1. How to use Newton’s Second Law

2. Newton’s first law predicts motion of objects with forces which are balanced.
Objects at rest
v = 0 m/s
Remain at rest
Forces are balanced
a = 0 m/s2
Objects in motion
v ≠ 0 m/s
Stay in motion at constant velocity

3. But when the forces are unbalanced
ACCELERATION results!
The acceleration depends on:
NET force
Mass of the object
The acceleration is in the same direction as the net force. Both are vectors!

4. Is there a net force? Yes or No?

5. Example Problem 1
Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

6. Example Problem 2
A 1380-kg car is moving due east with an initial speed of 27.0 m/s. After 8.00 s, the car has slowed down to 17.0 m/s. Find the magnitude and direction of the net force that produces the deceleration.

7. Force of GRAVITY
A ball is dropped from the top of a building. Find the force of gravity on the ball in terms of the mass of the object m and the acceleration of gravity g.
In general, the force of gravity on any object can be calculated with the formula
Fg = mg

8. So what’s so hard about Newton’s Second Law? Thinking question:
Is this object moving?
If so, in what direction?
20 N

9. What’s the net force?

10. Find the net force

11. Find the unknown forces

12. Now, use Newton’s 2nd Law:
Kate and Rob exert a 2.45 N rightward force on a kg cart to accelerate it across a low- friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart's acceleration?

13. A 200kg crate is being lifted by a steel cable.
If the crate is being lifted at a constant velocity of 2 m/s, what is the tension in the cable?
If the crate is being lifted with an acceleration of 3 m/s2 then what is the tension in the cable?
If the crate is now being lowered with an acceleration of 1 m/s2 what is the tension now?

14. Another example
In a Physics lab, Ernesto and Amanda apply a 34.5 N rightward force to a 4.52-kg cart to accelerate it across a horizontal surface at a rate of 1.28 m/s2. Determine the friction force acting upon the cart.

15. The parachute on a race car that weighs 8820N opens at the end of a quarter-mile run when the car is traveling 35 m/s.
What total retarding force must be supplied by the parachute to stop the car in a distance of 1100m?