Newton’s 2 nd and 3 rd Laws. Objectives Describe the acceleration of an object in terms of its mass and the net external force acting on it. Predict the direction and magnitude of the acceleration caused by a known net external force . Identify action-reaction pairs.
If there is NONET FORCE on an object, then it is at EQUILIBRIUM and either:
MOTIONLESS OR MOVING WITH CONSTANT VELOCITY
So a “net” or “unbalanced” force will
CHANGE AN OBJECT’S VELOCITY
Changing velocity means ACCELERATION
How much acceleration?
AMOUNT OF FORCE
MORE FORCE = MORE ACCELERATION
Acceleration is directly related to force
MASS OF OBJECT
MORE MASS = LESS ACCELERATION
Acceleration is inversely related to mass
“The acceleration of an object is directly proportional to
the net external force acting on the object and inversely
proportional to the mass of the object.”
Unit of force is theNEWTON (N)
A 2 kilogram box is pushed with a net, unbalanced force of 10 newtons.
What is the acceleration experienced by the box?
a = Fnet / m
a = (10 N) / (2 kg)
a = 5 m/s2
A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose?
A man getting out of a rowboat jumps north onto the dock. What happens to the boat?
There will be a force on the hose to the WEST
The boat will move to the SOUTH
Action: air pushing the balloon out.
Reaction: balloon pushing air in.
Action: bat hit the ball
Reaction: ball hit the bat
Action: ball hit the mitt
Reaction: mitt hit the ball
Force on the ground
FgAction/reaction forces vs. equilibrium forces
1. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?
2. For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ...
c. ... space is void of air and so there is no air resistance in space.
d. ... nonsense! Rockets do accelerate in space and have been able to do so for a long time.
3. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...
c. the same size as the acceleration of the bullet.