P5 - Forces

Contact force

Physical forces that occur only when two objects are directly touching

Non-contact force

Physical forces that act between objects without direct physical contact

Scalar

Only has magnitude

Vector

Has magnitude and direction

How can a resultant vestor be found?

Adding them (this value can be negative), if forces at right angle Pythagoras can be used

No resultant force =

No acceleration

Examples of scalar quantities

Distance, speed, mass, energy, temp

Examples of vector quantities

Displacement, velocity, acceleration, force, momentum

Weight

The force that gravity pulls an object down with

Weight =

Weight = mass x gravitational field strength

Gravitational Field Strength of the Earth

9.8 N/kg

Gravitational Field Strength of the moon

1.6 N/kg

Every 1kg has a weight of?

10N

If you hold an object or lift at a constant speed, you must be pushing up with how much force?

A force that is the same size as its weight

Work done

Energy transferred by a force

Work done =

Work done = force x distance

Any object that deforms elastically (returns to its original shape), obeys what?

Hooke’s law - force and extension are directly proportional

Force =

Force = spring constant x extension

Spring Practical

1. Add masses onto a spring to change force (100g = 1N)

2. Measure extension with ruler at each weight, plot F against e

3. Gradient of graph = spring constant

4. Avoid systematic error by lining 0cm with bottom of spring

5. Avoid parallax error by getting on eye level and measuring

6. Straight line through origin on graph = directly proportional relationship (obeys Hooke’s law)

Speed or velocity =

Speed or velocity = distance or displacement / time

The gradient of a distance/displacement time graph gives you the what?

Speed or velocity

1?, 2?, 3?

1 - slow
2 - stationary
3 - fast

Acceleration =

Acceleration = change in velocity / time

1?, 2?, 3?

1 - accelerating
2 - constant speed
3 - decelerating

Constant acceleration down

The area beneath a speed distance graph gives what?

The displacement

S U V A T

S - displacement (m)
U - initial velocity (m/s)
V - final velocity (m/s)
A - acceleration (m/s² )
T - time (s)

v =

v = u + at (similar to acceleration equation)

s =

s = ((u+v) /2) x t (essentially means v = d/t)

s = (also)

s = ut + ½ at² (usually u=0)

v² - u² =

v² - u² = 2as (this is the most common)

If starting at rest, u=what?

0 m/s e.g something dropped

If stopping, v=what?

0 m/s e.g car stopping

If only force involved is gravity, a=what?

9.8 m/s

Newtons 1st Law

If no resultant force acts on an object, its motion will be constant (velocity will not change)

Terminal Velocity

True when due to the drag force balancing the other force

Inertia

The tendency for an objects motion to stay constant if no force

Newtons 2nd law

F = ma, Force = mass x acceleration

F=ma practical

1. Accelerate trolley on track with slotted mass on string over pulley

2. Use light/photogates to measure acceleration

3. Change force by removing masses and placing on trolley (to keep the total mass the same)

4. Plot F against a, gradient = total mass

Newtons 3rd Law

For every action (force), there is an equal and opposite reaction (force) (this is not balanced forces)

Stopping distance =

Stopping distance = thinking distance + braking distance

What is thinking distance affected by?

Speed
Distractions
Alcohol
Drugs
Tiredness

What is braking distance affected by?

Speed
Conditions of brakes, tyres, road
Weather conditions

Momentum =

Momentum = mass x velocity

Momentum, scalar or vector?

Vector, it’s negative if v is in the negative direction

In any collision, total momentum is always what?

Conserved, but total kinetic energy is unlikely to be conserved

m₁ u₁ = m₁ v₁ + m₂ v₂

m₁ u₁ =mv

0 = m₁ v₁ + m₂ v₂

Recoil shortcut

The momentum of the two objects are equal and opposite so: m₁v₁ = (-) m₂v₂