Define it!
The tendency of an object to resist changes in its velocity.
Inertia
(#1a SG)
Newton’s 1st Law of Motion is also often called the law of inertia.
True
[An object in motion (or at rest) will tend to stay in motion (or at rest) until it is acted upon by an outside force.]
(p. 230)
An object in motion (or at rest) will tend to stay in motion (or at rest) until it is acted upon by an outside force. This is Newton’s ___ Law of Motion.
Newton’s 1st Law of Motion
(#2a SG)
An astronaut throws a ball in space with an initial velocity of 3m/sec west. In space, there is almost no air, so assume 0 friction. This is the ball’s velocity in a year.
3m/sec west
(no forces – not even friction – are operating on the ball)
(#3 SG)
You run north with a beanbag in your hands. The moment you pass a tree, you drop the beanbag. The beanbag lands: 1) next to 2) north of, or 3) south of the tree.
North of the tree
(Newton’s 1st Law: its velocity keeps going in the same direction)
(#4 SG)
This man is credited with many, many discoveries, including three laws of motion, a theory describing gravity, and calculus.
Sir Isaac Newton
(he spent as much time studying the Bible as he did studying science)
(p. 229)
A force that opposes motion, resulting from the contact of two surfaces.
Friction
(#1b SG)
Newton helped explain some of Aristotle’s errors about the physical nature of motion.
True
(p. 229)
When an object is acted on by one or more outside forces, the total force is equal to the mass of the object times the resulting acceleration. This is Newton’s ___ Law of Motion.
Newton’s 2nd Law of Motion
(#2b SG)
Wet roads can be slick, because the water fills in the grooves in the road, reducing this force between a car’s tires and the road.
Friction
(the water reduces how close the tire molecules can get to the road molecules)
(#7 SG)
You run past a friend standing at a tree with a beanbag in his hands. You barely tap the beanbag, causing it to fall. The beanbag lands: next to, north of, or south of the tree .
Next to the tree
(it has no initial velocity, so it simply falls to the ground)
(#5 SG)
You are on a horse, going at fast gallop. The horse gets spooked and stops quickly, and you fall off. Do you fall forward, backward, to the left, or to the right?
Forward
(you and the horse start at the same velocity, but the force of his feet stopping doesn’t stop your velocity.)
(OYO 10.1)
Friction that opposes motion once the motion has already started.
Kinetic friction
(#1c SG)
Aristotle incorrectly taught that objects “prefer” to be at rest. This was likely based on what he observed about objects in motion.
True
(The concept of an opposing force to an object’s motion, like friction, is not as observable in everyday experience.(p. 230-321)
For every action, there is an equal and opposite reaction. This is Newton’s ___ Law of Motion.
Newton’s 3rd Law of Motion
(#2c SG)
To slide a refrigerator across the floor, a man must exert an enormous amount of force. Once he does, keeping it moving does not take as much force because of this.
Static frictional force is greater than kinetic frictional force
(#8 SG)
A man is driving down the road with boxes in the back of his car when he hits the brakes to avoid a collision. The boxes slam into 1) the front or 2) back of the car.
The front of the car
(the boxes have the same velocity as the car, so they move forward relative to the car after the man hits the brakes)
(#6 SG)
A baseball player catches a fast-moving ball. You know the player exerted a force on the ball because of (1) this. The equal and opposite force required by Newton’s 3rd Law of Motion is (2) this. The evidence the player has of the force is (3) this.
(1) The ball’s velocity changed from fast-moving to stopped, showing acceleration, which means a force was exerted on the ball (2) the force exerted by the ball on the player, and (3) the player feels pain when he catches the ball.
(#16 SG)
Friction that opposes the initiation of motion.
Static friction
(#1d SG)
You try sliding different objects down an angled board to test frictional force. You find the frictional force is weaker for an eraser than for an ice cube.
False
(The strength of the frictional force between two objects depends on the physical characteristics of each one. An ice cube slides easier.)
(p. 238)
A child pushes hard against a large doghouse, but it doesn’t budge. The equal and opposite force that Newton’s 3rd Law of Motion says must happen in response is exerted by (1) this on (2) this.
(1) the doghouse on (2) the child
(#15 SG)
Friction exists because at the level of molecules and atoms, there’s no such surface as this.
A smooth surface
(the roughness at that level affects how closely the molecules can get to one another – the closer they can get, the stronger the friction)
(p. 239)
A girl pushes her toy with a constant velocity to the east. The static friction between the toy and the floor is 15 Newtons, while the kinetic friction is 10 Newtons. The child is exerting ___ Newtons of force to the ____.
10 Newtons of force to the east
[at constant velocity, acceleration is 0, and so is the total force since
Total force = (mass) x (acceleration of 0).
The girl exerts enough force to counteract kinetic friction, but no more. Since the toy is already moving, it is all kinetic.]
(#9 SG)
To learn to ice skate, a child is pushed by his father with an acceleration of 2.0 m/sec2 north. The child’s mass is 20 kg, so this is the force of the father’s push. (Assume 0 friction on the ice.)
40 Newtons north
[Since we are ignoring friction, the only force is the father’s, to the north.
Total force = (20 kg mass) x (2.0 m/sec2 acceleration) = 40 Newtons north]
(#10 SG)
The standard unit of force, defined as:
kg x m/sec2
is called this.
Newton
(named after Sir Isaac Newton, one of the most important scientists in the history of physics)
(p. 241)
Static friction can exert a force of up to 700 Newtons on a 500 kg box of bricks. The kinetic frictional force is 220 Newtons. A worker needs to use 700 Newtons of force to get the box moving and 270 Newtons of force to accelerate the box at 0.1 m/sec2 to the south.
True
(The static friction needed to get the box moving was provided. Once moving, Total force = (500 kg mass) x (0.1 m/sec2 acceleration) = 50 Newtons; This is the worker’s force (#? Newtons) – kinetic force (220 Newtons) = 50 Newtons total force; The worker’s force, then, must be = 220 + 50, or 270 Newtons south.]
(#12 SG)
A man leans up against a wall with a force of 20 Newtons to the east. The force exerted by the wall on the man is this.
20 Newtons west
(equal and opposite of the man’s force… according to Newton’s 3rd Law of Motion)
(#17 SG)
To get a 15 kg object moving west, a force of more than 25 Newtons is needed. Once it is moving, a force of only 20 Newtons accelerates the object 0.1 m/sec2 to the west. The force of 1) static friction is ___ and 2) kinetic friction is ___ .
Static friction = 25 Newtons east (provided)
Kinetic friction = 18.5 Newtons east
[once moving, Total force = (15 kg mass) x (0.1 m/sec2 acceleration) = 1.5 Newtons; This is the combination of the applied force (20* Newtons) and the kinetic force since it is moving – and kinetic is opposite of the applied force (20* Newtons – 1.5 Newtons) = 18.5 Newtons east.]
(#11 SG)
(*The answer key uses 25 here, but I believe it is an error.)
To move a 710 kg rock, 2 men exert force – one at 156 Newtons east and the other at 220 Newtons east. The rock accelerates at 0.20 m/sec2 east. Between the rock and the ground there are ____ Newtons of kinetic frictional force.
Kinetic frictional force = 234 Newtons west
[Total force = (710 kg mass) x (0.20 m/sec2 acceleration) = 142 Newtons; The men exert 376 Newtons east. Friction is subtracted and in the opposite (west) direction:
376 Newtons applied force east – #? Newtons kinetic frictional force west = 142 Newtons total force;
Kinetic frictional force = 376 Newtons applied force —142 Newtons total force = 234 Newtons west.
(#14 SG)
To move a rock out of the way, a gardener exerts slightly more than 100 Newtons of force. To keep it moving at a constant velocity eastward, he exerts just 45 Newtons of force east. Between the rock and the ground there are ____ Newtons of static frictional force and ___ Newtons of kinetic frictional force.
Static frictional force = 100 Newtons (provided)
Kinetic frictional force = 45 Newtons
[once moving, he kept it at a constant velocity eastward, so acceleration is 0 and Total force = (mass) x (acceleration of 0) = 0; The gardener exerts enough force to counteract kinetic friction, but no more. The kinetic friction, then, must be 45 Newtons.
(#13 SG)