Chapter 4- Newton’s Second Law of Motion
- Acceleration depends on net force
o a= F/m
o F= ma
- Why does stuff accelerate?
o An outside force is acting upon it.
- (1)A 1-kg rock is thrown at 10 m/s straight upward. Neglecting air
resistance, what is the net force that acts on it when it is halfway to the top of
- (2) What would be the net force halfway up, if you were to throw it at 100
o (1) Gravity is the net force when halfway to the top
o (2) Gravity- after leaving your hand the only force acting upon the ball
- Three identical blocks are pulled, as show, on a horizontal frictionless
surface. If tension in the rope held by the hands is 30 N, what is the tension in
the other ropes?
- Free fall
o Only one fact acts on object- gravity
o Comes in two “flavors” static and kinetic (sliding)
▪ Static, when stuff at rest and there is friction
- Why do we have groves in car tires and none in skateboard tires?
o Tires with grooves to displace and redirect water from between the
road surface and underside of the tire.
- Friction is always opposed to the motion
- When Sanjay pushed a refrigerator across a kitchen floor at a constant speed
the force of friction between the refrigerator and floor is
o Equal and opposite to Sanjay’s push
- When Sanjay pushes a refrigerator across a kitchen floor at an increasing
speed, the amount of friction between the fridge and floor is
o Less than Sanjay’s push
- Suppose in a high-flying airplane the captain announces over the cabin public
address system that the plane is flying at a constant 900 km/h and the thrust
of the engines is a constant 80,000 Newton’s
o The acceleration is zero because the velocity is constant because the
acceleration is zero, the net force is zero.
▪ Air resistance is 80,000
- A 400-kg bear grasping a vertical tree slides down a constant velocity. What
is the friction force that acts on the bear? o 400 kg * 10 m/s^2 (gravity)= 4,000 N
- Can you distinguish between mass and weight?
o Mass: matter in an object
o Weight: force due to gravity.
- Since an object weighs less on the surface of the Moon than on Earth’s
surface does it have less inertia on the Moon’s surface?
o Inertia is the same but the weight is different (less on the Moon)
- What relationship does mass have with inertia
o As mass increases so does inertia
- What relationship does mass have with weight? What’s the more
fundamental mass or weight?
o Mass in an intrinsic factor
- Checkpoint 6: Nellie Newton skydives from a high-flying helicopter. As she
falls faster and faster through the air, does her acceleration increase,
decrease, or remain the same?
o Acceleration decreases because net force decreases.
- Why is it that a cat that accidently falls from the top of a 50-story building
hits a safety net below no faster than if it fell from the 20-th story?
o The cat reaches terminal velocity at or before falling 20 stories and
therefore falling from a higher point doesn't cause the cat to go faster
- A falling skydiver of mass 100 kg experiences 500 N air resistances. The
acceleration of the skydiver is
o Acceleration= net force/ mass= 500 N/ 100 kg= 0.5 g
Clicker and Homework Questions
1. Does a 2-kg iron brick have twice as much inertia as a 1-kg iron brick?
2. Compare to a 1-kg block of solid iron, a 2-kg block of solid iron has the same
None of these
3. Consider a cart pushes along a rack with a certain force. If a force remains the
same while the mass of the cart decreases to half, the acceleration of the cart
4. Push a cart along a track so twice as much net force acts on it. If the
acceleration remains the same, what is a reasonable explanation?
A mass of the cart doubled
5. A skydiver steps