Question

warm - up:
consider these two scenarios:
scenario a: a skydiver falling through the air at a terminal (constant) velocity.
scenario b: a marble rolling down a ramp and speeding up.
question: which scenario involves balanced forces, and which involves unbalanced forces? for the unbalanced scenario, explain why the forces are unbalanced.
a car is driving at a constant speed on a straight, level road. according to newtons first law, which statement about the forces on the car is true?
a. the forces acting on the car are unbalanced, causing it to move forward.
b. the engine force is greater than the friction force, allowing it to maintain speed.
c. the forces acting on the car are balanced, and the net force is zero.
d. there are no forces acting on the car, which is why its speed doesnt change.
warm - up:
the picture shows two different forces acting on a car in motion.
what will be the effect of these forces on the motion of the car? fill in the blanks to complete the description. write your answers on the lines.
the forces are __________, and the car will __________.
a. balanced
b. not move
c. accelerate
d. slow down
e. unbalanced
which of the following describes a situation where an object is experiencing unbalanced forces?
a. a box being pushed across the floor at a constant speed.
b. an elevator holding steady between floors.
c. a bowling ball speeding up as it rolls down the ramp.
d. a marble resting on a flat, level table.

Explanation:

Warm - up (first part)

Scenario A

• Step1: Analyze forces in Scenario A

When a skydiver is falling at terminal (constant) velocity, the force of gravity (pulling down) is equal in magnitude and opposite in direction to the air - resistance force (pushing up). According to Newton's first law (\(F_{net}=\sum F = 0\) when \(a = 0\)), the forces are balanced.

Scenario B

• Step1: Analyze forces in Scenario B

A marble rolling down a ramp and speeding up has an unbalanced force. The component of the gravitational force along the ramp (\(F_{g\parallel}=mg\sin\theta\), where \(m\) is the mass of the marble, \(g\) is the acceleration due to gravity, and \(\theta\) is the angle of the ramp) is greater than the frictional force (\(F_f\)) acting up the ramp. So, \(F_{net}=F_{g\parallel}-F_f> 0\), and by Newton's second law (\(F_{net}=ma\)), since \(F_{net}
eq0\), \(a
eq0\) (the marble accelerates).

Question (second part - car moving at constant speed)

• Step1: Recall Newton's first law

Newton's first law states that an object at rest or in uniform motion (constant speed in a straight line) has a net force of zero (\(F_{net}=\sum F = 0\)). When a car is moving at a constant speed on a straight, level road, the forward force (from the engine, \(F_{engine}\)) is equal in magnitude and opposite in direction to the backward - acting forces (like friction, \(F_{friction}\)), and the normal force (\(F_N\)) from the road on the car is equal in magnitude and opposite in direction to the gravitational force (\(F_g = mg\)) on the car. So the forces acting on the car are balanced, and the net force is zero.

• Option A: Incorrect. If the forces were unbalanced (\(F_{net}

eq0\)), the car would accelerate (not move at constant speed).

• Option B: Incorrect. If \(F_{engine}>F_{friction}\), \(F_{net}

eq0\) and the car would accelerate (not maintain constant speed).

• Option C: Correct. As explained above, for constant - speed motion in a straight line, \(F_{net} = 0\) (forces are balanced).
• Option D: Incorrect. There are forces acting on the car (gravity, normal force, engine force, friction).

Friday (third part - forces on car in motion)

• Step1: Calculate the net force on the car

The forces on the car are \(F_1 = 90N\) (forward) and \(F_2=40N\) (backward). The net force \(F_{net}=F_1 - F_2=90N - 40N=50N\). Since \(F_{net}
eq0\), the forces are unbalanced.

• Step2: Determine the motion of the car

By Newton's second law (\(F_{net}=ma\)), when \(F_{net}>0\), the car will accelerate in the direction of the net force (forward).

Friday (fourth part - unbalanced forces)

• Step1: Analyze each option
• Option A: A box being pushed across the floor at a constant speed has \(F_{net}=0\) (balanced forces, since \(a = 0\)).
• Option B: An elevator holding steady between floors has \(F_{net}=0\) (balanced forces, \(a = 0\)).
• Option C: A bowling ball speeding up as it rolls down the ramp has \(F_{net}

eq0\) (unbalanced forces, because \(a
eq0\) and by \(F_{net}=ma\)).

• Option D: A marble resting on a flat, level table has \(F_{net}=0\) (balanced forces, \(a = 0\), \(F_N=F_g\)).

Answer:

• Warm - up (first part): Scenario A involves balanced forces. Scenario B involves unbalanced forces. In Scenario B, the net force is not zero (the component of gravitational force along the ramp is greater than the frictional force), so the marble accelerates.
• Question (second part): C. The forces acting on the car are balanced, and the net force is zero.
• Friday (third part): The forces are E. unbalanced, and the car will C. accelerate.
• Friday (fourth part): C. A bowling ball speeding up as it rolls down the ramp.