3- Forces

Question 1

types of forces and newtons third law

Answer 1

• distant forces = act over long distances e.g gravity, electrostatic attraction
• contact forces = only act on contact with other objects e.g friction, tension
• pairs of forces in newtons third law must be of the same type, act along the exact same line, and act on two separate bodies

Question 2

equilibrium and newtons first law

Answer 2

• object is in equilibrium when no external forces act on it, but due to the first law it can still be moving
• when multiple forces all act on an object and cancel out, equilibrium is still achieved as there is no resultant force
• when an object is in equilibrium these forces can be drawn as a free body diagram (usually a closed right-angled triangle about the center of mass)
• this means unknown forces acting on an object in equilibrium can be found using trigonometry

Question 3

fixed masses and multi-body systems

Answer 3

• newtons 2nd law only applies to fixed masses
• impulses can only be found for fixed masses
• in multi-body systems, resistive forces sum together, but only one body has the driving force.
• individual bodies in the system are accelerated by the tension pulling them forwards, and also experience the tension of every body behind it
• isolating the forces acting on each individual body and ignoring the effects of surrounding objects

Question 4

turning forces

Answer 4

• moment (Nm) = force*distance from pivot point
• equal and opposite forces can act in different directions to the pivot point, so the object will be in equilibrium, but it will be rotating and not accelerating (rotational equilibrium)
• sum of clockwise moments = sum of anti clockwise moments in rotational equilibrium