and f_k = μ_k·N to solve problems. The normal force, not the weight, determines friction — these are only equal on flat horizontal surfaces.
Key Points
- Static friction prevents an object from starting to move; it can range from 0 up to a maximum of μ_s·N.
- Kinetic friction acts on a moving object and is constant: f_k = μ_k·N.
- μ_s > μ_k: it takes more force to start an object moving than to keep it moving.
- Friction always acts parallel to the surface and opposite to the direction of motion (or intended motion).
A 20 kg box is being pushed across a floor at constant velocity with a horizontal force of 60 N. What is the coefficient of kinetic friction?
Constant velocity means zero acceleration, so the net force is zero. The applied force (60 N forward) must equal the kinetic friction force (60 N backward). Since f_k = μ_k·N and the normal force equals mg = 20 × 10 = 200 N, we solve: μ_k = f_k / N = 60 / 200 = 0.30. The coefficient of kinetic friction is 0.30 (unitless).
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What is Forces and Newton's Laws?
Forces and Newton's Laws is Unit 2 of Physics, covering Newton's three laws, free-body diagrams and friction.
How to study for Physics Unit 2?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 28+ review questions across 5 different game modes.