Week 4 Flashcards
Define force
Any action which tends to cause a change in motion or shape of a body
Properties of a force (4)
Line of action (path of force)
Magnitude (N)
Direction (degrees or radius)
Point of application
How do you calculate applied force
Applied force = Force of vertical component + force of horizontal component
Newtons Laws of motion (3)
1st law - Law of Inertia
2nd law - Law of Acceleration
3rd law - Law of Reaction
Newtons 1st Law
Law of inertia - An object will remain at rest or in motion unless acted upon by a net external force
Newtons 2nd law
Law of acceleration - The change in motion of an object is proportional to the
force impressed; and is made in the direction of the
straight line in which it is impressed
F = ma Force = mass x accelaration
Newtons 3rd law
Law of reaction - For every action there is an equal and opposite reaction
If object A exerts a force on object B, then object B exerts the same force on the object A but in the opposite direction
Name 5 types of forces
Weight force Muscle force Ground reaction force (GRF) Joint reaction force (JRF) Friction
Difference between weight and mass
Mass is the amount of matter an object has calculated in kg
Weight is the force experienced by an object due to gravity measured in newtons
Calculation for weight
Mass x acceleration due to gravity (always 9.81m/s2) on earth
Weight of a 2kg dumbell
2kg x 9.81 m/s2 = 19.62 N
How much does someone weigh if their mass is 130kg?
130kg x 9.81m/s2
1275.3 N
Explain friction forces
Resistance to movement between two surfaces in contact measured in Newtons (N)
Explain what happens to velocity when friction is applied
Displacement will slowly decrease over time as friction will be acting against it, therefore velocity will decrease over time
Equation for friction
Ffriction = µ*FN
Ffriction – Frictional Force (Newtons) - force acting parallel to the
surface which opposes motion
µ - Friction Coefficient (dimensionless) - varies with surface
FN – Normal Force (Newtons) - force (component of GRF) acting
perpendicular to the surface
How can the coefficient of friction vary (μ)
The hardness and roughness of a surface
μ = 0 = perfectly smooth/frictionless
What the difference be with a tyre on dry and wet road
Tyre on Dry Road µ ~ 1
Tyre on Wet Road µ = 0.2
If we push rectangular object on a surface with 0.5 coefficient of friction and it moves 1 metre, how far would that same object be pushed if it was flipped on a side with a smaller surface area touching the ground?
1 metre. Friction is independent of surface area.
If someone was sledding on a 200kg sleigh and the person on the sled was 75kg, what would the Normal Force be exerting against the sled?
275kg x 9.81m/s2
= 2697.75 N
In other words, it is exerting the exact same force back
What is static friction
Frictional force required to overcome inertia
What is dynamic friction
Kinetic or sliding friction
newtons 1st law
Why do gymnasts apply chalk (magnesium carbonate) to their hands?
To reduce the friction from the bar to the hands
What GRF does walking have on the body, measured in BW?
1-1.5 BW
What GRF does running have on the body, measured in BW?
2-3 BW
What GRF does jumping have on the body, measured in BW?
2-5+ BW
Does landing on a slope skiing decrease the GRF and why?
Yes. The GRF is decreased as as the bodyweight is not landing flat, therefore the GRF is decreased depending on the angle of the slope. Steeper the angle, the lower the GRF.
How much force does Doug have to push with to get the fridge to move?
Dougs force must be greated than the frictional force of the fridge. (Ffriction = µ*FN)
Fridge weighs 120kg. The µ is 0.5. How much force does he need to apply to move the fridge?
0.5 x (120kg x 9.81m/s2) = 588.6 N
Doug has to apply a force of <588.6 N to move the fridge
Which brick has greater frictional force?
Brick A lying on the length side with a force of 29 N
OR
Brick B lying on the top side with a force of 29 N
Both will have the same amount of Ff as frictional force is independent of surface area
Which brick has greater frictional force?
Brick C lying on the length side weighing 29 N
OR
Brick D lying on the length side weighing 87 N
Brick D will have greater frictional force as the weight of brick D is larger than Brick C as Ff = µ*FN
