Random Mechanics
One of Roberts shoes has an area of 0.003m2 and he weighs 900N.
Calculate the amount of pressure he exerts on the ground if he stands on both feet.
P= F/A
P= 900/0.006 = 150,000Nm-2
The equation for speed
What is v=d/t
The equation for Force
What is F=ma
The equation for work and its units
What is W=Fxd and Joules
The equation for potential energy and its units
What is Ep = mgh and Joules
A lawnmower with a mass of 36kg is being pushed forward with a horizontal force of 20N. The grass provides a friction force of 2N. Calculate the acceleration of the lawnmower
a = F/m
a = 18N/36kg = 0.5ms-2
The equation for acceleration
What is a=v/t
This is when all net forces equal 0N
The equation for Power and its units
What is P = W/t and Watts
The equation for Kinetic energy
What is Ek=1/2mv2
A child plays with a remote control car on concrete. The mass of the car is 400 grams. It starts from rest and travels a distance of 6m in 3 seconds. Calculate the average speed of the car over the 3s.
v = d/t
v = 6/3 = 2ms-1
This condition is met when Net Force and Net Torque = 0
Equilibrium
What is the difference between mass and weight and what are their units
Mass is the amount of matter an object has. It doesn't change on different planets. Its units are kg
Weight is the amount of matter x gravity. It changes depending on the planet you are on. Its units are N
What is Work?
Though energy is never destroyed, it can often be lost as these types of energy.
What is heat and sound?
Describe the motion and forces on a projectile after launching
Constant acceleration downwards, constant horizontal velocity. Vertical velocity starts upwards, decreases through 0 at the apex, then increases downwards.
In collisions, momentum is conserved assuming this.
What is No External Forces.
Draw on the whiteboard and label the forces on a car accelerating to the right
A car with thrust force (larger arrow) to the right, friction/drag (smaller arrow) to the left, gravity downwards and support upwards (same size as gravity arrow)
F = W/d
F= 250,000/20 = 12500N
Describe the conservation of energy of a woman pushing her child on a swing once she stops pushing.
A woman pushing her child on a swing, stops pushing. So the Ep and therefore Ek will gradually decrease and stop. The total energy transferred will be the same amount as the total energy she had at the start.
Calculate the flight time of a ball launched vertically at 19.6 ms-1
1/2 t = (vi/a)
t = 4 s
This can be increased by reducing the time of collision.
What is Force
This law describes the fact that two objects are subject to equal and opposite forces when touching.
A woman has a mass of 60kg. She climbs a 3m high ladder in 6s. Calculate how much power she developed climbing the ladder.
P= W/t
W=Fxd
W=(60x10) x 3 = 1800J
P=1800 / 6 = 300 Watts
An NZSS rally car has a mass of 1200kg. It travels for 5 seconds at 20ms-1.
Calculate the Kinetic energy of the car when it is travelling 20ms-1
Ek = 1/2mv2
Ek = 0.5 x 1200 x 202
Ek = 240,000J