Download Forces: Vectors and Free Body Diagrams

Dr Pusey
Learning Outcomes
Recall the SI units for force, weight, mass and gravity
Define the net or resultant force
Draw and calculate the net force by using a free body
diagram in one and two dimensions
Describe the motion of an object based on the net
force.
 A force is a push or a pull resulting
from an interaction between two
objects
 Force is a vector, requires a magnitude
and a direction
 Measured in Newtons
Force
Description
Friction
A force opposing the movement of an object
Tension
A force transmitted through string, rope, wire or cable
Normal
A support force
Air resistance
A force acting on an object as it travels through the air
(a type of friction)
Applied
A directional force (such as a push or pull) applied to
an object
Spring
The force exerted by a stretched or compressed
spring on any object attached to it
Weight
The downwards force exerted on the ground by
objects due to the pull of gravity
Force
Description
Gravity
The attractive force of gravity that
exists between two objects with mass.
It causes the weight force of an object.
The attractive or repulsive force
between two charged objects.
The attractive or repulsive force
between two magnets.
Electrical
Magnetic
Dr Pusey is walking his dog
Wally. When Wally wants to say
hello to another dog he pulls on
the lead with a force of 67.0 N at
an angle of 30.0° above the
horizontal.
Determine the horizontal and
vertical components of the force
exerted on Dr Pusey by Wally.
FH = F.cosθ
FH = 67.cos30
∴ FH = 58.0 N
Forwards
FV = F.sinθ
FV = 67.sin30
∴ FV = 33.5 N
Downwards
 When all the forces acting on an object
are balanced the object is said to be in
equilibrium.
 This means:
 an object that is not moving stays still
 an object that is moving continues to
move at the same speed and in the same
direction
 When the forces acting on an object are
not equal in size they are unbalanced.
 This means:
 an object that is not moving starts to move
 an object that is moving changes speed or
direction
 The overall force is called the net force
or the resultant force
Example 1
A dinosaur is chasing after some delicious prey. It
experiences an applied force from its feet of 500 N
and a friction force of 150 N, calculate the net force.
Example 1
A dinosaur is chasing after some delicious prey. It
experiences an applied force from its feet of 500 N
and a friction force of 150 N, calculate the net force.
150 N
500 N
Fnet = 500 N – 150 N = 350N forward
Example 2
 Calculate the net force for these situations:
Example 2
 Calculate the net force for these situations:
 When you need to analyse situations involving forces
it’s often useful to draw a force diagram
 Force diagrams (or free-body diagrams) are used to
show the magnitude and direction of all forces
acting upon an object in a given situation.
Drawing free body diagrams
1.
2.
3.
4.
5.
6.
Read the problem carefully
Draw the object (a box/dot will do)
Identify if the object is in equilibrium or not
Determine all the forces acting on the object
Determine the direction in which each force is acting
Draw and label each force as an arrow
Example 1
Draw a force diagram for a car accelerating to the
right/forwards.
Example 1
Draw a force diagram for a car accelerating to the
right/forwards.
Gravity
Example 1
Draw a force diagram for a car accelerating to the
right/forwards.
Normal Force (Due to the
reaction of the road holding
the car up)
Gravity
Example 1
Draw a force diagram for a car accelerating to the
right/forwards.
Normal Force (Due to the
reaction of the road holding
the car up)
Applied force (car engine)
Gravity
Example 1
Draw a force diagram for a car accelerating to the
right/forwards.
Normal Force (Due to the
reaction of the road holding
the car up)
Friction (Opposes motion). Since
the car is accelerating, this arrow
must be smaller than the applied
force (unbalanced)
Applied force (car engine)
Gravity
Your Turn
Draw a force diagram for a sky diver who has reached
terminal velocity (constant speed, no acceleration). He is
also being pushed by his wing suit to the right at a
constant speed
Draw a force diagram for a sky diver who has reached
terminal velocity (constant speed, no acceleration). He is also
being pushed by his wing suit to the right at a constant speed
Friction (Air resistance)
Friction (Drag)
Applied force (Thrust)
Gravity
Since he is not accelerating in any direction,
the addition of all these vectors should be ZERO
How did you go?
 Recall the SI units for force, weight, mass and gravity
 Define the net or resultant force
 Draw and calculate the net force by using a free body
diagram in one and two dimensions
 Describe the motion of an object based on the net
force.