1. Instructor: Dr. Tatiana Erukhimova
Physics 218: Mechanics
Instructor: Dr. Tatiana Erukhimova
Sections 807, 808, 809
Lecture 7

2. A can drops from a magnet just when a bullet is shot from a gun: Find the angle that the gun must be aimed at to hit the can.

3. A cannon at the origin points up at an angle θ with the x axis
A cannon at the origin points up at an angle θ with the x axis. A shell is fired which leaves the barrel with a velocity of magnitude Vm.
When does the shell reach its maximum height?
What is the maximum height?
What is the range (horizontal distance)?
What is the velocity of the shell when it hits the ground?

4. Hw Quiz
An object is at the point x = L and y = H at the time t=0. It has velocity given by
where c1 and c2 are constants. Find the position and acceleration as functions of time.

5. A faulty model rocket moves in the xy-plane (the positive y-direction is vertically upward). The rocket’s acceleration has components ax(t)=t2 and ay(t)=-t, where =2.50 m/s4, =9.00 m/s2, and =1.40 m/s3. At t=0 the rocket is at the origin and has velocity with and
Calculate the velocity and position vectors as functions of time.
What is the maximum height reached by the rocket?
What is the horizontal displacement of the rocket when it returns to y=0?

6. A physics professor did daredevil stunts in his spare time
A physics professor did daredevil stunts in his spare time. His last stunt was an attempt to jump across a river on a motorcycle. The takeoff ramp was inclined at 53.00, the river was 40.0 m wide, and the far bank was 15.0 m lower than the top of the ramp. The river itself was 100 m below the ramp. You can ignore air resistance. a) What should his speed have been at the top of the ramp to have just made it to the edge of the far bank? b) If his speed was only half the value found in (a), where did he land?

7. Dynamics Connection between force and motion
The concept of force gives us a quantitative description of the interaction between two bodies or between a body and its environment

8. Newton’s Laws
1st Law: A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration
2nd Law: The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object.
3rd Law: For every action there is an equal, but opposite reaction

9. 2nd Law From experiments we know: Force is a vector
The direction of acceleration vector is the same as the direction of the force vector
The magnitude of the force and acceleration are related by a constant which depends on number of blocks involved.

10. Newton’s second law
The vector acceleration of an object is in the same direction as the vector force applied to the object and the magnitudes are related by a constant called the mass of the object.

11. -is deviation from the natural length
Gravitational force
Normal force
Force exerted by a spring:
Hooke’s law: If spring is stretched or compressed by some small amount it exerted a force which is linearly proportional to the amount of stretching or compressing. The constant of proportionality is called the spring constant
-is deviation from the natural length

12. The force resisting the pull of the spring – friction
There is some maximum value the friction force can achieve, and once we apply a force greater than this maximum there is a net force on the object, so it accelerates.
The maximum of the force of friction varied linearly with the amount that the block pushes on the table.
 - coefficient of friction, is the vertical force exerted by the block on the table
The friction force only exists when there is another force trying to move an object

13. FFriction = mKineticN Kinetic Friction THIS IS NOT A VECTOR EQUATION!
For kinetic friction, it turns out that the larger the Normal Force the larger the friction. We can write
FFriction = mKineticN
Here m is a constant
Warning:
THIS IS NOT A VECTOR EQUATION!

14. A Recipe for Solving Problems
Sketch
Isolate the body (only external forces but not forces that one part of the object exert on another part)
2. Write down 2nd Newton’s law
Choose a coordinate system
Write 2nd Newton’s law in component form:
3. Solve for acceleration

15. Pulling Against Friction
A box of mass m is on a surface with coefficient of kinetic friction m. You pull with constant force FP at angle Q. The box does not leave the surface and moves to the right.
What is the magnitude of the acceleration?
What angle maximizes the acceleration? Q

16. Have a great day! Reading: Chapter 5,6
Hw: Chapter 6 problems and exercises