Forces & net force · First Law · F = ma · Mass, weight & elevators · Third Law · Free-body diagrams · Friction · Inclines & pulleys — 77 cards
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A push or a pull on an object, caused by an interaction with another object. It is a vector (size + direction), measured in newtons (N).
Contact forces need touching (push, normal, friction, tension, air resistance). Field forces act at a distance (gravity, magnetic, electric).
The object that exerts it (the table, the Earth, the rope). If you can't name an agent, the force isn't real.
The vector sum of all the forces acting on one object. In 1D: choose a positive direction and add with signs.
6 N right
Balanced: F_net = 0, so the motion doesn't change. Unbalanced: F_net ≠ 0, so the object accelerates.
The net force that gives a 1 kg mass an acceleration of 1 m/s². 1 N = 1 kg·m/s².
No. Nothing exerts it. The ball keeps moving because of inertia (First Law).
An object stays at rest, or keeps moving in a straight line at constant speed, unless a net external force acts on it.
F_net = 0 ⇔ a = 0 (v constant, including v = 0).
The tendency of an object to resist any change in its motion.
Mass. More mass means it's harder to start, stop or turn.
That a force is needed to keep an object moving, and rest is the natural state.
With no friction, a ball rolling onto a flat surface would roll on forever. Force is needed to change motion, not to keep it.
80 000 N. Constant velocity means F_net = 0.
Their bodies keep moving forward (inertia) until the seat or belt pushes them back.
Along the tangent, in a straight line. With no string force, nothing changes its velocity.
A frame of reference that isn't accelerating. The First Law holds in it. beyond the test
Acceleration is directly proportional to net force, inversely proportional to mass, and in the direction of the net force: F_net = m·a.
a ∝ F_net (mass fixed) and a ∝ 1/m (force fixed). Combined: a ∝ F_net/m, and the newton is defined so the constant is 1.
a doubles.
a halves.
Yes. Velocity can point elsewhere (a braking car: v forward, a backward).
Put F_net = 0 into a = F_net/m: a = 0, so v is constant.
F = ma = 30 000 × 1.5 = 45 000 N
a = F/m = 2.0 m/s². Towing an identical car: 1.0 m/s².
a = 27/9.0 = 3.0 m/s², so F = 1200 × 3.0 = 3600 N.
Separately on each axis: F_net,x = m·a_x and F_net,y = m·a_y.
F_net = m·v²/r, toward the center. It is supplied by real forces (tension, friction, gravity).
F_net = Δp/Δt, where p = mv is momentum. If m is constant this gives F = ma. beyond the test
Mass (kg) = amount of matter/inertia, the same everywhere. Weight (N) = the gravitational force on it, which depends on g.
In free fall the only force is W and a = g. N2: F_net = ma, so W = mg.
9.81 N (about 2.2 lb).
Mass 60 kg; weight 60 × 1.62 = 97 N.
a = W/m = mg/m = g. More weight, but also proportionally more mass (inertia).
The normal force N it pushes up on you with, not your weight.
N = m(g + a)
60 × 11.81 = 709 N (feels heavier).
More. Slowing while moving down means a points up.
0: "weightless". The weight is still mg; there's just no support force.
The constant speed reached when air resistance grows to equal the weight: F_net = 0, so a = 0.
When A exerts a force on B, B exerts a force on A that is equal in size and opposite in direction: F(A on B) = −F(B on A).
Write the action as "A exerts a force on B". The reaction is "B exerts a force on A". Swap A and B.
Same size · opposite direction · same type of force · act on different objects.
They act on different objects. Forces cancel only when they act on the same object.
The ball pulls Earth up with the same force.
No. Both act on the book and are different types. They're equal because of the First Law (a = 0).
The book pushes down on the table.
The road pushes the tire forward, and that is what drives the car.
The same force. The bug accelerates far more, because a = F/m.
Those two forces act on different objects. The horse pushes back on the ground and the ground pushes the horse forward.
That's an internal force. Only outside forces accelerate a system.
A sketch of one object showing every force acting on it as an arrow from the object.
Perpendicular to the surface, pushing away from it.
Along the rope, pulling away from the object.
Parallel to the surface, opposing sliding (or the tendency to slide).
Only weight (down) and normal (up). Nothing pushes it forward.
Only weight, down. Its net force is down even while it's rising.
N = 98.1 + 20 = 118 N. N isn't always mg.
T = mg/2 = 58.9 N.
Static: surfaces not sliding, f_s adjusts up to μ_s·N. Kinetic: surfaces sliding, f_k = μ_k·N (constant).
μ_s. That's why it's harder to start a box moving than to keep it moving.
60 N. Static friction matches the push; 88 N is only the maximum.
None. It's a ratio of two forces (f/N).
Static friction from the ground (the tire or foot isn't sliding).
Equal to kinetic friction: F = μ_k·N (F_net = 0).
μ_k = a/g = 1.2/9.81 ≈ 0.12. The mass cancels.
mg sinθ down the slope; mg cosθ into the slope.
N = mg cosθ (if no other perpendicular forces).
a = g sinθ. It doesn't depend on mass.
a = g(sinθ − μ_k cosθ)
The steepest angle at which a block can rest: tanθ = μ_s.
The size of the acceleration and the tension.
a = (m₂ − m₁)g/(m₁ + m₂)
T = 2m₁m₂g/(m₁ + m₂), between m₁g and m₂g.
a = (m₂ − μ_k m₁)g/(m₁ + m₂)
No, less, while m₂ accelerates down: T = m₂(g − a).
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