1. Do Now:
1. Draw a free body diagram (all the forces) acting on a water skier being pulled by a boat.
2. What is the net force acting on this object?
4 N
5 N
3 N
1 N
3 N

2. Principles of Physics - Foederer
Newton’s Laws
Principles of Physics - Foederer

3. Newton’s 2nd Law (N2L) Fnet = net force (N) m = mass (kg)
a = acceleration(m/s2)
so F = ma
(kg) (m/s2) = Newton

4. When Fnet = 0, acceleration = 0 The object is in equilibrium
When acceleration = 0
When Fnet ≠ 0, object is accelerating
object is either not moving or
moving at constant speed
-changing speed
-changing direction

5. Equilibrium Examples
1. Determine if the object below is in equilibrium
40 kg
200 N
The object is not in equilibrium because Fnet = 200 N
2. Calculate the acceleration of the object
Fnet = ma
200N = 40 kg (a)
a = 5 m/s2

6. Equilibrium Examples
1. Determine if the object below is in equilibrium
100 kg
100 N
150 N
250 N
The object is in equilibrium because Fnet = 0
2. Calculate the acceleration of the object
Fnet = ma
0 = 100 kg (a)
a = 0