1. Physical Science Ms. Pollock 2008-2009
Newton’s Second Law
Physical Science
Ms. Pollock

2. Force, Mass, and Acceleration
Throwing and dropping, different forces
Greater force = greater acceleration
Different masses, different forces
Greater mass = greater acceleration
Force, mass, and acceleration related mathematically in Newton’s Second Law
Acceleration (m/s2) = net force (in newtons)
mass (in kg)
A = fnet m

3. Newton's Second Law Example
You push a friend on a sled. Your friend and the sled together have a mass of 70 kg. If the net force on the sled is 35 N, what is the sled’s acceleration?
A = fnet ÷ m
A = 35 N ÷ 70 kg
A = 0.5 m/s2

4. Calculating Net Force with the Second Law
Force calculated, if mass and acceleration known
Fnet = ma
Example: When a tennis player hits a ball with a mass of 0.06 kg, the acceleration is 5,000 m/s2. What is the force exerted on the ball?
F = ma
F = (0.06 kg)(5,000 m/s2)
F = 300 N

5. Friction
Even after force is applied, objects slow down and eventually stop.
Deceleration implies that a force is acting on the object.
Friction – force that opposes the sliding motion of two surfaces that are touching each other
Amount of friction dependent on kinds of surfaces and force pressing surfaces together
Caused by microwelds between surfaces in contact

6. Friction Bumps weld together Larger force = stronger microwelds
A force must be applied to break the microwelds and allow one surface to move over the other.

7. Static Friction No motion = zero acceleration = no net force
Static friction – frictional force that prevents two surfaces from sliding past each other
Microwelds between objects that cannot be broken, so object does not move.

8. Sliding Friction
With added force, the object moves, but when the force is removed, it stops.
Microwelds broken but reformed as objects slide
Sliding friction – force that opposes the motion of two surfaces sliding past each other

9. Rolling Friction
Frictional force between rolling object and the surface it rolls on
Wheel and surface deformed to allow object to roll
Static friction acting over deformed area
Also causes rolling object to slow down and stop

10. Air Resistance Falling object pulled downward by gravity
Air resistance – friction- like force that opposes motion of objects moving through air
Without air resistance, all objects would fall with the same acceleration.
Amount dependent on speed, size, and shape of object

11. Terminal Velocity
Acceleration causes an object’s speed to increase as it falls.
Increasing speed increases the air resistance on the object.
Air resistance will eventually balance the force of gravity.
Acceleration becomes zero, so speed becomes constant.
Terminal velocity – highest speed a falling object will reach

12. Terminal Velocity Depends on size, shape, and mass of falling object
Greater surface area = greater air resistance