P2 Forces & Motion Flashcards
What is Newton’s first law of motion?
An object will continue move with constant velocity (speed and direction) unless a resultant force acts on it
What is Newton’s second law of motion?
Force = mass x acceleration F = ma
What is Newton’s third law of motion?
Forces always occur in pairs that are equal and opposite
What happens if the forces acting on an object are balanced?
It will not accelerate - its speed and direction remain constant
What is the equation fro speed (or velocity)?
Speed = distance / time
What is the equation for acceleration?
a = (v - u) / t
v - final velocity
u - initial velocity
What does a straight line on a distance-time graph tell us?
A straight line means constant speed.
The gradient is the size of the speed
What does a straight line on a velocity-time graph tell us?
A straight line means constant acceleration
The gradient is the size of the acceleration
It can be positive (acceleration) or negative (deceleration)
How can we calculate the distance travelled from a velocity-time graph?
The distance travelled is the area under the graph
What is the unit for velocity?
metres per second
m/s
What is the unit for acceleration?
metres per second per second
m/s2
What can be said about the forces acting on a car moving at constant speed?
For constant speed, the forces are balanced. This means the driving force is exactly balanced by the resistive forces (air resistance and friction)
What is the equation for stopping distance?
Stopping distance = thinking distance + braking distance
What factors affect think ing distance?
The speed of the car and the driver’s reaction time. The reaction time is affected by:
- tiredness
- alcohol consumption
- drug consumption
- distractions
What factors affect braking distance?
Braking distance is affected by the mass and speed of the car and also the quality of the brakes and the road conditions. Any conditions that reduce friction will increase the braking distance.
What is the equation for weight?
Weight = mass x gravitational field strength
(N) (kg) (N/kg)
W = mg
What happens to the air resistance acting on an object when its speed increases?
As speed increases, air resistance increases
What is terminal velocity?
When a falling object (parachutist) is going so fast that the air resistance acting exactly balances the weight. There is no overall force and so a constant velocity - terminal velocity.
Similar to the maximum speed of a vehicle - air resistance matches the maximum driving force
What is Hooke’s Law?
Force = spring constant x extension
(N) (N/m) (m)
F = ke
Define work done
Work done is the amount of energy transferred
Work done = force x distance moved in direction of the force
(J) (N) (m)
WD = F x d
Define power
Power is the rate of doing work (rate of energy transfer)
Power = energy / time
(W) (J)/(s)
P = E /t
What is gravitational potential energy?
Gravitational potential energy is the work done in moving an object to a greater height
Ep = weight x height
Ep = mgh
What is the equation for kinetic energy?
kinetic energy = ½ x mass x (velocity)2
Ek = ½mv2
How can we calculate braking distance?
Braking distance depends on mass, velocity and braking force:
The brakes do work, they remove kinetic energy so that:
Loss in Ek = work done by the brakes
Ek = FbD - where Fb is braking force and D is braking distance, so
½mv2 = FbD
