Physics: Mechanics and Kinematics Flashcards

A car accelerates uniformly from rest to 20 m/s in 5 seconds. What is its acceleration?

Answer: 4 m/s²

Acceleration a = Δv/Δt = (20-0)/5 = 4 m/s².

According to Newton's Third Law, when a horse pulls a cart forward, the cart pulls the horse backward with equal force. Why does the horse-cart system still accelerate forward?

Answer: Newton's Third Law action-reaction pairs act on different objects; the net force on the horse from the ground exceeds the backward pull from the cart

Action-reaction forces always act on different objects and cannot cancel each other. The horse accelerates because the ground pushes it forward (friction) more than the cart pulls it back. The NET force on the horse-cart system is analyzed separately.

A 2 kg object slides down a frictionless incline of 30°. What is its acceleration along the incline? (g = 10 m/s²)

Answer: 5 m/s²

The component of gravity along the incline: a = g·sin30° = 10 × 0.5 = 5 m/s². Mass cancels out.

The unit of momentum in the SI system is:

Answer: kg·m/s

Momentum p = mv. Units: kg × m/s = kg·m/s. This is also equivalent to N·s (Newton-second), since N = kg·m/s².

In projectile motion (抛体运动), what happens to the horizontal velocity component throughout the flight (ignoring air resistance)?

Answer: It remains constant

In projectile motion without air resistance, the horizontal direction has no force acting on it, so horizontal velocity remains constant throughout the flight (Newton's First Law).

What does the area under a velocity-time (v-t) graph represent?

Answer: Displacement

The area under a v-t graph represents displacement (位移). This follows from the definition: displacement = average velocity × time, and graphically this is the area under the v-t curve.