IB MYP 4-5 Physics- Frictional forces- Study Notes - New Syllabus
IB MYP 4-5 Physics-Frictional forces- Study Notes
Key Concepts
- Frictional forces
Normal & Frictional Forces
Normal Force
The normal force is the contact force exerted by a surface on an object resting on it. It acts perpendicular to the surface, preventing the object from passing through it.
Symbol: \( F_N \)
SI Unit: Newton (\( N \))
- The normal force balances the perpendicular component of an object’s weight when the surface is horizontal.
- It can change if an additional vertical force is applied (e.g., pushing down or pulling up on the object).
- On inclined planes, the normal force is less than the object’s weight and is given by:
\( F_N = mg \cos\theta \)
Example:
A 10 kg box rests on a horizontal table. Find the normal force acting on it.
▶️ Answer/Explanation
Given: \( m = 10\ \text{kg},\ g = 9.8\ \text{m/s}^2 \)
Since the table is horizontal and no vertical forces other than weight act: \( F_N = mg = (10)(9.8) = 98\ \text{N} \)
\(\boxed{F_N = 98\ \text{N}}\)
Example:
A 15 kg block is placed on a \( 30^\circ \) incline. Find the normal force.
▶️ Answer/Explanation
Given: \( m = 15\ \text{kg},\ g = 9.8\ \text{m/s}^2,\ \theta = 30^\circ \)
Normal force: \( F_N = mg\cos\theta = (15)(9.8)\cos 30^\circ \)
\( F_N = 147 \times 0.866 = 127.3\ \text{N} \)
\(\boxed{F_N \approx 127.3\ \text{N}}\)
Frictional Forces
- Friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact.
- It acts parallel to the surfaces in contact and in the opposite direction of motion.
- It is a contact force caused by microscopic irregularities and intermolecular forces between surfaces.
Types of Friction:
- Static friction (\(f_s\)) – The frictional force that prevents motion when a force is applied to a stationary object.
- Kinetic (sliding) friction (\(f_k\)) – The frictional force acting when an object is sliding over a surface.
- Rolling friction – The frictional force resisting the motion when a body rolls over a surface (usually much smaller than sliding friction).
Formulas:
\( f_s \leq \mu_s N \)
\( f_k = \mu_k N \)
- \( \mu_s \) and \( \mu_k \) are the coefficients of static and kinetic friction (dimensionless).
- \( N \) is the normal force.
Key Notes:
- Static friction adjusts up to its maximum limit to prevent motion.
- Kinetic friction remains constant for given surfaces and normal force once motion starts.
- Friction converts kinetic energy into heat and sometimes sound.
- Lubricants reduce friction; rough surfaces increase it.
Example:
A \( 10 \ \text{kg} \) box rests on a horizontal surface with \( \mu_s = 0.4 \) and \( \mu_k = 0.3 \). Calculate the maximum static friction force.
▶️ Answer/Explanation
Normal force:
\( N = m g = 10 \times 9.8 = 98 \ \text{N} \)
Maximum static friction:
\( f_s^{\text{max}} = \mu_s N = 0.4 \times 98 = 39.2 \ \text{N} \)
Final Answer: \( \boxed{39.2 \ \text{N}} \)
Example:
The same box is sliding on the surface. Calculate the kinetic friction force.
▶️ Answer/Explanation
Normal force (same as before):
\( N = 98 \ \text{N} \)
Kinetic friction:
\( f_k = \mu_k N = 0.3 \times 98 = 29.4 \ \text{N} \)
Final Answer: \( \boxed{29.4 \ \text{N}} \)