Forces

FORCES Sec 4 http://www.uq.edu.au/_School_Science_Lessons/PushPull0.GIF

BTEC, you should be able to:
Describe the effects of a force
Recall the relationship resultant force = mass x acceleration (F = m x a)
Solve related problems with above relationships
Understand mass, weight and acceleration
Understand weight and able to differentiate weight and mass
Recall the relationship weight = mass x gravity Understand Inertia

Force
Think of force as a PUSH or PULL
The block was pushed/pulled by tractor
Both blocks experience a force to the right.
Force felt by block is provided by tractor
PUSH PULL

Types of Forces
Magnetic force
force due to magnetic attraction and repulsion
Weight
force acting on a body due to gravitational pull
Tension
force at the ends of a stretched string, spring or rope
Contact force
force that appears only when there is contact between two bodies
Friction
force due to roughness of surfaces

Force
When a force acts on an object, it tends to cause the object to accelerate.
Other effects of force are:
Decelerate an object
Change the direction of motion of a body
Change the shape and size of a body

Effect of force
Make a stationary object move

Effect of force
Change the speed of a body

Effect of force
Change the direction of motion of a body without its speed

Effect of force
Change the shape and size of a body

Understanding Net Force
What happens if an object is pushed by two set of forces as shown above?
The result is that the object experiences a SUM of the forces, which is 10 + 5 = 15 N.
In this case, we say that the net force is 15 N.
10 N 5 N = 15 N

Understanding Net Force
What happens if an object is pushed by two set of opposite forces as shown above?
The result is that the object experiences a SUM of the forces, which is 10 - 5 = 5 N.
In this case, we say that the net force is 5 N.
10 N 5 N = 5 N

Net force
What is the net force acting on objects below?
A 2N 8N A 6N B 2N 10N B 12N C 8N 8N C No net force D 3N 5N D 2N 10N

Net force
What is the net force acting on objects below?
8N A 6N B 12N C No net force D 2N A 2N 8N B 2N 10N C 8N D 3N 5N 10N

Resolving Forces in 2D

Force
Force can be calculated with this simple equation:
Net Force Mass Acceleration (N) (kg) (m/s 2 ) SI Unit: newton ; symbol : N

Quick Check
A car of mass 1500 kg accelerates at 2 m/s 2 . Calculate the forward force provided by the engine.
F = m x a
= 1500 x 2
= 3000 N

Quick Check
A 1800 kg van is reversing. Backward force provided by the engine is 2400 N. Calculate the backward acceleration of the van.
F = m x a
2400 = 1800 x a
a = 2400 / 1800
= 1.33 m/s 2

Quick Check
A boat of mass 500kg is moving against the flow of a river. Current of river produces a force of 1000 N. Engine of the boat produces a forward force of 3000 N. What is the net force acting on the boat?
Net force = 3000 – 1000 = 2000 N
What is the acceleration of the boat?
F = m x a
2000 = 500 x a
a = 2000 / 500 = 4 m/s 2
3000 N 1000 N

Quick Check
A small rocket with mass 800 kg has a weight of 8000 N. At take-off, the rocket engine produces an upward force of 10 000 N.
What is the net upward force acting on the rocket during take-off?
Net force, F = 10,000 – 8000
= 2,000 N
8000 N 10,000 N

Quick Check
What is the acceleration of the rocket?
F = m x a 
a = F/m
= 2000 N / 800 kg
= 2.5 m/s 2
8000 N 10,000 N

Mass and Weight
In the study of mass and weight, you first need to forget the day-to-day understanding of weight.
For example, “the weight of a pupil is 50 kg” is WRONG! FORGET THIS, at least temporarily.
Mass of an object can be understood as the amount of atoms that makes up the object.*
* This is not strictly correct, but as a beginner it is useful to think of it this way

Mass
Mass of a handphone is always 0.2 kg, in Singapore, or US or Brazil or even on the Moon or Mars.
A 1.5 liter PET bottle of Coke will always have the same mass even if the drink is poured out into a container. (shape of liquid has changed but not the mass)

Mass
In Physics is defined as (this means that if asked in exams, please write this statement down)
A measure of the amount of matter in a body
Symbol: m
SI Unit: kilogram (kg)
Mass of an object cannot be changed by its:
Shape
Location

Weight
Weight of an object is the effect of gravity on it.
For instance, a box has a mass of 1 kg. (this means that the box has a great number of atoms worth 1 kg)
For a 1kg box on Earth, it weighs 10 N. However if the box is on the Moon, where the gravity is only 1/6 of Earth, the box weighs only 10/6 N.

Weight
In other words, you might not be able to lift a 60kg dumb-bell on Earth but on the Moon, you possibly can because it is now appears lighter.
On Earth, the dumb-bell weighs 600 N while on the Moon is only 100 N.

Weight
Weight of an object can be measured by simply multiplying the mass of a object with the gravity of its location.
Weight = Mass x Gravitational of object of object strength (N) (kg) (N/kg)

Quick check
A book has a mass of 1.5 kg. What is its weight on Earth?
W = m x g
= 1.5 x 10
= 15 N
If gravitational acceleration of Moon is 1/6 of Earth’s, how much is the book’s weight on the Moon.
W = m x g
= 1.5 x (1/6 x 10)
= 2.5 N
Ask yourself, what is the mass of the book on the Moon? Has it changed? Can it change?

Quick check
What is the mass of a boy who weighs 450N on the surface of the Earth?
W = m x g  m = W/g
= 450/10
= 45 kg

Friction
A force that opposes motion
Block is pulled to the right by force F
So, block tends to move to the right
But friction acts on the block towards the left
Force, F Friction

Friction
Consider that the friction is constant. This means for two particular surfaces in contact, the friction force between is fixed. For other surfaces, the value of frictional force could be different. *
* not strictly true, but for N-levels, consider it so
Force, F Friction

Friction
When force applied is more than friction, the box will start moving. In fact it will accelerate.
If the force applied is removed, the box will stop moving due to friction, since it opposes motion.
However if the force is reduced such that it is equal to friction, the result is that the box will move at constant speed.
Force, F Friction

Friction – What’s Good about it?
Allows us to walk
Allows a car to move
Allows us to hold a chopstick

Friction – What’s bad about it?
Causes wear and tear – brake pads of vehicles, soles of shoes
Reduces efficiency of performance – traveling

Solutions to overcome friction
Moving on air cushion or electromagnetic fields
Use rollers or wheels
Lubricate surface
Smoothen surface
Using materials with very low frictional resistance

Quick check
A 5kg box is pushed with a forward force of 10N on a rough surface with friction of 4N, what will be the acceleration of the box?
Net force on box = 10 – 4 = 6N
F = m x a  a = F/m
= 6/5
= 1.2 m/s 2
5 kg 10 N 4 N

Quick check
Tom pushes a crate of mass 20kg across a floor at constant velocity of 0.2m/s by exerting a horizontal force of 50N.
What is the resultant force acting on crate?
Since velocity is constant, there is no acceleration, so (by Newton’s 2 nd Law) resultant force is 0.
20 kg 50 N Friction ? N 0.2m/s

Quick check
What is the frictional force acting on crate?
Since resultant force = 0 N, frictional force must also be 50 N, but in opposite direction.
20 kg 50 N Friction ? N 0.2m/s

Quick check
Tom now increases his force to 80N.
What is the new resultant force on crate?
Frictional force remains at 50 N.
Resultant force = 80 – 50 = 30 N
20 kg 80 N Friction ? N

Quick check
What is the acceleration of the case?
F = m x a  a = F/m
= 30 / 20
= 1.5 m/s 2
20 kg 80 N Friction ? N

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Forces

by meenng on Sep 17, 2007

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