Mechanical Aptitude Forces and Motion Principles Questions and Answers 2 — Questions and Answers

Question 1: A 5 kg object is accelerating at 3 m/s². What is the net force acting on it?

8 N
15 N (Correct answer)
1.67 N
2 N

Correct answer: 15 N

Using Newton's Second Law (F = ma): F = 5 kg × 3 m/s² = 15 N. The net force acting on the object is 15 Newtons.

Newton's Second Law of Motion states that the net force acting on an object equals its mass multiplied by its acceleration (F = ma). This is perhaps the most important equation in classical mechanics. For this problem: F = 5 kg × 3 m/s² = 15 N. The unit Newton (N) is defined as kg·m/s², confirming the dimensional consistency. This law also tells us that for a constant force, doubling the mass halves the acceleration, and for a constant mass, doubling the force doubles the acceleration.

Question 2: What is the term for the tendency of an object to resist changes in its state of motion?

Momentum
Friction
Inertia (Correct answer)
Gravity

Correct answer: Inertia

Inertia is the property of matter that resists changes in motion. An object at rest tends to stay at rest, and an object in motion tends to stay in motion at constant velocity, unless acted upon by an external force (Newton's First Law).

Inertia is the inherent property of all matter to resist acceleration — any change in velocity (speed or direction). The measure of an object's inertia is its mass: more massive objects are harder to start moving, stop, or change direction. Newton's First Law (Law of Inertia) formalizes this: without a net external force, an object maintains its current velocity. This is why passengers lurch forward when a car brakes suddenly — their bodies tend to continue at the car's original speed due to inertia.

Question 3: When two objects collide and stick together, what type of collision has occurred?

Elastic collision
Inelastic collision (Correct answer)
Perfectly elastic collision
Static collision

Correct answer: Inelastic collision

When objects collide and stick together, it is a perfectly inelastic collision. In this type, momentum is conserved but kinetic energy is not — some is converted to heat, sound, and deformation. This is the maximum loss of kinetic energy possible in a collision.

Collisions are classified by how much kinetic energy is conserved. In a perfectly inelastic (or perfectly plastic) collision, the objects merge and move as one unit afterward, resulting in maximum kinetic energy loss while still conserving momentum. In elastic collisions, both momentum and kinetic energy are conserved (objects bounce apart). Real-world collisions fall between these extremes. A car crash where vehicles crumple together is nearly perfectly inelastic. The 'lost' kinetic energy transforms into deformation of materials, heat, sound, and other forms.

Question 4: An object is thrown straight up into the air. At the highest point of its trajectory, what is its velocity?

Maximum
Equal to initial velocity
Zero (Correct answer)
Negative

Correct answer: Zero

At the highest point, the object momentarily stops before falling back down, so its velocity is zero. Gravity decelerates the object on the way up and accelerates it on the way down.

When an object is thrown vertically upward, gravity acts as a constant decelerating force opposing its motion. The object's velocity decreases linearly over time until it reaches zero at the peak of its trajectory. At this instant, the object has zero velocity but not zero acceleration — gravity still acts on it at 9.8 m/s² downward. Immediately after this instant, the object begins accelerating downward. This is a common misconception: many assume that because velocity is zero, acceleration must also be zero, but force (and thus acceleration) is independent of velocity.

Question 5: What is the relationship between action and reaction forces according to Newton's Third Law?

They act on the same object
They are equal in magnitude and opposite in direction (Correct answer)
The reaction is always greater
They cancel each other out

Correct answer: They are equal in magnitude and opposite in direction

Newton's Third Law states that for every action, there is an equal and opposite reaction. These forces are equal in magnitude but opposite in direction, and they always act on different objects.

Newton's Third Law states that forces always occur in pairs: when object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. Critically, these forces act on different objects, which is why they do not cancel out. When you push against a wall, the wall pushes back on you with equal force. When Earth pulls you down with gravity, you pull Earth up with equal force (though Earth's enormous mass means its acceleration is negligible). This law is the principle behind rocket propulsion — exhaust gases push backward, and the rocket is pushed forward.

Question 6: A car traveling at 60 km/h has how much kinetic energy compared to the same car at 30 km/h?

Twice as much
Three times as much
Four times as much (Correct answer)
The same amount

Correct answer: Four times as much

Kinetic energy is proportional to the square of velocity (KE = ½mv²). Doubling the speed from 30 to 60 km/h means the kinetic energy increases by a factor of (60/30)² = 4.

The kinetic energy formula KE = ½mv² shows that energy scales with the square of velocity. When speed doubles from 30 to 60 km/h, the velocity ratio is 2:1, and squaring gives 4:1. This has profound safety implications: a car at 60 km/h needs four times the stopping distance of one at 30 km/h (assuming the same braking force), because four times the energy must be dissipated. This quadratic relationship explains why high-speed collisions are disproportionately more destructive than low-speed ones.

Mechanical Aptitude Forces and Motion Principles Questions and Answers 2

A 5 kg object is accelerating at 3 m/s².
What is the net force acting on it?