Lesson 3: Chapters 4&5 : Force and Newton's Laws of Motion

Compared to the force needed to start sliding a crate across a rough level floor, the force needed to keep it sliding once it is moving is

less

Does using a heavier mass increase, decrease, or maintain the same coefficient of static friction?

maintains the same

Using the reference frame defined in the lesson (positive quantities to the right are positive), an object moving to the left and slowing down has

negative displacement, negative velocity, and positive acceleration.

tension

-the pull exerted by a rope or cable attached to an object. -direction of force: Away from the object (in the same direction as the rope or cable. -symbol: FT

A net force of 8 N on an object will cause a 4 kg object to accelerate

2 m/s2

If the mass of an object does not change, a constant net force on the object produces constant

acceleration

A falling object is said to reach terminal speed when

its air resistance equals the force of gravity on it.

friction

-A contact force that opposes motion. -symbol: Ff

Newton's first law of motion

-The law of inertia -Without outside forces (such as gravity or friction), an object at rest will remain at rest, while an object in motion will never stop or deviate from its course

net force

-The sum of all forces acting on an object -F=ma -net force is always in the same direction as the acceleration -When the net force on the object is zero, the object is said to be in equilibrium. -A constant velocity equals zero acceleration, meaning no net force on the object exists

A 10 N box rests on a flat lab station. A student pushes it horizontally with increasing force until it moves. The box just moves when 1.2 N of force is applied. The coefficient of static friction is closest to _______________.

0.12 (static friction can be equal to or less than the force of the object being pushed. once it exceeds that force, the object will move) 1.2N=0.12x10N or static friction (1.2N)=coefficient of static friction (0.12) times the magnitude of the normal force (10N)

What is the net force on an object that has a mass of 6 kg and accelerates at .25 m/s2?

1.5 N

The diagram below represents a block sliding across a flat level table at a constant speed. All forces are shown except the frictional force. The force straight down is 2.5 N, the force perpendicular to the surface is 2.5 N, and the force from a push is 1.7 N. What is the magnitude of the frictional force on the block?

1.7N

You are in the back of a friend's pickup truck when it stalls on a hill. You jump out, get behind the truck, and push with all your might (300 N) in a vain attempt to keep the truck from accelerating down the hill. The force the truck exerts on you is

300N

A force of 26 N is needed to overcome a frictional force of 5 N to accelerate a 3 kg mass across a floor. What is the acceleration of the mass?

7 m/s2 subtract the frictional force from total force to get the net force(21N) then plug that into F=ma (21=3a or a=7m/s2)

static equilibrium

A condition where there are no net external forces acting upon an object and it is at rest

Applied Force

A push or pull on the object. -symbol: FA

Newton's Third Law of Motion

For every action there is an equal and opposite reaction

weight

The product of the mass of the object and the acceleration due to gravity (mg). w=mg symbol: Fg

A student pushes a desk horizontally with constant non-zero velocity along a rough surface with significant friction. Which of the following is true?

The student applies a force equal to the frictional force.

free fall acceleration

When the net external force on an object is its weight-the only force acting on the object is the force of gravity about -10m/sec squared

When a rifle is fired, the rifle exerts a force on the bullet, and the bullet

exerts a force of equal magnitude on the rifle.

normal force

force applied to an object by another object that acts perpendicular to the surface of the object symbol: FN

Action at a distance force example

force due to gravity

Drag

force opposing thrust

dynamic equilibrium

state where an object is moving at a constant velocity: net force is still zero since there is no acceleration

Newton's Second Law of Motion

the acceleration of an object is equal to the net force acting on the object divided by the object's mass. Acceleration and net force are directly proportional, meaning if the net force increases, then the acceleration will also increase and vice versa.

A bug splatters on the windshield of a moving automobile. Which object, the automobile or the bug, experiences the greater force of impact?

the forces are the same