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Mass and Weight• What is the difference between the terms
mass and weight?
• What are some of the important effects of gravity?
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Mass and Weight• Mass – the amount of material. Measured in Kilograms (kg).
This is the same anywhere in the universe.
• Weight – the force which attracts a body. Measured in Newtons by a Newton meter (N). This is caused by the pull of gravity.
• Therefore, 1kg bag of sugar has the same mass on Earth and the moon, but a different weight (10N on Earth, 1N on the moon) as the gravity on the moon is less.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
GravityGravity has three important effects:
1.On the surface of a planet, it makes all things accelerate towards the ground (on Earth it is about 10m/s2)
2.It gives everything a weight.3.It keeps planets, moons and satellites in
their orbit.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Very important formulaRemember, mass is measured in kilograms
(kg) and weight is measured in Newtons (N).
So, you can work out the weight of an object using:
W = m x g (W = weight, m = mass, g = gravity strength)
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Very important formulaW = m x g (W = weight, m = mass, g = gravity strength)
Q: What is the weight, in newtons, of a 5kg mass, both on Earth and on the moon?(‘g’ on moon is 1.6N/kg, ‘g’ on Earth is 10N/kg)
A: W = m x g. Earth = 5 x 10 = 50N, Moon = 5 x 1.6 = 8N.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
The Three Laws of Motion
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Sir Isaac Newton1643 - 1727
First Law: Balanced Forces mean no change in velocity.
• Remember: Velocity is the speed in a certain direction.
• So: As long as there is no change in the forces on an object, it will either:
1.Stay stationary2.Travel at a constant speed.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
First Law: Balanced Forces mean no change in velocity.
Why?
Team 1 Team 2
6N 6N
The resultant force is 0 (both forces are balanced).
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
This means that if there is a resultant force (i.e. the forces are unbalanced) there will be movement.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Why?
Team 1 Team 2
6N 4N
The resultant force is 2N towards team 1
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Second Law: A Resultant Force means acceleration.
Any resultant force will produce acceleration, and is calculated by:
F = ma (F = Force, m = mass, a = acceleration)
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Remember:
1. The bigger the force, the greater the acceleration or deceleration.
2. The bigger the mass, the smaller the acceleration.
3. To get a big mass to accelerate as fast as a small mass it needs a bigger force.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
Task:1. A car of mass 1750kg has an engine which
provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
a. When first setting off from rest (assume that there is no friction at rest)
b. When at 70mph
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Second Law: A Resultant Force means acceleration.
2. A car of mass 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
a. When first setting off from rest
Resultant Force = 5200Na = F/m = 5200/1750 = 3.0m/s2
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
5200N
Second Law: A Resultant Force means acceleration.
2. A car of mass 1750kg has an engine which provides a driving force of 5200N. At 70mph the drag force in 5150N. Find its acceleration:
b. When at 70mph
Resultant Force = 5200 – 5150 = 50Na = F/m = 50/1750 = 0.03 m/s2
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
5200N5150N
Third Law: Reaction Forces
Think about this one!
This law says “if an object exerts a force on to another object, then that object exerts an equal and opposite force back”.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
So:
If you push pram, the pram will push back against you just as hard.
When you stop pushing, so does the pram.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Q: If this is happening, then how do we ever go anywhere?
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Third Law: Reaction Forces
Think about:
Skater ‘A’ pushes Skater ‘B’ (the action force).
Skater ‘A’ feels an equal and opposite force from Skater ‘B’s hands (the reaction force)
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Skater A Skater B
Third Law: Reaction Forces
So:
Both Skaters feel the same sized force, in opposite directions.
Therefore, they accelerate in opposite directions away from each other.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Skater A Skater B
Third Law: Reaction Forces
Another thought:
Think about swimming. You push against the water
with your arms and legs. The water pushes you in
the opposite direction, with the same force.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Task: How does mass affect acceleration?
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,
Are you now able to….
I still feel unsure. I need some more help to understand.
I feel ok. I need to do some more work to check my understanding.
I am happy and feel I understand and can explain the main points.
Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,Keywords: Gravity, Mass, Weight, Balanced, Resultant, Reaction,