Newton's 2nd & 3rd Law
If you double the net force acting on an object while keeping its mass constant, what happens to its acceleration?
Doubles
What is the difference between mass and weight? What are their units?
Mass is the amount of matter/inertia (in kg) and stays constant everywhere; weight is the force of gravity (in N) and changes with location.
When an elevator is moving upward at a constant speed of 4 m/s, how does a scale reading compare to your actual weight?
The scale reading equals your actual weight (acceleration is zero).
Can internal forces within a defined system accelerate the center of mass of that system?
No. Only net external forces can accelerate a system.
As you pull a sled with a constant force F but decrease the rope's angle relative to the ground, what happens to the horizontal component of your pull?
It increases (F_x = F cos θ increases as θ decreases towards 0°).
A bug hits the windshield of a fast-moving bus. How does the magnitude of the force exerted by the bus on the bug compare to the force exerted by the bug on the bus?
They are equal in magnitude (Newton's Third Law pair).
Which type of friction is generally larger for the same two surfaces: static friction or kinetic (sliding) friction?
Static friction (maximum static friction exceeds kinetic friction).
If an elevator accelerates upward at 2 m/s², does your apparent weight (scale reading) increase, decrease, or stay the same?
Increases (scale reads greater than actual weight).
A person inside a stalled car pushes hard on the dashboard. Why doesn't the car move?
The push is an internal force within the person-car system, so equal and opposite reaction forces cancel out within the system boundary.
Which component of gravity acts parallel to an inclined ramp and pulls objects down the slope?
F_{g,||} = F_g sin θ (or mg sin θ).
Identify the action-reaction Third Law pair for the downward gravitational force that Earth exerts on a 15 N book resting on a table.
The upward gravitational force that the book exerts on the Earth.
An 800 N astronaut on Earth travels to the Moon, where g = 1.6 m/s². What is the astronaut's mass on the Moon?
80 kg (mass on Earth = 800 / 10 = 80 kg, which does not change on the Moon).
An elevator cable snaps, causing the elevator to fall freely downward with a = 10 m/s². What does a scale read for a 60 kg person inside?
0 N (apparent weightlessness).
A 30 N force pulls two connected 5 kg blocks across a frictionless floor. What is the tension in the string between them?
15 N (Total mass = 10 kg, a = 3 m/s²; middle string pulls rear 5 kg block: T = 5 × 3 = 15 N).
As a ramp becomes steeper (angle θ increases), what happens to the normal force acting on a block sitting on the ramp?
It decreases (F_N = F_g cos θ decreases as θ increases).
A 10 N horizontal force pulls a 2 kg block to the right across a frictionless floor. What is the acceleration of the block?
5 m/s² to the right (a = F/m = 10 / 2 = 5 m/s²).
You push a 20 kg crate horizontally with 60 N of force, and it slides at a constant velocity. What is the magnitude of kinetic friction acting on the crate?
60 N (at constant velocity, net force is zero, so friction equals applied force).
A 10 kg fish hangs from a spring scale in an elevator accelerating upward at 3 m/s². What is the scale reading in newtons? (Use g = 10 m/s²)
130 N (F_scale = m(g + a) = 10(10 + 3) = 130 N).
You push a 30 kg block (Block A) which pushes a 10 kg block (Block B) with a total force of 80 N. What is the contact force that Block A exerts on Block B?
20 N (Block B is 1/4 of total system mass, so contact force = 1/4 × 80 N = 20 N).
A 10 kg block sits on a frictionless 30° incline. What is its acceleration down the ramp? (sin 30° = 0.5, g = 10 m/s²)
5 m/s² (a = g sin θ = 10 × 0.5 = 5 m/s²).
Why do a heavy bowling ball and a light tennis ball accelerate at the exact same rate in a vacuum, even though gravity pulls harder on the bowling ball?
The bowling ball has proportionally more mass (inertia/resistance to acceleration), so the ratio of force to mass (F/m) remains constant.
Does flipping a wooden block from its smallest side onto its largest side change the sliding friction force when pulled across a table? Explain.
No. Surface contact area generally does not affect the force of friction.
Describe the direction of motion and behavior (speeding up/slowing down) of two scenarios where an elevator causes a scale to read LESS than your actual weight.
When accelerating downward (starting to descend from top floor) AND when decelerating upward (slowing down as you reach top floor).
A person of weight 600 N stands on a platform supported by a rope that loops over a single overhead pulley back to their hands. What tension must they pull with to remain stationary in equilibrium?
300 N (The rope supports the person-platform system twice, so 2T = 600 N).
A 500 N force pulls a block at 37° above the horizontal. If cos 37° = 0.8 and sin 37° = 0.6, find the horizontal and vertical components of the pulling force.
F_x = 400 N, F_y = 300 N.