Download Force due to gravity: A field force (a vector quantity) that always is

Weight and the Normal Force
Lyzinski Physics
CRHS-South
Force due to gravity and the normal force
Force due to gravity: A field force (a vector quantity) that always is directed towards the
center of the earth.
Weight: The magnitude of the Force due to gravity
Fg  W  mg
gearth = 9.8 m/s2
gmoon = 1.6 m/s2
Mass vs. weight
Mass: A measure of an objects inertia
(its tendency to resist a change in
its motion). Inherent property of
an object.
Weight: Decreases as you move away from the
center of the earth. NOT an inherent
property of an object
Normal Force: A “reactionary” contact force exerted on one object by another in a direction
perpendicular () to the surface of contact.
1) Compare the weight of a 60 kg person on the earth with the weight of the same person on
the moon. Then, describe a quick (but very costly) way for dieters
at NASA to lose weight.
2) A bully is pushing a boy against a locker as shown with a force of
500N. The angle between his arms and the ground is 40o. Draw a free-body diagram of
the boy and then find the normal force between the boy and the wall.
3) A 200 kg block, on a VERY rough surface, is being pulled/pushed by two people, one on
each end. The first person is pulling with a force of 20 N at an angle of 40o above the
ground, while the other person is pushing with a 30 N force at an angle of 50o above the
ground. Find the normal force on the block.
4) In terms of the normal force, explain what is necessary for an object to physically “lift”
off of the ground.
5) A 45 kg box is sitting AT REST on the ground. Find…
Newton’s 2nd Law with
Weight and the Normal Force
a) the weight (force due to
gravity) of the block.
b) the normal force on the
block.
c) the force of friction acting
on the block.
6) A 30 kg box is sitting at rest on a frictionless surface. A 100 N force PUSHES down on
the the block at an angle of 30o below the horizontal. Find….
a) the normal force acting on
the block.
b) the acceleration of the
block.
c) the block’s velocity after
being pulled for 5 seconds.
7) A 50 kg block sits on the ground. A force PULLS on the block at an angle of 45o above
the horizontal. What is….
a) the weight of the block.
b) the normal force on the
block just as the block lifts
off the ground.
c) the pulling force necessary
to just barely lift the block
off the ground.
8) A 5g bullet is fired into a large oak tree. The bullet leaves the gun at a speed of 300
m/s. If the block of wood slows the bullet down in 5ms (5 milliseconds, or 5
thousandths of a second), then …
a) what is the “retarding”
force on the bullet.
b) how far into the tree
does the bullet
penetrate?
9) A 2000 kg car is driving down the road at a speed of 30 m/s. It suddenly slams on its
brakes and stops in 40 m. Assuming a constant acceleration, find:
a) the force applied by
the brakes on the car.
b) the time required to
stop the car.
10) A block of wood is sliding across a frictionless surface at a speed of 10 m/s.
Suddenly, it hits a shaggy carpet, which supplies a constant frictional force of 50N that
stops the block in 3 seconds. Find the mass of the block.
11) Three forces are applied to a 500 kg box that is sitting at rest on a surface. The first
force is 200 Newtons applied downward and to the left (a push) at an angle of 30 o
below the horizontal. The 2nd force of 100 Newtons is applied upward and to the right
at an angle of 40 o above the horizontal (a pull). The third and final force is 150
Newtons downward and to the right (another push) at an angle of 80o below the
horizontal. Find…
a)
b)
c)
d)
the weight of the block.
the normal force acting on the block.
the acceleration of the block.
the distance that the block has covered when it reaches a velocity of 15 m/s.