Chapter 3: Forces

The amount of force needed to change an object’s motion depends upon the object’s mass and the acceleration used during the change.

Newton’s Second Law

F = force, units are Newtons (N)m = mass, units MUST be kga = acceleration, units MUST be m/s2

Examples:1. How much force is needed to accelerate a

1000 kg truck at a rate of 3 m/s2 ?

2. A force of 10 N is used to push a 10 kg box. What is its acceleration?

3. Applying a force of 10 N to an object causes the object to accelerate at a rate of 5 m/s2. What is the mass of the object?

F = ma F = 3000 NF = (1000 kg)(3 m/s2)

F = ma

F = ma

10 N = (10 kg)a a = 1 m/s2

10 N = m(5 m/s2) m = 2 kg

Types of forcesFriction: a force that occurs when two touching objects move past each other. Frictional force is always in the opposite direction to the motion.

Ff FappFapp Ff

More downward force (weight) pushes the bumps closer together.

This causes the static friction to increase.

Force More force

Microwelds form where bumps come into contact. If the applied force is not big enough to break the microwelds, the object will not move.

Fapp

Types of forcesGravitational: occurs between any two objects and depends only the masses of the objects and the distance between them

•The three Fg vectors show how the masses of the Earth, the moon and the astronaut attract each other.

The vector between the astronaut and the Moon

The vector between the Moon and the Earth

1. Which one is the most significant to the astronaut?

2. To the Moon?

Fg

Fg

Fg

Force of gravity = weight

Close to the Earth’s surface, all objects are accelerated by gravity at a rate of 9.8 m/s2

0 s

1 s

2 s

5 m

20 m

Fg

Fg = mgg = 9.8

m/s2

Projectile motionvy

vx

vy = 0

vx

vx

vy

There is no acceleration in the x direction: vx is constantGravity changes vy. In the first half of the throw, the upward motion of the ball is slowed down by gravity.In the second half of the throw, gravity accelerates the ball’s vy.