P2 – Forces and Motion Lesson 4: Forces 1. Lesson Objectives To know the difference between mass and weight. To be able to calculate mass and weight.

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.


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.


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)


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.


The Three Laws of Motion


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.


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).


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.


Why?

Team 1 Team 2

6N 4N

The resultant force is 2N towards team 1




Any resultant force will produce acceleration, and is calculated by:

F = ma (F = Force, m = mass, a = 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.



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



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


5200N


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


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”.



So:

If you push pram, the pram will push back against you just as hard.

When you stop pushing, so does the pram.



Q: If this is happening, then how do we ever go anywhere?



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)


Skater A Skater B


So:

Both Skaters feel the same sized force, in opposite directions.

Therefore, they accelerate in opposite directions away from each other.


Skater A Skater B


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.


Task: How does mass affect acceleration?


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.


P2 – Forces and Motion Lesson 4: Forces 1. Lesson Objectives To know the difference between mass and weight. To be able to calculate mass and weight.

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