A box initially at rest is pushed with a force of 30 N to the right while friction applies a force of 30 N to the left. What happens to the motion of the box?
The box remains at rest.
This question tests Newton's First Law of Motion (the law of inertia), which states that a body continues in its state of rest or of uniform motion in a straight line unless acted upon by a net (unbalanced) external force. The crucial word is net: several forces can act at once, and what governs the motion is their vector sum, not any single force.
Here two forces act along the same horizontal line:
- An applied push of 30 N toward the right.
- A frictional force of 30 N toward the left (friction always opposes the tendency of relative motion).
Since they are equal in magnitude and opposite in direction, they cancel exactly. Taking rightward as positive, net force = (+30) + (−30) = 0 N. With zero net force, the acceleration is also zero because a = Fnet/m = 0.
The box was initially at rest. Zero acceleration means its velocity cannot change, so a body at rest stays at rest. Therefore the box remains at rest — the forces are said to be balanced.
The box cannot accelerate to the right, since that would require a leftover rightward force, but friction has neutralised the push. It cannot slow down and stop, because it was never moving in the first place. And it cannot move in the opposite direction, because the net force is exactly zero, not directed leftward. (In reality static friction only rises up to a maximum; the problem simply tells us it matches the push at 30 N, keeping the object stationary.)
Similar Questions
- The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. This rule is known as:
- A constant force acts on an object of mass $10\text{ kg}$ for a duration of $2\text{ seconds}$. It increases the object's velocity from $5\text{ metres/second}$ to $10\text{ metres/second}$. Find the magnitude of the applied force. Now, if the force is applied for a duration of $5\text{ seconds}$, what would be the final velocity of the object?
- What is the equation for Newton's second law of motion?
- What will happen to a person's weight when he is in a moving elevator?
What will be the resultant force if a body of mass 10 kg is moving with an acceleration of 5 m/sec2?
Action and reaction forces are exerted on which bodies during an interaction?
Important Questions from Laws of Motion
A uniform meter scale of mass 0.24 kg is made of steel. It is kept on two wedges, W1 and W2 , in a horizontal position. W1 is at a distance of 0.2 m from one of its ends, while W2 is at distance of 0.4 m from the other end. If the force on the scale is N1 due to W1 and N2 due to W2, then : (take g =10·0 m s-2
Rocket works on the principle of:
A rocket is launched to travel vertically upward with a constant velocity of 20 m/s. After travelling for 35 seconds, the rocket develops a snag and its fuel supply is cut off. The rocket then travels like a free body. The height achieved by it is:
According to Newton's third law of motion, mark the correct option.
1. Action and reaction act on different bodies and so they can be cancelled out.
2. The internal action and reaction forces between different parts of a body do, however, sum to zero.
If Force (F), velocity (V) and time (T) are taken as the fundamental dimensions, instead of mass, length and time, what will be dimensions of linear Momentum (P) ?
