A swimmer pushes backward against the pool wall with a force of 300 N. What force does the wall exert on the swimmer?

0 N — the wall cannot push back because it is fixed 300 N forward on the swimmer 600 N forward on the swimmer 300 N backward on the swimmer

A rocket engine expels hot gas downward at high speed. What propels the rocket upward?

Air pressure beneath the rocket pushing it upward The engine heating the rocket body and making it lighter The reaction force of the expelled gas pushing the rocket upward Gravity being temporarily cancelled by the engine

You are trying to push a heavy box across the floor. You apply 80 N but the box does not move. What type of friction is acting and what is its magnitude?

Kinetic friction of 80 N opposing your push Static friction of 80 N opposing your push Static friction of 40 N opposing your push No friction — the box is not moving so friction does not exist yet

An astronaut has a mass of 70 kg. What is their weight on Earth (g = 9.8 m/s²)?

70 N 9.8 N 686 N 7.14 N

The same astronaut travels to Mars, where g = 3.7 m/s². What is true about their mass and weight on Mars compared to Earth?

Both mass and weight stay the same Their mass stays at 70 kg; their weight decreases to approximately 259 N Their mass decreases; their weight stays the same Both mass and weight decrease to approximately 259 kg/N

Why is static friction generally greater than kinetic friction?

Gravity increases as objects begin to move Kinetic friction increases with velocity Surface asperities interlock more deeply when surfaces are stationary, requiring more force to start motion than to maintain it Static friction is always equal to kinetic friction; this premise is incorrect

In designing a car's braking system, engineers want to avoid the wheels locking up (skidding). Why is controlled braking (wheels still turning) better than locked-wheel skidding?

Skidding generates more heat, which melts the road and creates drag Rolling tyres use static friction, which is greater than the kinetic friction of a skidding tyre, providing better deceleration Locked wheels produce less noise, so drivers can hear hazards better Skidding stretches the tyres and increases their surface area

Action-reaction pairs always act on:

The same object in opposite directions Two different objects The same object in the same direction Objects that are not touching each other

A book rests on a table. A student says: 'The normal force from the table is the reaction to the book's weight, so they form a Newton's 3rd law pair.' Is this correct?

Yes — the normal force and weight are always a Newton's 3rd law pair on stationary objects Yes — whenever two forces are equal and opposite they form a Newton's 3rd law pair No — the weight (Earth pulling book down) and the reaction to weight (book pulling Earth up) form the pair; the normal force is a separate interaction No — Newton's 3rd law only applies to objects in motion

The friction coefficient between rubber and dry asphalt is about 0.8, while between rubber and wet asphalt it is about 0.4. What practical engineering implication does this have for vehicle safety systems?

Speed limits should be reduced by exactly 10% on wet roads Braking distance on wet roads is approximately double that on dry roads at the same speed Wet roads have no effect on stopping distance if the tyres are new Friction coefficient does not affect stopping distance, only tyre wear

A bird sits on a power line and pushes down with its weight. The power line pulls the bird up with tension. Which of the following correctly identifies a Newton's 3rd law action-reaction pair?

Gravity pulls bird down; normal force from the wire pushes bird up Bird pushes down on wire; wire pushes up on bird Tension in the wire pulls both ends outward Earth pulls bird down; bird pulls Earth down