Newton’s 3rd Law & Forces in
Two Dimensions
Mr. Kuffer
Concepts of Physics
Newton’s 3rd Law
Name:______Period:____
- In the interaction between a hammer and the nail it hits…
- Is the forces exerted on the nail?
- On the hammer?
- How many forces occur in this interaction?
- When the hammer exerts a force on the nail…
- How does the amount of force compare with that of the nail on the hammer?
- When you walk along the floor…
- What pushes you along?
- When you swim, you push water backwards – call this the action. What is the reaction force?
- When a rifle is fired…
- How does the size of the force of the rifle on the bullet compare with the force of the bullet on the rifle?
- How does the acceleration of the rifle compare with that of the bullet? (defend your answer)
- Your weight is the result of the gravitational force of the earth on your body. What is the corresponding reaction force?
- Why can’t you hit a feather in midair with a force of 200N?
- A bullet is fired from a rifle. The mass of the bullet is .0075 kg. The mass of the rifle is 4.2 kg. The bullet pushes the rifle with a force of -6.5 N (recoil force).
- What is the force by the rifle on the bullet?
- Draw a Free-Body Diagram of the scenario (indicate the magnitude and direction of the forces and accelerations)
Force Acceleration
- What is the acceleration of the bullet and the rifle?
Bullet: Rifle:
- Not counting air resistance, how fast will the bullet be traveling in 0.7 seconds?
Newton’s 3rd Law
Name:______Date:______
1. While driving, Anna Litical observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?
2. Rockets are unable to accelerate in space because ...
- there is no air in space for the rockets to push off of.
- there is no gravity is in space.
- there is no air resistance in space.
- ... nonsense! Rockets do accelerate in space.
3. A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...
- greater than the acceleration of the bullet.
- smaller than the acceleration of the bullet.
- the same size as the acceleration of the bullet.
4. In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. The physics student is pulling
- with more force when the rope is attached to the wall.
- with more force when the rope is attached to the Strongman.
- the same force in each case.
Newton’s 3rd Law
- State Newton’s 3rd law.
- Give three everyday examples of his 3rd law.
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- A bullet (mass = 8g) is fired from a rifle (mass = 3.5 kg). The bullet pushes the rifle with a force of -6.5 N.
- What is the force by the rifle on the bullet?
- Draw a sketch of the scenario (indicate direction and magnitude of the forces and accelerations)
- What is the acceleration of the bullet and the rifle?
- Not counting air resistance, how fast will the bullet be traveling in .67 seconds?
- Not counting air resistance, how far will the bullet travel in .67 seconds?
- A 2.5 kg object falls to the earth. Calculate the acceleration of both the earth and the object. (Mass of the earth is 5.96 x 1024 kg)
Recall:
Newton says ‘For every action there is an equal and opposite reaction’
FAonB = -FBonA
Native American Lady Bug2008 F-150
Mass = 3.5 gMass = 2289 kg (5000 lbs)
Velocity = 4.8 m/s, SouthVelocity = 24.6 m/s (55mph), North
Force supplied by the truck = ______Force supplied by the bug = -10 N
AND THE WINNER IS…
Let’s talk acceleration…
What is the bug’s acceleration?
What is the truck’s acceleration?
HHHMMMmm?