P2 Flashcards
How does friction work?
driving force = friction force ; steady speed
driving force > friction force ; accelerate
driving force < friction force ; decelerate
How can moving vehicles and falling objects reach a terminal velocity?
- driving force > friction force
- Resistance is directly proportional to the velocity of the object
- Acceleration is decreased until friction = driving force
How are cars designed to have less drag?
Streamlined.
What is Inertia?
The measure of how difficult it is change an object’s velocity.
What is Inertia dependent on?
The mass of the object. Larger the mass, larger the Inertia, harder it is to change the velocity of an object.
What is Newton’s Third Law?
Reaction forces are equal and opposite.
What is a vector?
It has size and direction.
What is a vector?
It has size and direction.
What is a vector?
It has size and direction.
What causes a change in Momentum?
Resultant force causes it to accelerate.
Acceleration is change in velocity over time,
so: force = mass x change in velocity / time
Mass x change in velocity = change in momentum
Calculate the momentum of a 220,000 kg aeroplane that is travelling at 250m/s.
220,000 x 250 = 550,000,00 kg m/s
Calculate the force a tennis racket has to apply to a 58g tennis ball to accelerate it from rest to 34m/s in 11.6 ms.
Convert units: 58g -> 0.058kg, 11.6ms -> 0.0116
Change in momentum: (0.058 x 34) - (0.058 x 0) = 1.972
Force: 1.972 / 0.0116 = 170N
What does the conservation of momentum mean?
Momentum Before = Momentum After
Ball A has a mass of 0.08kg while Ball B has a mass of 0.36kg. Ball A moves towards Ball B at 9m/s in the same direction which Ball B moves at 3m/s.
The two balls collide. Ball A stays stationary and Ball B moves away. Calculate the velocity of Ball B after the collision.
Total momentum before: momentum of ball A and B
= (0.08 x 9) + (0.36 x 3) = 1.8 ,, because before = after
Total momentum after = 1.8 / 0.36 = 5m/s
What are elastic collisions?
Total energy in the kinetic energy stores of objects are same before and after collisions. (Energy is conserved)
What is an inelastic collision?
Some of the energy in the kinetic energy stores are transferred to other stores. (e.g by heating or sounds)
A 2kg trolley travelling at 1.5m/s collides with a 3kg stationary trolley. They move away together at constant speed. Find the final velocity of the two trolleys.
(2 x 1.5) + (3 x 0) = 3kg m/s
3/ (2+3) = 0.6 m/s
How do you calculate gravity force?
Gravity force (N) = mass (kg) x gravitational field strength, g (N/kg) W = m x g
Calculate the weight of a 70 kg mass on Earth?
70 x 10 = 700N
What is the weight of a 2kg chicken both on the Earth
g = 10N/ kg),, and on the moon (g= 1.6N/ kg
Earth = 2 x 10 = 20 N Moon = 2 x 1.6 = 3.2 N
How do you calculate gravitational potential energy?
PE = mhg potential energy(J) = mass(kg) x height (m) x gravitational field strength,g (N/kg)
When would an object have energy in its gravitational potential energy store?
When an object is at any height above the Earth’s surface.
Under what circumstance does an object have energy in its kinetic energy store?
When an object is moving.
What is the formula to calculating kinetic energy?
KE = 0.5mv^2 Kinetic energy (J) = 0.5 x mass (kg) x speed (m/s)^2
What is the equivalent to the amount of work done?
Energy transferred.
What is the formula to calculate the amount of work done?
W = F x d
What is the definition of power?
The rate of doing work.
What is the formula of power?
P = W/ t
Power (W) / work done (J) / time (s)
A book sliding across a table has 1.25 J of energy in its kinetic energy store. Friction from the table provides a constant force of 5N. Calculate the distance travelled by the book before it stops.
W = F x d
1.25 / 5 = 0.25 m
What is an elastic deformation?
When an object returns to its original shape after the forces are removed.
What is the formula for extension?
F = xk
Force exerted by a spring (N) = extension (m) x spring constant (N/m)
What happens after the object reaches the limit of proportionality and continues to increase?
The object starts to deform plastically and won’t spring back to its original shape.
A spring has a natural length of 0.32 m. When a force of 10N is applied to the spring, it’s length becomes 0.40 m. Calculate the spring constant if the spring.
10/ (0.40 - 0.32) = 125 N/m
What is the equation for the energy stored in an object’s elastic potential energy?
E = 0.5kx^2
Energy transferred in stretching (J) = 0.5 x spring constant (N/m) x extension^2
What is a moment?
The turning effect of a force.
What is the formula for moment of a force?
M = F x d
Moment of a force (Nm) = force (N) x distance (m)
How could you get the maximum moment?
Explain how it works.
Push at right angles (90 degrees). Pushing at any other angle means a smaller moment because the perpendicular distance between the line of actions and the pivot is smaller.
What do levers act as?
Force multipliers.
How do levers work?
They increase the distance from the pivot that the force is applied, so less input force is needed to get the same moment.
What is the formula to calculate input force?
Input force/ output force =
distance of output force from pivot/ distance of input force from pivot
What are the use of gears?
They fit together to transfer turning effects.
How could you calculate pressure?
P = F/A
Pressure (Pa) = force normal to a surface (N)/ area of that surface (m^2)
Why are liquids incompressible?
Force applied to the water is transmitted to other parts.
How do hydraulic systems work?
They are used as force multipliers- use a smaller force to create a bigger force with pistons.
