1. Unit 2: Forces in Physics
VHS Physics
Unit 2: Forces in Physics

2. Unit 2: Forces in Physics
Learning Targets:
2.A: Force of Gravity
2.B: Friction
2.C: Hooke’s Law
Argument Lab: Friction

3. Force!
Every time you push, pull, twist or squeeze something you exert a force on it.
A force is a push or a pull
Forces are measured in a unit called the newton (N)

4. 2. A Force of Gravity Gravitational Force
The force of gravity pulls on you all the time.
Any two bodies in the universe exert a gravitational force on each other. The amount of force they exert depends upon how massive the bodies are and how far apart they are.
Two things to keep in mind about gravity
It cannot be “shut off”
Its always an attractive force, not a repulsive force.

5. Gravitational Field Strength
Gravity acts on objects without touching them. It is an action-at-a-distance force. This happens because gravity creates a gravitational field around a body that depends on the mass of the object.
We quantify gravitational field (Fg) strength using this equation:
g = Fg / m
g is gravity (9.81 N/kg at earths surface) and m is mass

6. Weight
As we discussed in Unit 1, objects fall with the same acceleration regardless of their mass. Other forces such as air resistance may slow an object down, but the acceleration due to gravity remains constant.
Gravitational force is equal to the product of an objects mass and the acceleration due to gravity.
Fg = m x g
When we calculate gravitational force acting on an object, we are calculating its weight.

7. Quick Check – Gravity as a Force
What is the force of gravity on a 90 kg person? What is the weight of that person?
If a person experiences a 637 N force of gravity on Earth’s surface, what is the person’s mass?
A 75 kg person would experience a force of gravity of a N on the Moon. What is the gravitational field strength on the Moon?

8. Quick Check Answers
8.8 x 102 N
64.9 kg
1.7 N/kg

9. Measuring the Force of Gravity
Gravity causes unsupported objects to fall toward Earth. The usual way to measure the force of gravity is to balance it with another force acting upward.
Example: when you stand on a bathroom scale, gravity pulls you downward. A coiled spring inside the scale pushes upward and balances the force of gravity.

10. Measuring the force of gravity
We will be using a spring scale to measure gravity in class – the amount of stretch in a spring scale is directly proportional to the force of gravity on the object.

11. Newton’s Law of Universal Gravitation
Every body in the universe attracts every other body with a force that is:
directly proportional to the product of the masses of the two bodies, and
is inversely proportional to the square of the distance between the centers of mass of the two bodies

12. Newton’s law of universal gravitation
G = gravitational constant
m1 and m2 = masses of the two bodies attracting each other
r = distance between the centers of the two bodies
G = 6.67 x N x m2 / kg2

13. Earth’s Gravitational Force
To calculate the force of gravity on a mass (m), you could multiply the mass by the gravitational field strength (g). F = mg
You could also use the law of universal gravitation

14. Check in! – paper copies in the folder