In which situation is the object experiencing unbalanced forces? A) A box resting on a horizontal floor. B) A car slowing as it
2 answers:
Answer:
B) A car slowing as it reaches a traffic light.
Explanation:
Newton's second law states that an object which experiences unbalanced forces has an acceleration, according to the equation:
where
F is the net force acting on the object
m is its mass
a is its acceleration
When the forces are unbalanced, it means that the net force F is different from zero, so the acceleration of the object (a) is also different from zero.
Now let's analyze each situation:
A) A box resting on a horizontal floor. --> The object is at rest, so it is not accelerating. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice
B) A car slowing as it reaches a traffic light. --> The car is slowing down, this means that it has an acceleration: therefore, it is experiencing unbalanced forces. So, this is the correct choice.
C) A car with its cruise control set to 30 km/h. --> The car is moving at constant velocity, so it has zero acceleration. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice
D) A penny at terminal velocity after falling off of a building. --> The penny is at terminal velocity, which means that it is moving at constant velocity, so it has zero acceleration. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice.
Therefore, the correct choice is
B) A car slowing as it reaches a traffic light.
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Answer
Radius of the wheel r = 2.1 m
Moment of inertia I = 2500 Kg m²
Tangential force applied F = 18 N
Time interval t = 16 s
Initial angular speed ω1 = 0
Final angular speed ω2 = ?
Let α be the angular acceleration.
Torque applied τ = Iα
F r = Iα
Angular acceleration α = F r/I
=
= 0.015 rad/s²
(a)From rotational kinematic relation
Final angular speed ω₂ = ω₁ + αt
= 0 + (0.015 rad/s^2 * 16 s)
= 0.24 rad/s
(b) Work done W = 0.5 Iω₂² - (1/2)Iω₁²
= 0.5*( 2500 Kg m²)(0.24 rad/s)^2 - 0
= 72 J
(c) Average power supplied by the child P = W/t =
= 4.5 watt