Question

physics test review – 8th grade
newton’s second law of motion (a= f / m)

• the formula for force is f = _ m × _ a= _ / _, m= _ / _
• if mass increases, ___ force is needed to keep the same acceleration.
• if the same force is applied, the ___ (lighter/heavier) object will accelerate more.
• net force = _ force – _ force (like friction or air resistance).

newton’s first law of motion (inertia)

• objects at rest will stay at rest, and objects in motion will stay in motion unless ___
• in bumper cars, passengers keep moving forward when the car stops suddenly because of ___
• a ball rolling on the ground eventually stops because of ___.

newton’s third law of motion (action-reaction)

• for every action, there is an _ and _ reaction.
• example: a rocket goes up because gases push ___.
• when you push down on a diving board, the board pushes back in the ___ direction.

balanced vs. unbalanced forces

• balanced forces mean there is ___ change in motion.
• unbalanced forces cause an object to _, _, or change ___.
• if net force = 0, forces are _. if net force ≠ 0, forces are _.

motion, speed, and graphs

• the formula for speed is speed = _ ÷ _.
• a student runs 100 m in 12.5 seconds. speed = ___ m/s.
• on a motion graph: steeper slope = _ speed, flat line = _ motion.

Explanation:

Newton’s Second Law of Motion

• The formula for force is \( F = m \times a \), \( a=\frac{F}{m} \), \( m=\frac{F}{a} \) (from \( F = ma \), rearranged for \( a \) and \( m \)).
• If mass increases, more force is needed (since \( F = ma \), for constant \( a \), \( F \) is proportional to \( m \)).
• If the same force is applied, the lighter object will accelerate more (from \( a=\frac{F}{m} \), smaller \( m \) gives larger \( a \)).
• Net Force = Applied force – Resistive force (e.g., friction, air resistance).

Newton’s First Law of Motion (Inertia)

• Objects at rest will stay at rest, and objects in motion will stay in motion unless acted upon by an unbalanced force.
• In bumper cars, passengers keep moving forward when the car stops suddenly because of inertia (tendency to maintain motion).
• A ball rolling on the ground eventually stops because of friction (resistive force opposing motion).

Newton’s Third Law of Motion (Action - Reaction)

• For every action, there is an equal and opposite reaction.
• Example: A rocket goes up because gases push downward (action - reaction: rocket pushes gases down, gases push rocket up).
• When you push down on a diving board, the board pushes back in the upward direction (action - reaction).

Balanced vs. Unbalanced Forces

• Balanced forces mean there is no change in motion (net force = 0).
• Unbalanced forces cause an object to speed up, slow down, or change direction (net force ≠ 0, so acceleration occurs).
• If net force = 0, forces are balanced. If net force ≠ 0, forces are unbalanced.

Motion, Speed, and Graphs

• The formula for speed is \( \text{Speed} = \frac{\text{Distance}}{\text{Time}} \).
• A student runs 100 m in 12.5 seconds. Speed \( = \frac{100}{12.5}=8 \) m/s.
• On a motion graph: Steeper slope = faster speed (slope = \( \frac{\text{distance}}{\text{time}}=\text{speed} \)), flat line = no (or constant) motion (distance not changing over time).

Filled - in Answers (Key Blanks):

• Newton’s Second Law: \( F = \boldsymbol{m \times a} \), \( a=\boldsymbol{\frac{F}{m}} \), \( m=\boldsymbol{\frac{F}{a}} \); more; lighter; Applied, Resistive.
• Newton’s First Law: acted upon by an unbalanced force; inertia; friction.
• Newton’s Third Law: equal, opposite; downward; upward.
• Balanced vs. Unbalanced: no; speed up, slow down, direction; balanced, unbalanced.
• Motion, Speed, Graphs: Distance, Time; 8; faster, no.

Answer:

Newton’s Second Law of Motion

• The formula for force is \( F = m \times a \), \( a=\frac{F}{m} \), \( m=\frac{F}{a} \) (from \( F = ma \), rearranged for \( a \) and \( m \)).
• If mass increases, more force is needed (since \( F = ma \), for constant \( a \), \( F \) is proportional to \( m \)).
• If the same force is applied, the lighter object will accelerate more (from \( a=\frac{F}{m} \), smaller \( m \) gives larger \( a \)).
• Net Force = Applied force – Resistive force (e.g., friction, air resistance).

Newton’s First Law of Motion (Inertia)

• Objects at rest will stay at rest, and objects in motion will stay in motion unless acted upon by an unbalanced force.
• In bumper cars, passengers keep moving forward when the car stops suddenly because of inertia (tendency to maintain motion).
• A ball rolling on the ground eventually stops because of friction (resistive force opposing motion).

Newton’s Third Law of Motion (Action - Reaction)

• For every action, there is an equal and opposite reaction.
• Example: A rocket goes up because gases push downward (action - reaction: rocket pushes gases down, gases push rocket up).
• When you push down on a diving board, the board pushes back in the upward direction (action - reaction).

Balanced vs. Unbalanced Forces

• Balanced forces mean there is no change in motion (net force = 0).
• Unbalanced forces cause an object to speed up, slow down, or change direction (net force ≠ 0, so acceleration occurs).
• If net force = 0, forces are balanced. If net force ≠ 0, forces are unbalanced.

Motion, Speed, and Graphs

• The formula for speed is \( \text{Speed} = \frac{\text{Distance}}{\text{Time}} \).
• A student runs 100 m in 12.5 seconds. Speed \( = \frac{100}{12.5}=8 \) m/s.
• On a motion graph: Steeper slope = faster speed (slope = \( \frac{\text{distance}}{\text{time}}=\text{speed} \)), flat line = no (or constant) motion (distance not changing over time).

Filled - in Answers (Key Blanks):

• Newton’s Second Law: \( F = \boldsymbol{m \times a} \), \( a=\boldsymbol{\frac{F}{m}} \), \( m=\boldsymbol{\frac{F}{a}} \); more; lighter; Applied, Resistive.
• Newton’s First Law: acted upon by an unbalanced force; inertia; friction.
• Newton’s Third Law: equal, opposite; downward; upward.
• Balanced vs. Unbalanced: no; speed up, slow down, direction; balanced, unbalanced.
• Motion, Speed, Graphs: Distance, Time; 8; faster, no.