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
a net force of 0
What is a balanced and stationary cars net force
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
What is a straight vertical line on a speed/time graph
Constant Speed
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
The definition of work
What is "work is done on an object when energy is transferred to that object"
The rearranged kinetic energy equation to find velocity
What is (square root) 2xEk/m
A skydiver jumps out of the plane. Describe her motion AND the vertical forces acting on her just BEFORE the parachute opens.
The forces are balanced, drag has increased to the same as gravity. She is at terminal velocity, which is constant speed.
What is a curved line going downwards on a distance time graph?
Deceleration
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.
Describe the motion AND the vertical forces acting on her when her parachute opens.
She decelerates rapidly because the forces become hugely unbalanced and the drag/air resistance force becomes huge compared to gravity.
How do you work out the distance travelled using a speed time graph?
Use...
Triangle = 1/2bxh
Square = bxh
Add all sections together
A rally car has a mass of 1500kg. The engine is producing a thrust of 7500N. (ignoring the effect of friction) explain what would happen if the thrust force of the engine was doubled.
The car would accelerate as the weight force has not changed.
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