Inertia and Newton's First
What is inertia?
The tendency of an object to resist changes in its motion.
Define force.
A push or pull that can change an object's motion.
State Newton’s Second Law.
F=ma
Define a two-body problem.
A system where two objects interact through forces like tension or contact.
State Newton’s Third Law.
For every action, there is an equal and opposite reaction.
Define projectile.
An object moving through the air under the influence of gravity alone.
What happens to an object in motion with no net force?
It continues moving at constant velocity.
Balanced vs. unbalanced forces.
Balanced forces cause no acceleration; unbalanced forces cause acceleration.
Acceleration: 10 kg, 30 N.
3 m/s2
Treating two objects as one.
Add their masses and apply Newton’s Second Law using the total mass.
Force pair definition.
Two equal and opposite forces acting on different objects.
What force acts after launch?
Gravity (ignoring air resistance).
Why does a hockey puck slow down?
Friction acts on it; in ideal Newton’s First Law, no friction would be present.
(diagram)
Mass: F = 4000 N, a = 2 m/s².
2000 kg
Acceleration of two objects: force = 20 N, mass = 4 kg.
a=20/4=5 m/s2
Walking force pairs.
Your foot pushes backward on the ground; the ground pushes forward on you.
Time in air: launched at 10 m/s horizontally from 5 m height.
t = 1.01 s
Why do passengers move forward when a car stops?
Their inertia keeps them moving forward while the car stops.
Net force if 15N are applied right and 10N are applied left.
5N to the right.
How Newton's second applies to Free Body Diagrams.
Net force from the diagram determines acceleration using F=ma.
Why analyze individual forces?
To find internal forces (like tension) or understand how each object accelerates.
Why a gun recoils.
Bullet is pushed forward; gun is pushed backward with equal force.
Initial vertical velocity: 20 m/s at 30°.
vy=10 m/s
Real-world example where inertia keeps an object at rest.
(ans vary)
A box is pushed across a floor with a 40 N force at a 30° angle downward toward the floor. The box experiences 10 N of friction. Draw or describe the free body diagram and determine the horizontal net force acting on the box.
Net horizontal: 24.6N to the right
An object experiences three forces:
20 N to the right
15 N to the left
25 N upward
If the object has a mass of 5 kg, determine the magnitude of its acceleration.
5.1 m/s2
2 kg & 3 kg blocks, pulled with 15 N; find tension.
acc = 3m/s2
tension = 6N
Why equal forces when pushing against a wall, even if the wall doesn’t move.
The wall exerts an equal reaction force, but its massive support structure prevents noticeable acceleration.
Time to reach max height: 15 m/s at 45°.
t=1.08 s