physics - unit 3 - forces and newton's laws

if you pound a feather with a hammer, which one feels a greater force?

the size of the force is always exactly the same on both of them (newton’s 3rd law - for every action, there is an equal and opposite reaction)

a satellite is in orbit around the earth. which one feels the greater force due to their mutual gravitational attraction?

earth and the satellite feel exactly the same force (newton’s 3rd law - for every action, there is an equal and opposite reaction)

a crate is sliding down an inclined ramp at a constant speed of 0.55 m/s. the net force acting on this crate must point

in no direction because it’s zero (newton’s 2nd law - constant speed means acceleration is zero, and zero acceleration means the net force is zero)

a bucket is being lowered by a very light rope with a constant downward velocity. the tension in the rope must be

equal to the weight of the bucket (newton’s 2nd law - constant velocity means acceleration is zero, and zero acceleration means the net force is zero, so the forces (tensions and weight) must be equal in magnitude

an object of mass m rests on a flat table. the earth pulls on this object with a force of magnitude mg. what is the reaction force to this pull?

the object pulling upward on the earth with force mg (newton’s 3rd law - earth pulls down on the object with force mg, and the reaction force is the object pulling up on the earth with an equal force)

a person is using a rope to lower a 5.0-N bucket into a well with a constant speed of 2.0 m/s. what is the magnitude of the force exerted by the rope on the bucket?

5.0 N (newton’s 2nd law - since bucket moves at constant speed, acceleration is zero, meaning net force is zero, so the upward force from the rope is equal to the downward gravitational force which is 5.0 N)

the acceleration due to gravity is lower on the moon than on earth. which one of the following statements is true about the mass and weight of an astronaut on the moon's surface, compared to earth?

mass is the same, weight is less (mass stays same everywhere, acceleration due to gravity (g) changes depending on location)

a 200-N sled of slides down a frictionless hillside that rises at 37° above the horizontal. what is the magnitude of the force that the surface of the hill exerts on the sled?

160 N (pressing against surface = normal force is always perpendicular to the surface, so cosine is used—Fn=200(Fg is F=mg, but is already converted to N) x cos(37))

a block of mass m sits at rest on a rough inclined ramp that makes an angle θ with the horizontal. what must be true about force of static friction f on the block?

f = mgsin0 (parallel, therefore use sin so f = mgsin0)